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Writing a Literature Review

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A literature review is a document or section of a document that collects key sources on a topic and discusses those sources in conversation with each other (also called synthesis ). The lit review is an important genre in many disciplines, not just literature (i.e., the study of works of literature such as novels and plays). When we say “literature review” or refer to “the literature,” we are talking about the research ( scholarship ) in a given field. You will often see the terms “the research,” “the scholarship,” and “the literature” used mostly interchangeably.

Where, when, and why would I write a lit review?

There are a number of different situations where you might write a literature review, each with slightly different expectations; different disciplines, too, have field-specific expectations for what a literature review is and does. For instance, in the humanities, authors might include more overt argumentation and interpretation of source material in their literature reviews, whereas in the sciences, authors are more likely to report study designs and results in their literature reviews; these differences reflect these disciplines’ purposes and conventions in scholarship. You should always look at examples from your own discipline and talk to professors or mentors in your field to be sure you understand your discipline’s conventions, for literature reviews as well as for any other genre.

A literature review can be a part of a research paper or scholarly article, usually falling after the introduction and before the research methods sections. In these cases, the lit review just needs to cover scholarship that is important to the issue you are writing about; sometimes it will also cover key sources that informed your research methodology.

Lit reviews can also be standalone pieces, either as assignments in a class or as publications. In a class, a lit review may be assigned to help students familiarize themselves with a topic and with scholarship in their field, get an idea of the other researchers working on the topic they’re interested in, find gaps in existing research in order to propose new projects, and/or develop a theoretical framework and methodology for later research. As a publication, a lit review usually is meant to help make other scholars’ lives easier by collecting and summarizing, synthesizing, and analyzing existing research on a topic. This can be especially helpful for students or scholars getting into a new research area, or for directing an entire community of scholars toward questions that have not yet been answered.

What are the parts of a lit review?

Most lit reviews use a basic introduction-body-conclusion structure; if your lit review is part of a larger paper, the introduction and conclusion pieces may be just a few sentences while you focus most of your attention on the body. If your lit review is a standalone piece, the introduction and conclusion take up more space and give you a place to discuss your goals, research methods, and conclusions separately from where you discuss the literature itself.

Introduction:

• An introductory paragraph that explains what your working topic and thesis is
• A forecast of key topics or texts that will appear in the review
• Potentially, a description of how you found sources and how you analyzed them for inclusion and discussion in the review (more often found in published, standalone literature reviews than in lit review sections in an article or research paper)
• Summarize and synthesize: Give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: Don’t just paraphrase other researchers – add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically Evaluate: Mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: Use transition words and topic sentence to draw connections, comparisons, and contrasts.

Conclusion:

• Summarize the key findings you have taken from the literature and emphasize their significance
• Connect it back to your primary research question

How should I organize my lit review?

Lit reviews can take many different organizational patterns depending on what you are trying to accomplish with the review. Here are some examples:

• Chronological : The simplest approach is to trace the development of the topic over time, which helps familiarize the audience with the topic (for instance if you are introducing something that is not commonly known in your field). If you choose this strategy, be careful to avoid simply listing and summarizing sources in order. Try to analyze the patterns, turning points, and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred (as mentioned previously, this may not be appropriate in your discipline — check with a teacher or mentor if you’re unsure).
• Thematic : If you have found some recurring central themes that you will continue working with throughout your piece, you can organize your literature review into subsections that address different aspects of the topic. For example, if you are reviewing literature about women and religion, key themes can include the role of women in churches and the religious attitude towards women.
• Qualitative versus quantitative research
• Empirical versus theoretical scholarship
• Divide the research by sociological, historical, or cultural sources
• Theoretical : In many humanities articles, the literature review is the foundation for the theoretical framework. You can use it to discuss various theories, models, and definitions of key concepts. You can argue for the relevance of a specific theoretical approach or combine various theorical concepts to create a framework for your research.

What are some strategies or tips I can use while writing my lit review?

Any lit review is only as good as the research it discusses; make sure your sources are well-chosen and your research is thorough. Don’t be afraid to do more research if you discover a new thread as you’re writing. More info on the research process is available in our "Conducting Research" resources .

As you’re doing your research, create an annotated bibliography ( see our page on the this type of document ). Much of the information used in an annotated bibliography can be used also in a literature review, so you’ll be not only partially drafting your lit review as you research, but also developing your sense of the larger conversation going on among scholars, professionals, and any other stakeholders in your topic.

Usually you will need to synthesize research rather than just summarizing it. This means drawing connections between sources to create a picture of the scholarly conversation on a topic over time. Many student writers struggle to synthesize because they feel they don’t have anything to add to the scholars they are citing; here are some strategies to help you:

• It often helps to remember that the point of these kinds of syntheses is to show your readers how you understand your research, to help them read the rest of your paper.
• Writing teachers often say synthesis is like hosting a dinner party: imagine all your sources are together in a room, discussing your topic. What are they saying to each other?
• Look at the in-text citations in each paragraph. Are you citing just one source for each paragraph? This usually indicates summary only. When you have multiple sources cited in a paragraph, you are more likely to be synthesizing them (not always, but often
• Read more about synthesis here.

The most interesting literature reviews are often written as arguments (again, as mentioned at the beginning of the page, this is discipline-specific and doesn’t work for all situations). Often, the literature review is where you can establish your research as filling a particular gap or as relevant in a particular way. You have some chance to do this in your introduction in an article, but the literature review section gives a more extended opportunity to establish the conversation in the way you would like your readers to see it. You can choose the intellectual lineage you would like to be part of and whose definitions matter most to your thinking (mostly humanities-specific, but this goes for sciences as well). In addressing these points, you argue for your place in the conversation, which tends to make the lit review more compelling than a simple reporting of other sources.

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• How to Write a Literature Review | Guide, Examples, & Templates

How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates, and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

• Quick Run-through
• Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and its scholarly context
• Develop a theoretical framework and methodology for your research
• Position your work in relation to other researchers and theorists
• Show how your research addresses a gap or contributes to a debate
• Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models, and methods?
• Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

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To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

• Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically evaluate: mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

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If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
• Reproducibility

 Statistics

• Null hypothesis
• Statistical power
• Probability distribution
• Effect size
• Poisson distribution

Research bias

• Optimism bias
• Cognitive bias
• Implicit bias
• Hawthorne effect
• Anchoring bias
• Explicit bias

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarize yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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Literature Reviews

What this handout is about.

This handout will explain what literature reviews are and offer insights into the form and construction of literature reviews in the humanities, social sciences, and sciences.

Introduction

OK. You’ve got to write a literature review. You dust off a novel and a book of poetry, settle down in your chair, and get ready to issue a “thumbs up” or “thumbs down” as you leaf through the pages. “Literature review” done. Right?

Wrong! The “literature” of a literature review refers to any collection of materials on a topic, not necessarily the great literary texts of the world. “Literature” could be anything from a set of government pamphlets on British colonial methods in Africa to scholarly articles on the treatment of a torn ACL. And a review does not necessarily mean that your reader wants you to give your personal opinion on whether or not you liked these sources.

What is a literature review, then?

A literature review discusses published information in a particular subject area, and sometimes information in a particular subject area within a certain time period.

A literature review can be just a simple summary of the sources, but it usually has an organizational pattern and combines both summary and synthesis. A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information. It might give a new interpretation of old material or combine new with old interpretations. Or it might trace the intellectual progression of the field, including major debates. And depending on the situation, the literature review may evaluate the sources and advise the reader on the most pertinent or relevant.

But how is a literature review different from an academic research paper?

The main focus of an academic research paper is to develop a new argument, and a research paper is likely to contain a literature review as one of its parts. In a research paper, you use the literature as a foundation and as support for a new insight that you contribute. The focus of a literature review, however, is to summarize and synthesize the arguments and ideas of others without adding new contributions.

Why do we write literature reviews?

Literature reviews provide you with a handy guide to a particular topic. If you have limited time to conduct research, literature reviews can give you an overview or act as a stepping stone. For professionals, they are useful reports that keep them up to date with what is current in the field. For scholars, the depth and breadth of the literature review emphasizes the credibility of the writer in his or her field. Literature reviews also provide a solid background for a research paper’s investigation. Comprehensive knowledge of the literature of the field is essential to most research papers.

Who writes these things, anyway?

Literature reviews are written occasionally in the humanities, but mostly in the sciences and social sciences; in experiment and lab reports, they constitute a section of the paper. Sometimes a literature review is written as a paper in itself.

Let’s get to it! What should I do before writing the literature review?

If your assignment is not very specific, seek clarification from your instructor:

• Roughly how many sources should you include?
• What types of sources (books, journal articles, websites)?
• Should you summarize, synthesize, or critique your sources by discussing a common theme or issue?
• Should you evaluate your sources?
• Should you provide subheadings and other background information, such as definitions and/or a history?

Find models

Look for other literature reviews in your area of interest or in the discipline and read them to get a sense of the types of themes you might want to look for in your own research or ways to organize your final review. You can simply put the word “review” in your search engine along with your other topic terms to find articles of this type on the Internet or in an electronic database. The bibliography or reference section of sources you’ve already read are also excellent entry points into your own research.

Narrow your topic

There are hundreds or even thousands of articles and books on most areas of study. The narrower your topic, the easier it will be to limit the number of sources you need to read in order to get a good survey of the material. Your instructor will probably not expect you to read everything that’s out there on the topic, but you’ll make your job easier if you first limit your scope.

Keep in mind that UNC Libraries have research guides and to databases relevant to many fields of study. You can reach out to the subject librarian for a consultation: https://library.unc.edu/support/consultations/ .

And don’t forget to tap into your professor’s (or other professors’) knowledge in the field. Ask your professor questions such as: “If you had to read only one book from the 90’s on topic X, what would it be?” Questions such as this help you to find and determine quickly the most seminal pieces in the field.

Consider whether your sources are current

Some disciplines require that you use information that is as current as possible. In the sciences, for instance, treatments for medical problems are constantly changing according to the latest studies. Information even two years old could be obsolete. However, if you are writing a review in the humanities, history, or social sciences, a survey of the history of the literature may be what is needed, because what is important is how perspectives have changed through the years or within a certain time period. Try sorting through some other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to consider what is currently of interest to scholars in this field and what is not.

Strategies for writing the literature review

Find a focus.

A literature review, like a term paper, is usually organized around ideas, not the sources themselves as an annotated bibliography would be organized. This means that you will not just simply list your sources and go into detail about each one of them, one at a time. No. As you read widely but selectively in your topic area, consider instead what themes or issues connect your sources together. Do they present one or different solutions? Is there an aspect of the field that is missing? How well do they present the material and do they portray it according to an appropriate theory? Do they reveal a trend in the field? A raging debate? Pick one of these themes to focus the organization of your review.

Convey it to your reader

A literature review may not have a traditional thesis statement (one that makes an argument), but you do need to tell readers what to expect. Try writing a simple statement that lets the reader know what is your main organizing principle. Here are a couple of examples:

The current trend in treatment for congestive heart failure combines surgery and medicine. More and more cultural studies scholars are accepting popular media as a subject worthy of academic consideration.

Consider organization

You’ve got a focus, and you’ve stated it clearly and directly. Now what is the most effective way of presenting the information? What are the most important topics, subtopics, etc., that your review needs to include? And in what order should you present them? Develop an organization for your review at both a global and local level:

First, cover the basic categories

Just like most academic papers, literature reviews also must contain at least three basic elements: an introduction or background information section; the body of the review containing the discussion of sources; and, finally, a conclusion and/or recommendations section to end the paper. The following provides a brief description of the content of each:

• Introduction: Gives a quick idea of the topic of the literature review, such as the central theme or organizational pattern.
• Body: Contains your discussion of sources and is organized either chronologically, thematically, or methodologically (see below for more information on each).
• Conclusions/Recommendations: Discuss what you have drawn from reviewing literature so far. Where might the discussion proceed?

Organizing the body

Once you have the basic categories in place, then you must consider how you will present the sources themselves within the body of your paper. Create an organizational method to focus this section even further.

To help you come up with an overall organizational framework for your review, consider the following scenario:

You’ve decided to focus your literature review on materials dealing with sperm whales. This is because you’ve just finished reading Moby Dick, and you wonder if that whale’s portrayal is really real. You start with some articles about the physiology of sperm whales in biology journals written in the 1980’s. But these articles refer to some British biological studies performed on whales in the early 18th century. So you check those out. Then you look up a book written in 1968 with information on how sperm whales have been portrayed in other forms of art, such as in Alaskan poetry, in French painting, or on whale bone, as the whale hunters in the late 19th century used to do. This makes you wonder about American whaling methods during the time portrayed in Moby Dick, so you find some academic articles published in the last five years on how accurately Herman Melville portrayed the whaling scene in his novel.

Now consider some typical ways of organizing the sources into a review:

• Chronological: If your review follows the chronological method, you could write about the materials above according to when they were published. For instance, first you would talk about the British biological studies of the 18th century, then about Moby Dick, published in 1851, then the book on sperm whales in other art (1968), and finally the biology articles (1980s) and the recent articles on American whaling of the 19th century. But there is relatively no continuity among subjects here. And notice that even though the sources on sperm whales in other art and on American whaling are written recently, they are about other subjects/objects that were created much earlier. Thus, the review loses its chronological focus.
• By publication: Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on biological studies of sperm whales if the progression revealed a change in dissection practices of the researchers who wrote and/or conducted the studies.
• By trend: A better way to organize the above sources chronologically is to examine the sources under another trend, such as the history of whaling. Then your review would have subsections according to eras within this period. For instance, the review might examine whaling from pre-1600-1699, 1700-1799, and 1800-1899. Under this method, you would combine the recent studies on American whaling in the 19th century with Moby Dick itself in the 1800-1899 category, even though the authors wrote a century apart.
• Thematic: Thematic reviews of literature are organized around a topic or issue, rather than the progression of time. However, progression of time may still be an important factor in a thematic review. For instance, the sperm whale review could focus on the development of the harpoon for whale hunting. While the study focuses on one topic, harpoon technology, it will still be organized chronologically. The only difference here between a “chronological” and a “thematic” approach is what is emphasized the most: the development of the harpoon or the harpoon technology.But more authentic thematic reviews tend to break away from chronological order. For instance, a thematic review of material on sperm whales might examine how they are portrayed as “evil” in cultural documents. The subsections might include how they are personified, how their proportions are exaggerated, and their behaviors misunderstood. A review organized in this manner would shift between time periods within each section according to the point made.
• Methodological: A methodological approach differs from the two above in that the focusing factor usually does not have to do with the content of the material. Instead, it focuses on the “methods” of the researcher or writer. For the sperm whale project, one methodological approach would be to look at cultural differences between the portrayal of whales in American, British, and French art work. Or the review might focus on the economic impact of whaling on a community. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed. Once you’ve decided on the organizational method for the body of the review, the sections you need to include in the paper should be easy to figure out. They should arise out of your organizational strategy. In other words, a chronological review would have subsections for each vital time period. A thematic review would have subtopics based upon factors that relate to the theme or issue.

Sometimes, though, you might need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. Put in only what is necessary. Here are a few other sections you might want to consider:

• Current Situation: Information necessary to understand the topic or focus of the literature review.
• History: The chronological progression of the field, the literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Methods and/or Standards: The criteria you used to select the sources in your literature review or the way in which you present your information. For instance, you might explain that your review includes only peer-reviewed articles and journals.

Questions for Further Research: What questions about the field has the review sparked? How will you further your research as a result of the review?

Begin composing

Once you’ve settled on a general pattern of organization, you’re ready to write each section. There are a few guidelines you should follow during the writing stage as well. Here is a sample paragraph from a literature review about sexism and language to illuminate the following discussion:

However, other studies have shown that even gender-neutral antecedents are more likely to produce masculine images than feminine ones (Gastil, 1990). Hamilton (1988) asked students to complete sentences that required them to fill in pronouns that agreed with gender-neutral antecedents such as “writer,” “pedestrian,” and “persons.” The students were asked to describe any image they had when writing the sentence. Hamilton found that people imagined 3.3 men to each woman in the masculine “generic” condition and 1.5 men per woman in the unbiased condition. Thus, while ambient sexism accounted for some of the masculine bias, sexist language amplified the effect. (Source: Erika Falk and Jordan Mills, “Why Sexist Language Affects Persuasion: The Role of Homophily, Intended Audience, and Offense,” Women and Language19:2).

Use evidence

In the example above, the writers refer to several other sources when making their point. A literature review in this sense is just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence to show that what you are saying is valid.

Be selective

Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the review’s focus, whether it is thematic, methodological, or chronological.

Use quotes sparingly

Falk and Mills do not use any direct quotes. That is because the survey nature of the literature review does not allow for in-depth discussion or detailed quotes from the text. Some short quotes here and there are okay, though, if you want to emphasize a point, or if what the author said just cannot be rewritten in your own words. Notice that Falk and Mills do quote certain terms that were coined by the author, not common knowledge, or taken directly from the study. But if you find yourself wanting to put in more quotes, check with your instructor.

Summarize and synthesize

Remember to summarize and synthesize your sources within each paragraph as well as throughout the review. The authors here recapitulate important features of Hamilton’s study, but then synthesize it by rephrasing the study’s significance and relating it to their own work.

Keep your own voice

While the literature review presents others’ ideas, your voice (the writer’s) should remain front and center. Notice that Falk and Mills weave references to other sources into their own text, but they still maintain their own voice by starting and ending the paragraph with their own ideas and their own words. The sources support what Falk and Mills are saying.

Use caution when paraphrasing

When paraphrasing a source that is not your own, be sure to represent the author’s information or opinions accurately and in your own words. In the preceding example, Falk and Mills either directly refer in the text to the author of their source, such as Hamilton, or they provide ample notation in the text when the ideas they are mentioning are not their own, for example, Gastil’s. For more information, please see our handout on plagiarism .

Revise, revise, revise

Draft in hand? Now you’re ready to revise. Spending a lot of time revising is a wise idea, because your main objective is to present the material, not the argument. So check over your review again to make sure it follows the assignment and/or your outline. Then, just as you would for most other academic forms of writing, rewrite or rework the language of your review so that you’ve presented your information in the most concise manner possible. Be sure to use terminology familiar to your audience; get rid of unnecessary jargon or slang. Finally, double check that you’ve documented your sources and formatted the review appropriately for your discipline. For tips on the revising and editing process, see our handout on revising drafts .

Works consulted

We consulted these works while writing this handout. This is not a comprehensive list of resources on the handout’s topic, and we encourage you to do your own research to find additional publications. Please do not use this list as a model for the format of your own reference list, as it may not match the citation style you are using. For guidance on formatting citations, please see the UNC Libraries citation tutorial . We revise these tips periodically and welcome feedback.

Anson, Chris M., and Robert A. Schwegler. 2010. The Longman Handbook for Writers and Readers , 6th ed. New York: Longman.

Jones, Robert, Patrick Bizzaro, and Cynthia Selfe. 1997. The Harcourt Brace Guide to Writing in the Disciplines . New York: Harcourt Brace.

Lamb, Sandra E. 1998. How to Write It: A Complete Guide to Everything You’ll Ever Write . Berkeley: Ten Speed Press.

Rosen, Leonard J., and Laurence Behrens. 2003. The Allyn & Bacon Handbook , 5th ed. New York: Longman.

Troyka, Lynn Quittman, and Doug Hesse. 2016. Simon and Schuster Handbook for Writers , 11th ed. London: Pearson.

You may reproduce it for non-commercial use if you use the entire handout and attribute the source: The Writing Center, University of North Carolina at Chapel Hill

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Literature Reviews

• What is a literature review?
• Steps in the Literature Review Process
• Define your research question
• Determine inclusion and exclusion criteria
• Choose databases and search
• Review Results
• Synthesize Results
• Analyze Results
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What is a Literature Review?

A literature or narrative review is a comprehensive review and analysis of the published literature on a specific topic or research question. The literature that is reviewed contains: books, articles, academic articles, conference proceedings, association papers, and dissertations. It contains the most pertinent studies and points to important past and current research and practices. It provides background and context, and shows how your research will contribute to the field. 

A literature review should: 

• Provide a comprehensive and updated review of the literature;
• Explain why this review has taken place;
• Articulate a position or hypothesis;
• Acknowledge and account for conflicting and corroborating points of view

From  S age Research Methods

Purpose of a Literature Review

A literature review can be written as an introduction to a study to:

• Demonstrate how a study fills a gap in research
• Compare a study with other research that's been done

Or it can be a separate work (a research article on its own) which:

• Organizes or describes a topic
• Describes variables within a particular issue/problem

Limitations of a Literature Review

Some of the limitations of a literature review are:

• It's a snapshot in time. Unlike other reviews, this one has beginning, a middle and an end. There may be future developments that could make your work less relevant.
• It may be too focused. Some niche studies may miss the bigger picture.
• It can be difficult to be comprehensive. There is no way to make sure all the literature on a topic was considered.
• It is easy to be biased if you stick to top tier journals. There may be other places where people are publishing exemplary research. Look to open access publications and conferences to reflect a more inclusive collection. Also, make sure to include opposing views (and not just supporting evidence).

Source: Grant, Maria J., and Andrew Booth. “A Typology of Reviews: An Analysis of 14 Review Types and Associated Methodologies.” Health Information & Libraries Journal, vol. 26, no. 2, June 2009, pp. 91–108. Wiley Online Library, doi:10.1111/j.1471-1842.2009.00848.x.

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Hannah Chapman Tripp : Biology, Neuroscience

Carolyn Cunningham : Human Development & Family Sciences, Psychology, Sociology

Larayne Dallas : Engineering

Janelle Hedstrom : Special Education, Curriculum & Instruction, Ed Leadership & Policy ​

Susan Macicak : Linguistics

Imelda Vetter : Dell Medical School

For help in other subject areas, please see the guide to library specialists by subject .

Periodically, UT Libraries runs a workshop covering the basics and library support for literature reviews. While we try to offer these once per academic year, we find providing the recording to be helpful to community members who have missed the session. Following is the most recent recording of the workshop, Conducting a Literature Review. To view the recording, a UT login is required.

• October 26, 2022 recording
• Last Updated: Oct 26, 2022 2:49 PM
• URL: https://guides.lib.utexas.edu/literaturereviews

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How To Write A Literature Review - A Complete Guide

Table of Contents

A literature review is much more than just another section in your research paper. It forms the very foundation of your research. It is a formal piece of writing where you analyze the existing theoretical framework, principles, and assumptions and use that as a base to shape your approach to the research question.

Curating and drafting a solid literature review section not only lends more credibility to your research paper but also makes your research tighter and better focused. But, writing literature reviews is a difficult task. It requires extensive reading, plus you have to consider market trends and technological and political changes, which tend to change in the blink of an eye.

Now streamline your literature review process with the help of SciSpace Copilot. With this AI research assistant, you can efficiently synthesize and analyze a vast amount of information, identify key themes and trends, and uncover gaps in the existing research. Get real-time explanations, summaries, and answers to your questions for the paper you're reviewing, making navigating and understanding the complex literature landscape easier.

In this comprehensive guide, we will explore everything from the definition of a literature review, its appropriate length, various types of literature reviews, and how to write one.

What is a literature review?

A literature review is a collation of survey, research, critical evaluation, and assessment of the existing literature in a preferred domain.

Eminent researcher and academic Arlene Fink, in her book Conducting Research Literature Reviews , defines it as the following:

“A literature review surveys books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated.

Literature reviews are designed to provide an overview of sources you have explored while researching a particular topic, and to demonstrate to your readers how your research fits within a larger field of study.”

Simply put, a literature review can be defined as a critical discussion of relevant pre-existing research around your research question and carving out a definitive place for your study in the existing body of knowledge. Literature reviews can be presented in multiple ways: a section of an article, the whole research paper itself, or a chapter of your thesis.

A literature review does function as a summary of sources, but it also allows you to analyze further, interpret, and examine the stated theories, methods, viewpoints, and, of course, the gaps in the existing content.

As an author, you can discuss and interpret the research question and its various aspects and debate your adopted methods to support the claim.

What is the purpose of a literature review?

A literature review is meant to help your readers understand the relevance of your research question and where it fits within the existing body of knowledge. As a researcher, you should use it to set the context, build your argument, and establish the need for your study.

What is the importance of a literature review?

The literature review is a critical part of research papers because it helps you:

• Gain an in-depth understanding of your research question and the surrounding area
• Convey that you have a thorough understanding of your research area and are up-to-date with the latest changes and advancements
• Establish how your research is connected or builds on the existing body of knowledge and how it could contribute to further research
• Elaborate on the validity and suitability of your theoretical framework and research methodology
• Identify and highlight gaps and shortcomings in the existing body of knowledge and how things need to change
• Convey to readers how your study is different or how it contributes to the research area

How long should a literature review be?

Ideally, the literature review should take up 15%-40% of the total length of your manuscript. So, if you have a 10,000-word research paper, the minimum word count could be 1500.

Your literature review format depends heavily on the kind of manuscript you are writing — an entire chapter in case of doctoral theses, a part of the introductory section in a research article, to a full-fledged review article that examines the previously published research on a topic.

Another determining factor is the type of research you are doing. The literature review section tends to be longer for secondary research projects than primary research projects.

What are the different types of literature reviews?

All literature reviews are not the same. There are a variety of possible approaches that you can take. It all depends on the type of research you are pursuing.

Here are the different types of literature reviews:

Argumentative review

It is called an argumentative review when you carefully present literature that only supports or counters a specific argument or premise to establish a viewpoint.

Integrative review

It is a type of literature review focused on building a comprehensive understanding of a topic by combining available theoretical frameworks and empirical evidence.

Methodological review

This approach delves into the ''how'' and the ''what" of the research question —  you cannot look at the outcome in isolation; you should also review the methodology used.

Systematic review

This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research and collect, report, and analyze data from the studies included in the review.

Meta-analysis review

Meta-analysis uses statistical methods to summarize the results of independent studies. By combining information from all relevant studies, meta-analysis can provide more precise estimates of the effects than those derived from the individual studies included within a review.

Historical review

Historical literature reviews focus on examining research throughout a period, often starting with the first time an issue, concept, theory, or phenomenon emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and identify future research's likely directions.

Theoretical Review

This form aims to examine the corpus of theory accumulated regarding an issue, concept, theory, and phenomenon. The theoretical literature review helps to establish what theories exist, the relationships between them, the degree the existing approaches have been investigated, and to develop new hypotheses to be tested.

Scoping Review

The Scoping Review is often used at the beginning of an article, dissertation, or research proposal. It is conducted before the research to highlight gaps in the existing body of knowledge and explains why the project should be greenlit.

State-of-the-Art Review

The State-of-the-Art review is conducted periodically, focusing on the most recent research. It describes what is currently known, understood, or agreed upon regarding the research topic and highlights where there are still disagreements.

Can you use the first person in a literature review?

When writing literature reviews, you should avoid the usage of first-person pronouns. It means that instead of "I argue that" or "we argue that," the appropriate expression would be "this research paper argues that."

Do you need an abstract for a literature review?

Ideally, yes. It is always good to have a condensed summary that is self-contained and independent of the rest of your review. As for how to draft one, you can follow the same fundamental idea when preparing an abstract for a literature review. It should also include:

• The research topic and your motivation behind selecting it
• A one-sentence thesis statement
• An explanation of the kinds of literature featured in the review
• Summary of what you've learned
• Conclusions you drew from the literature you reviewed
• Potential implications and future scope for research

Here's an example of the abstract of a literature review

Is a literature review written in the past tense?

Yes, the literature review should ideally be written in the past tense. You should not use the present or future tense when writing one. The exceptions are when you have statements describing events that happened earlier than the literature you are reviewing or events that are currently occurring; then, you can use the past perfect or present perfect tenses.

How many sources for a literature review?

There are multiple approaches to deciding how many sources to include in a literature review section. The first approach would be to look level you are at as a researcher. For instance, a doctoral thesis might need 60+ sources. In contrast, you might only need to refer to 5-15 sources at the undergraduate level.

The second approach is based on the kind of literature review you are doing — whether it is merely a chapter of your paper or if it is a self-contained paper in itself. When it is just a chapter, sources should equal the total number of pages in your article's body. In the second scenario, you need at least three times as many sources as there are pages in your work.

Quick tips on how to write a literature review

To know how to write a literature review, you must clearly understand its impact and role in establishing your work as substantive research material.

You need to follow the below-mentioned steps, to write a literature review:

• Outline the purpose behind the literature review
• Search relevant literature
• Examine and assess the relevant resources
• Discover connections by drawing deep insights from the resources
• Structure planning to write a good literature review

1. Outline and identify the purpose of  a literature review

As a first step on how to write a literature review, you must know what the research question or topic is and what shape you want your literature review to take. Ensure you understand the research topic inside out, or else seek clarifications. You must be able to the answer below questions before you start:

• How many sources do I need to include?
• What kind of sources should I analyze?
• How much should I critically evaluate each source?
• Should I summarize, synthesize or offer a critique of the sources?
• Do I need to include any background information or definitions?

Additionally, you should know that the narrower your research topic is, the swifter it will be for you to restrict the number of sources to be analyzed.

2. Search relevant literature

Dig deeper into search engines to discover what has already been published around your chosen topic. Make sure you thoroughly go through appropriate reference sources like books, reports, journal articles, government docs, and web-based resources.

You must prepare a list of keywords and their different variations. You can start your search from any library’s catalog, provided you are an active member of that institution. The exact keywords can be extended to widen your research over other databases and academic search engines like:

• Google Scholar
• Microsoft Academic
• Science.gov

Besides, it is not advisable to go through every resource word by word. Alternatively, what you can do is you can start by reading the abstract and then decide whether that source is relevant to your research or not.

Additionally, you must spend surplus time assessing the quality and relevance of resources. It would help if you tried preparing a list of citations to ensure that there lies no repetition of authors, publications, or articles in the literature review.

3. Examine and assess the sources

It is nearly impossible for you to go through every detail in the research article. So rather than trying to fetch every detail, you have to analyze and decide which research sources resemble closest and appear relevant to your chosen domain.

While analyzing the sources, you should look to find out answers to questions like:

• What question or problem has the author been describing and debating?
• What is the definition of critical aspects?
• How well the theories, approach, and methodology have been explained?
• Whether the research theory used some conventional or new innovative approach?
• How relevant are the key findings of the work?
• In what ways does it relate to other sources on the same topic?
• What challenges does this research paper pose to the existing theory
• What are the possible contributions or benefits it adds to the subject domain?

Be always mindful that you refer only to credible and authentic resources. It would be best if you always take references from different publications to validate your theory.

Always keep track of important information or data you can present in your literature review right from the beginning. It will help steer your path from any threats of plagiarism and also make it easier to curate an annotated bibliography or reference section.

4. Discover connections

At this stage, you must start deciding on the argument and structure of your literature review. To accomplish this, you must discover and identify the relations and connections between various resources while drafting your abstract.

A few aspects that you should be aware of while writing a literature review include:

• Rise to prominence: Theories and methods that have gained reputation and supporters over time.
• Constant scrutiny: Concepts or theories that repeatedly went under examination.
• Contradictions and conflicts: Theories, both the supporting and the contradictory ones, for the research topic.
• Knowledge gaps: What exactly does it fail to address, and how to bridge them with further research?
• Influential resources: Significant research projects available that have been upheld as milestones or perhaps, something that can modify the current trends

Once you join the dots between various past research works, it will be easier for you to draw a conclusion and identify your contribution to the existing knowledge base.

5. Structure planning to write a good literature review

There exist different ways towards planning and executing the structure of a literature review. The format of a literature review varies and depends upon the length of the research.

Like any other research paper, the literature review format must contain three sections: introduction, body, and conclusion. The goals and objectives of the research question determine what goes inside these three sections.

Nevertheless, a good literature review can be structured according to the chronological, thematic, methodological, or theoretical framework approach.

Literature review samples

1. Standalone

2. As a section of a research paper

How SciSpace Discover makes literature review a breeze?

SciSpace Discover is a one-stop solution to do an effective literature search and get barrier-free access to scientific knowledge. It is an excellent repository where you can find millions of only peer-reviewed articles and full-text PDF files. Here’s more on how you can use it:

Find the right information

Find what you want quickly and easily with comprehensive search filters that let you narrow down papers according to PDF availability, year of publishing, document type, and affiliated institution. Moreover, you can sort the results based on the publishing date, citation count, and relevance.

Assess credibility of papers quickly

When doing the literature review, it is critical to establish the quality of your sources. They form the foundation of your research. SciSpace Discover helps you assess the quality of a source by providing an overview of its references, citations, and performance metrics.

Get the complete picture in no time

SciSpace Discover’s personalized suggestion engine helps you stay on course and get the complete picture of the topic from one place. Every time you visit an article page, it provides you links to related papers. Besides that, it helps you understand what’s trending, who are the top authors, and who are the leading publishers on a topic.

Make referring sources super easy

To ensure you don't lose track of your sources, you must start noting down your references when doing the literature review. SciSpace Discover makes this step effortless. Click the 'cite' button on an article page, and you will receive preloaded citation text in multiple styles — all you've to do is copy-paste it into your manuscript.

Final tips on how to write a literature review

A massive chunk of time and effort is required to write a good literature review. But, if you go about it systematically, you'll be able to save a ton of time and build a solid foundation for your research.

We hope this guide has helped you answer several key questions you have about writing literature reviews.

Would you like to explore SciSpace Discover and kick off your literature search right away? You can get started here .

Frequently Asked Questions (FAQs)

1. how to start a literature review.

• What questions do you want to answer?

• What sources do you need to answer these questions?

• What information do these sources contain?

• How can you use this information to answer your questions?

2. What to include in a literature review?

• A brief background of the problem or issue

• What has previously been done to address the problem or issue

• A description of what you will do in your project

• How this study will contribute to research on the subject

3. Why literature review is important?

The literature review is an important part of any research project because it allows the writer to look at previous studies on a topic and determine existing gaps in the literature, as well as what has already been done. It will also help them to choose the most appropriate method for their own study.

4. How to cite a literature review in APA format?

To cite a literature review in APA style, you need to provide the author's name, the title of the article, and the year of publication. For example: Patel, A. B., & Stokes, G. S. (2012). The relationship between personality and intelligence: A meta-analysis of longitudinal research. Personality and Individual Differences, 53(1), 16-21

5. What are the components of a literature review?

• A brief introduction to the topic, including its background and context. The introduction should also include a rationale for why the study is being conducted and what it will accomplish.

• A description of the methodologies used in the study. This can include information about data collection methods, sample size, and statistical analyses.

• A presentation of the findings in an organized format that helps readers follow along with the author's conclusions.

6. What are common errors in writing literature review?

• Not spending enough time to critically evaluate the relevance of resources, observations and conclusions.

• Totally relying on secondary data while ignoring primary data.

• Letting your personal bias seep into your interpretation of existing literature.

• No detailed explanation of the procedure to discover and identify an appropriate literature review.

7. What are the 5 C's of writing literature review?

• Cite - the sources you utilized and referenced in your research.

• Compare - existing arguments, hypotheses, methodologies, and conclusions found in the knowledge base.

• Contrast - the arguments, topics, methodologies, approaches, and disputes that may be found in the literature.

• Critique - the literature and describe the ideas and opinions you find more convincing and why.

• Connect - the various studies you reviewed in your research.

8. How many sources should a literature review have?

When it is just a chapter, sources should equal the total number of pages in your article's body. if it is a self-contained paper in itself, you need at least three times as many sources as there are pages in your work.

9. Can literature review have diagrams?

• To represent an abstract idea or concept

• To explain the steps of a process or procedure

• To help readers understand the relationships between different concepts

10. How old should sources be in a literature review?

Sources for a literature review should be as current as possible or not older than ten years. The only exception to this rule is if you are reviewing a historical topic and need to use older sources.

11. What are the types of literature review?

• Argumentative review

• Integrative review

• Methodological review

• Systematic review

• Meta-analysis review

• Historical review

• Theoretical review

• Scoping review

• State-of-the-Art review

12. Is a literature review mandatory?

Yes. Literature review is a mandatory part of any research project. It is a critical step in the process that allows you to establish the scope of your research, and provide a background for the rest of your work.

But before you go,

• Six Online Tools for Easy Literature Review
• Evaluating literature review: systematic vs. scoping reviews
• Systematic Approaches to a Successful Literature Review
• Writing Integrative Literature Reviews: Guidelines and Examples

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• What is a Literature Review? | Guide, Template, & Examples

What is a Literature Review? | Guide, Template, & Examples

Published on 22 February 2022 by Shona McCombes . Revised on 7 June 2022.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research.

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarise sources – it analyses, synthesises, and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

Why write a literature review, examples of literature reviews, step 1: search for relevant literature, step 2: evaluate and select sources, step 3: identify themes, debates and gaps, step 4: outline your literature review’s structure, step 5: write your literature review, frequently asked questions about literature reviews, introduction.

• Quick Run-through
• Step 1 & 2

When you write a dissertation or thesis, you will have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and scholarly context
• Develop a theoretical framework and methodology for your research
• Position yourself in relation to other researchers and theorists
• Show how your dissertation addresses a gap or contributes to a debate

You might also have to write a literature review as a stand-alone assignment. In this case, the purpose is to evaluate the current state of research and demonstrate your knowledge of scholarly debates around a topic.

The content will look slightly different in each case, but the process of conducting a literature review follows the same steps. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research objectives and questions .

If you are writing a literature review as a stand-alone assignment, you will have to choose a focus and develop a central question to direct your search. Unlike a dissertation research question, this question has to be answerable without collecting original data. You should be able to answer it based only on a review of existing publications.

Make a list of keywords

Start by creating a list of keywords related to your research topic. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list if you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can use boolean operators to help narrow down your search:

Read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

To identify the most important publications on your topic, take note of recurring citations. If the same authors, books or articles keep appearing in your reading, make sure to seek them out.

You probably won’t be able to read absolutely everything that has been written on the topic – you’ll have to evaluate which sources are most relevant to your questions.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models and methods? Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• How does the publication contribute to your understanding of the topic? What are its key insights and arguments?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible, and make sure you read any landmark studies and major theories in your field of research.

You can find out how many times an article has been cited on Google Scholar – a high citation count means the article has been influential in the field, and should certainly be included in your literature review.

The scope of your review will depend on your topic and discipline: in the sciences you usually only review recent literature, but in the humanities you might take a long historical perspective (for example, to trace how a concept has changed in meaning over time).

Remember that you can use our template to summarise and evaluate sources you’re thinking about using!

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It’s important to keep track of your sources with references to avoid plagiarism . It can be helpful to make an annotated bibliography, where you compile full reference information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

You can use our free APA Reference Generator for quick, correct, consistent citations.

To begin organising your literature review’s argument and structure, you need to understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly-visual platforms like Instagram and Snapchat – this is a gap that you could address in your own research.

There are various approaches to organising the body of a literature review. You should have a rough idea of your strategy before you start writing.

Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarising sources in order.

Try to analyse patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organise your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text, your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

If you are writing the literature review as part of your dissertation or thesis, reiterate your central problem or research question and give a brief summary of the scholarly context. You can emphasise the timeliness of the topic (“many recent studies have focused on the problem of x”) or highlight a gap in the literature (“while there has been much research on x, few researchers have taken y into consideration”).

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, make sure to follow these tips:

• Summarise and synthesise: give an overview of the main points of each source and combine them into a coherent whole.
• Analyse and interpret: don’t just paraphrase other researchers – add your own interpretations, discussing the significance of findings in relation to the literature as a whole.
• Critically evaluate: mention the strengths and weaknesses of your sources.
• Write in well-structured paragraphs: use transitions and topic sentences to draw connections, comparisons and contrasts.

In the conclusion, you should summarise the key findings you have taken from the literature and emphasise their significance.

If the literature review is part of your dissertation or thesis, reiterate how your research addresses gaps and contributes new knowledge, or discuss how you have drawn on existing theories and methods to build a framework for your research. This can lead directly into your methodology section.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a dissertation , thesis, research paper , or proposal .

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarise yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your  dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

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What Is A Literature Review?

A plain-language explainer (with examples).

By:  Derek Jansen (MBA) & Kerryn Warren (PhD) | June 2020 (Updated May 2023)

If you’re faced with writing a dissertation or thesis, chances are you’ve encountered the term “literature review” . If you’re on this page, you’re probably not 100% what the literature review is all about. The good news is that you’ve come to the right place.

Literature Review 101

• What (exactly) is a literature review
• What’s the purpose of the literature review chapter
• How to find high-quality resources
• How to structure your literature review chapter
• Example of an actual literature review

What is a literature review?

The word “literature review” can refer to two related things that are part of the broader literature review process. The first is the task of  reviewing the literature  – i.e. sourcing and reading through the existing research relating to your research topic. The second is the  actual chapter  that you write up in your dissertation, thesis or research project. Let’s look at each of them:

Reviewing the literature

The first step of any literature review is to hunt down and  read through the existing research  that’s relevant to your research topic. To do this, you’ll use a combination of tools (we’ll discuss some of these later) to find journal articles, books, ebooks, research reports, dissertations, theses and any other credible sources of information that relate to your topic. You’ll then  summarise and catalogue these  for easy reference when you write up your literature review chapter. 

The literature review chapter

The second step of the literature review is to write the actual literature review chapter (this is usually the second chapter in a typical dissertation or thesis structure ). At the simplest level, the literature review chapter is an  overview of the key literature  that’s relevant to your research topic. This chapter should provide a smooth-flowing discussion of what research has already been done, what is known, what is unknown and what is contested in relation to your research topic. So, you can think of it as an  integrated review of the state of knowledge  around your research topic. 

What’s the purpose of a literature review?

The literature review chapter has a few important functions within your dissertation, thesis or research project. Let’s take a look at these:

Purpose #1 – Demonstrate your topic knowledge

The first function of the literature review chapter is, quite simply, to show the reader (or marker) that you  know what you’re talking about . In other words, a good literature review chapter demonstrates that you’ve read the relevant existing research and understand what’s going on – who’s said what, what’s agreed upon, disagreed upon and so on. This needs to be  more than just a summary  of who said what – it needs to integrate the existing research to  show how it all fits together  and what’s missing (which leads us to purpose #2, next). 

Purpose #2 – Reveal the research gap that you’ll fill

The second function of the literature review chapter is to  show what’s currently missing  from the existing research, to lay the foundation for your own research topic. In other words, your literature review chapter needs to show that there are currently “missing pieces” in terms of the bigger puzzle, and that  your study will fill one of those research gaps . By doing this, you are showing that your research topic is original and will help contribute to the body of knowledge. In other words, the literature review helps justify your research topic.  

Purpose #3 – Lay the foundation for your conceptual framework

The third function of the literature review is to form the  basis for a conceptual framework . Not every research topic will necessarily have a conceptual framework, but if your topic does require one, it needs to be rooted in your literature review. 

For example, let’s say your research aims to identify the drivers of a certain outcome – the factors which contribute to burnout in office workers. In this case, you’d likely develop a conceptual framework which details the potential factors (e.g. long hours, excessive stress, etc), as well as the outcome (burnout). Those factors would need to emerge from the literature review chapter – they can’t just come from your gut! 

So, in this case, the literature review chapter would uncover each of the potential factors (based on previous studies about burnout), which would then be modelled into a framework. 

Purpose #4 – To inform your methodology

The fourth function of the literature review is to  inform the choice of methodology  for your own research. As we’ve  discussed on the Grad Coach blog , your choice of methodology will be heavily influenced by your research aims, objectives and questions . Given that you’ll be reviewing studies covering a topic close to yours, it makes sense that you could learn a lot from their (well-considered) methodologies.

So, when you’re reviewing the literature, you’ll need to  pay close attention to the research design , methodology and methods used in similar studies, and use these to inform your methodology. Quite often, you’ll be able to  “borrow” from previous studies . This is especially true for quantitative studies , as you can use previously tried and tested measures and scales. 

How do I find articles for my literature review?

Finding quality journal articles is essential to crafting a rock-solid literature review. As you probably already know, not all research is created equally, and so you need to make sure that your literature review is  built on credible research . 

We could write an entire post on how to find quality literature (actually, we have ), but a good starting point is Google Scholar . Google Scholar is essentially the academic equivalent of Google, using Google’s powerful search capabilities to find relevant journal articles and reports. It certainly doesn’t cover every possible resource, but it’s a very useful way to get started on your literature review journey, as it will very quickly give you a good indication of what the  most popular pieces of research  are in your field.

One downside of Google Scholar is that it’s merely a search engine – that is, it lists the articles, but oftentimes  it doesn’t host the articles . So you’ll often hit a paywall when clicking through to journal websites. 

Thankfully, your university should provide you with access to their library, so you can find the article titles using Google Scholar and then search for them by name in your university’s online library. Your university may also provide you with access to  ResearchGate , which is another great source for existing research. 

Remember, the correct search keywords will be super important to get the right information from the start. So, pay close attention to the keywords used in the journal articles you read and use those keywords to search for more articles. If you can’t find a spoon in the kitchen, you haven’t looked in the right drawer. 

Need a helping hand?

How should I structure my literature review?

Unfortunately, there’s no generic universal answer for this one. The structure of your literature review will depend largely on your topic area and your research aims and objectives.

You could potentially structure your literature review chapter according to theme, group, variables , chronologically or per concepts in your field of research. We explain the main approaches to structuring your literature review here . You can also download a copy of our free literature review template to help you establish an initial structure.

In general, it’s also a good idea to start wide (i.e. the big-picture-level) and then narrow down, ending your literature review close to your research questions . However, there’s no universal one “right way” to structure your literature review. The most important thing is not to discuss your sources one after the other like a list – as we touched on earlier, your literature review needs to synthesise the research , not summarise it .

Ultimately, you need to craft your literature review so that it conveys the most important information effectively – it needs to tell a logical story in a digestible way. It’s no use starting off with highly technical terms and then only explaining what these terms mean later. Always assume your reader is not a subject matter expert and hold their hand through a journe y of the literature while keeping the functions of the literature review chapter (which we discussed earlier) front of mind.

Example of a literature review

In the video below, we walk you through a high-quality literature review from a dissertation that earned full distinction. This will give you a clearer view of what a strong literature review looks like in practice and hopefully provide some inspiration for your own. 

Wrapping Up

In this post, we’ve (hopefully) answered the question, “ what is a literature review? “. We’ve also considered the purpose and functions of the literature review, as well as how to find literature and how to structure the literature review chapter. If you’re keen to learn more, check out the literature review section of the Grad Coach blog , as well as our detailed video post covering how to write a literature review . 

Psst… there’s more!

This post is an extract from our bestselling Udemy Course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

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16 Comments

Thanks for this review. It narrates what’s not been taught as tutors are always in a early to finish their classes.

Thanks for the kind words, Becky. Good luck with your literature review 🙂

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This is review is amazing. I benefited from it a lot and hope others visiting this website will benefit too.

Timothy T. Chol [email protected]

Thank you very much for the guiding in literature review I learn and benefited a lot this make my journey smooth I’ll recommend this site to my friends

This was so useful. Thank you so much.

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The review is really helpful to me especially during this period of covid-19 pandemic when most universities in my country only offer online classes. Great stuff

Great Brief Explanation, thanks

So helpful to me as a student

GradCoach is a fantastic site with brilliant and modern minds behind it.. I spent weeks decoding the substantial academic Jargon and grounding my initial steps on the research process, which could be shortened to a couple of days through the Gradcoach. Thanks again!

This is an amazing talk. I paved way for myself as a researcher. Thank you GradCoach!

Well-presented overview of the literature!

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• UConn Library
• Literature Review: The What, Why and How-to Guide
• Introduction

Literature Review: The What, Why and How-to Guide — Introduction

• Getting Started
• How to Pick a Topic
• Strategies to Find Sources
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

What are Literature Reviews?

So, what is a literature review? "A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries." Taylor, D.  The literature review: A few tips on conducting it . University of Toronto Health Sciences Writing Centre.

Goals of Literature Reviews

What are the goals of creating a Literature Review?  A literature could be written to accomplish different aims:

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 

Baumeister, R. F., & Leary, M. R. (1997). Writing narrative literature reviews .  Review of General Psychology , 1 (3), 311-320.

What kinds of sources require a Literature Review?

• A research paper assigned in a course
• A thesis or dissertation
• A grant proposal
• An article intended for publication in a journal

All these instances require you to collect what has been written about your research topic so that you can demonstrate how your own research sheds new light on the topic.

Types of Literature Reviews

What kinds of literature reviews are written?

Narrative review: The purpose of this type of review is to describe the current state of the research on a specific topic/research and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weakness, and gaps are identified. The review ends with a conclusion section which summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.

• Example : Predictors and Outcomes of U.S. Quality Maternity Leave: A Review and Conceptual Framework:  10.1177/08948453211037398  

Systematic review : "The authors of a systematic review use a specific procedure to search the research literature, select the studies to include in their review, and critically evaluate the studies they find." (p. 139). Nelson, L. K. (2013). Research in Communication Sciences and Disorders . Plural Publishing.

• Example : The effect of leave policies on increasing fertility: a systematic review:  10.1057/s41599-022-01270-w

Meta-analysis : "Meta-analysis is a method of reviewing research findings in a quantitative fashion by transforming the data from individual studies into what is called an effect size and then pooling and analyzing this information. The basic goal in meta-analysis is to explain why different outcomes have occurred in different studies." (p. 197). Roberts, M. C., & Ilardi, S. S. (2003). Handbook of Research Methods in Clinical Psychology . Blackwell Publishing.

• Example : Employment Instability and Fertility in Europe: A Meta-Analysis:  10.1215/00703370-9164737

Meta-synthesis : "Qualitative meta-synthesis is a type of qualitative study that uses as data the findings from other qualitative studies linked by the same or related topic." (p.312). Zimmer, L. (2006). Qualitative meta-synthesis: A question of dialoguing with texts .  Journal of Advanced Nursing , 53 (3), 311-318.

• Example : Women’s perspectives on career successes and barriers: A qualitative meta-synthesis:  10.1177/05390184221113735

Literature Reviews in the Health Sciences

• UConn Health subject guide on systematic reviews Explanation of the different review types used in health sciences literature as well as tools to help you find the right review type
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What is a literature review?

A literature review discusses published information in a particular subject area. Often part of the introduction to an essay, research report or thesis, the literature review is literally a "re" view or "look again" at what has already been written about the topic, wherein the author analyzes a segment of a published body of knowledge through summary, classification, and comparison of prior research studies, reviews of literature, and theoretical articles. Literature reviews provide the reader with a bibliographic history of the scholarly research in any given field of study. As such,  as new information becomes available, literature reviews grow in length or become focused on one specific aspect of the topic.

A literature review can be just a simple summary of the sources, but usually contains an organizational pattern and combines both summary and synthesis. A summary is a recap of the important information of the source, whereas a synthesis is a re-organization, or a reshuffling, of that information. The literature review might give a new interpretation of old material or combine new with old interpretations. Or it might trace the intellectual progression of the field, including major debates. Depending on the situation, the literature review may evaluate the sources and advise the reader on the most pertinent or relevant.

A literature review is NOT:

• An annotated bibliography – a list of citations to books, articles and documents that includes a brief description and evaluation for each citation. The annotations inform the reader of the relevance, accuracy and quality of the sources cited.
• A literary review – a critical discussion of the merits and weaknesses of a literary work.
• A book review – a critical discussion of the merits and weaknesses of a particular book.
• Teaching Information Literacy Reframed: 50+ Framework-Based Exercises for Creating Information-Literate Learners
• The UNC Writing Center – Literature Reviews
• The UW-Madison Writing Center: The Writer’s Handbook – Academic and Professional Writing – Learn How to Write a Literature Review

What is the difference between a literature review and a research paper?

The focus of a literature review is to summarize and synthesize the arguments and ideas of others without adding new contributions, whereas academic research papers present and develop new arguments that build upon the previously available body of literature.

How do I write a literature review?

There are many resources that offer step-by-step guidance for writing a literature review, and you can find some of them under Other Resources in the menu to the left. Writing the Literature Review: A Practical Guide suggests these steps:

• Chose a review topic and develop a research question
• Locate and organize research sources
• Select, analyze and annotate sources
• Evaluate research articles and other documents
• Structure and organize the literature review
• Develop arguments and supporting claims
• Synthesize and interpret the literature
• Put it all together

What is the purpose of writing a literature review?

Literature reviews serve as a guide to a particular topic: professionals can use literature reviews to keep current on their field; scholars can determine credibility of the writer in his or her field by analyzing the literature review.

As a writer, you will use the literature review to:

• See what has, and what has not, been investigated about your topic
• Identify data sources that other researches have used
• Learn how others in the field have defined and measured key concepts
• Establish context, or background, for the argument explored in the rest of a paper
• Explain what the strengths and weaknesses of that knowledge and ideas might be
• Contribute to the field by moving research forward
• To keep the writer/reader up to date with current developments in a particular field of study
• Develop alternative research projects
• Put your work in perspective
• Demonstrate your understanding and your ability to critically evaluate research in the field
• Provide evidence that may support your own findings
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What is a literature review?

A literature review is an integrated analysis -- not just a summary-- of scholarly writings and other relevant evidence related directly to your research question.  That is, it represents a synthesis of the evidence that provides background information on your topic and shows a association between the evidence and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

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1. Choose a topic. Define your research question.

Your literature review should be guided by your central research question.  The literature represents background and research developments related to a specific research question, interpreted and analyzed by you in a synthesized way.

• Make sure your research question is not too broad or too narrow.  Is it manageable?
• Begin writing down terms that are related to your question. These will be useful for searches later.
• If you have the opportunity, discuss your topic with your professor and your class mates.

2. Decide on the scope of your review

How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

• This may depend on your assignment.  How many sources does the assignment require?

3. Select the databases you will use to conduct your searches.

Make a list of the databases you will search. 

Where to find databases:

• use the tabs on this guide
• Find other databases in the Nursing Information Resources web page
• More on the Medical Library web page
• ... and more on the Yale University Library web page

4. Conduct your searches to find the evidence. Keep track of your searches.

• Use the key words in your question, as well as synonyms for those words, as terms in your search. Use the database tutorials for help.
• Save the searches in the databases. This saves time when you want to redo, or modify, the searches. It is also helpful to use as a guide is the searches are not finding any useful results.
• Review the abstracts of research studies carefully. This will save you time.
• Use the bibliographies and references of research studies you find to locate others.
• Check with your professor, or a subject expert in the field, if you are missing any key works in the field.
• Ask your librarian for help at any time.
• Use a citation manager, such as EndNote as the repository for your citations. See the EndNote tutorials for help.

Review the literature

Some questions to help you analyze the research:

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions.
• Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited? If so, how has it been analyzed?

Tips: 

• Review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
• Create a matrix of the studies for easy analysis, and synthesis, across all of the studies.
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A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE : Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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• Subject Guides

Systematic, Scoping, and Other Literature Reviews: Overview

• Project Planning

What Is a Systematic Review?

Regular literature reviews are simply summaries of the literature on a particular topic. A systematic review, however, is a comprehensive literature review conducted to answer a specific research question. Authors of a systematic review aim to find, code, appraise, and synthesize all of the previous research on their question in an unbiased and well-documented manner. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) outline the minimum amount of information that needs to be reported at the conclusion of a systematic review project. 

Other types of what are known as "evidence syntheses," such as scoping, rapid, and integrative reviews, have varying methodologies. While systematic reviews originated with and continue to be a popular publication type in medicine and other health sciences fields, more and more researchers in other disciplines are choosing to conduct evidence syntheses. 

This guide will walk you through the major steps of a systematic review and point you to key resources including Covidence, a systematic review project management tool. For help with systematic reviews and other major literature review projects, please send us an email at  [email protected] .

Getting Help with Reviews

Organization such as the Institute of Medicine recommend that you consult a librarian when conducting a systematic review. Librarians at the University of Nevada, Reno can help you:

• Understand best practices for conducting systematic reviews and other evidence syntheses in your discipline
• Choose and formulate a research question
• Decide which review type (e.g., systematic, scoping, rapid, etc.) is the best fit for your project
• Determine what to include and where to register a systematic review protocol
• Select search terms and develop a search strategy
• Identify databases and platforms to search
• Find the full text of articles and other sources
• Become familiar with free citation management (e.g., EndNote, Zotero)
• Get access to you and help using Covidence, a systematic review project management tool

Doing a Systematic Review

• Plan - This is the project planning stage. You and your team will need to develop a good research question, determine the type of review you will conduct (systematic, scoping, rapid, etc.), and establish the inclusion and exclusion criteria (e.g., you're only going to look at studies that use a certain methodology). All of this information needs to be included in your protocol. You'll also need to ensure that the project is viable - has someone already done a systematic review on this topic? Do some searches and check the various protocol registries to find out. 
• Identify - Next, a comprehensive search of the literature is undertaken to ensure all studies that meet the predetermined criteria are identified. Each research question is different, so the number and types of databases you'll search - as well as other online publication venues - will vary. Some standards and guidelines specify that certain databases (e.g., MEDLINE, EMBASE) should be searched regardless. Your subject librarian can help you select appropriate databases to search and develop search strings for each of those databases.  
• Evaluate - In this step, retrieved articles are screened and sorted using the predetermined inclusion and exclusion criteria. The risk of bias for each included study is also assessed around this time. It's best if you import search results into a citation management tool (see below) to clean up the citations and remove any duplicates. You can then use a tool like Rayyan (see below) to screen the results. You should begin by screening titles and abstracts only, and then you'll examine the full text of any remaining articles. Each study should be reviewed by a minimum of two people on the project team. 
• Collect - Each included study is coded and the quantitative or qualitative data contained in these studies is then synthesized. You'll have to either find or develop a coding strategy or form that meets your needs. 
• Explain - The synthesized results are articulated and contextualized. What do the results mean? How have they answered your research question?
• Summarize - The final report provides a complete description of the methods and results in a clear, transparent fashion. 

Adapted from

Types of reviews, systematic review.

These types of studies employ a systematic method to analyze and synthesize the results of numerous studies. "Systematic" in this case means following a strict set of steps - as outlined by entities like PRISMA and the Institute of Medicine - so as to make the review more reproducible and less biased. Consistent, thorough documentation is also key. Reviews of this type are not meant to be conducted by an individual but rather a (small) team of researchers. Systematic reviews are widely used in the health sciences, often to find a generalized conclusion from multiple evidence-based studies. 

Meta-Analysis

A systematic method that uses statistics to analyze the data from numerous studies. The researchers combine the data from studies with similar data types and analyze them as a single, expanded dataset. Meta-analyses are a type of systematic review.

Scoping Review

A scoping review employs the systematic review methodology to explore a broader topic or question rather than a specific and answerable one, as is generally the case with a systematic review. Authors of these types of reviews seek to collect and categorize the existing literature so as to identify any gaps.

Rapid Review

Rapid reviews are systematic reviews conducted under a time constraint. Researchers make use of workarounds to complete the review quickly (e.g., only looking at English-language publications), which can lead to a less thorough and more biased review. 

Narrative Review

A traditional literature review that summarizes and synthesizes the findings of numerous original research articles. The purpose and scope of narrative literature reviews vary widely and do not follow a set protocol. Most literature reviews are narrative reviews. 

Umbrella Review

Umbrella reviews are, essentially, systematic reviews of systematic reviews. These compile evidence from multiple review studies into one usable document. 

Grant, Maria J., and Andrew Booth. “A Typology of Reviews: An Analysis of 14 Review Types and Associated Methodologies.” Health Information & Libraries Journal , vol. 26, no. 2, 2009, pp. 91-108. doi: 10.1111/j.1471-1842.2009.00848.x .

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What is a Literature Review? How to Write It (with Examples)

A literature review is a critical analysis and synthesis of existing research on a particular topic. It provides an overview of the current state of knowledge, identifies gaps, and highlights key findings in the literature. 1 The purpose of a literature review is to situate your own research within the context of existing scholarship, demonstrating your understanding of the topic and showing how your work contributes to the ongoing conversation in the field. Learning how to write a literature review is a critical tool for successful research. Your ability to summarize and synthesize prior research pertaining to a certain topic demonstrates your grasp on the topic of study, and assists in the learning process. 

Table of Contents

• What is the purpose of literature review? 
• a. Habitat Loss and Species Extinction: 
• b. Range Shifts and Phenological Changes: 
• c. Ocean Acidification and Coral Reefs: 
• d. Adaptive Strategies and Conservation Efforts: 
• How to write a good literature review 
• Choose a Topic and Define the Research Question: 
• Decide on the Scope of Your Review: 
• Select Databases for Searches: 
• Conduct Searches and Keep Track: 
• Review the Literature: 
• Organize and Write Your Literature Review: 
• Frequently asked questions 

What is a literature review?

A well-conducted literature review demonstrates the researcher’s familiarity with the existing literature, establishes the context for their own research, and contributes to scholarly conversations on the topic. One of the purposes of a literature review is also to help researchers avoid duplicating previous work and ensure that their research is informed by and builds upon the existing body of knowledge.

What is the purpose of literature review?

A literature review serves several important purposes within academic and research contexts. Here are some key objectives and functions of a literature review: 2  

• Contextualizing the Research Problem: The literature review provides a background and context for the research problem under investigation. It helps to situate the study within the existing body of knowledge. 
• Identifying Gaps in Knowledge: By identifying gaps, contradictions, or areas requiring further research, the researcher can shape the research question and justify the significance of the study. This is crucial for ensuring that the new research contributes something novel to the field. 
• Understanding Theoretical and Conceptual Frameworks: Literature reviews help researchers gain an understanding of the theoretical and conceptual frameworks used in previous studies. This aids in the development of a theoretical framework for the current research. 
• Providing Methodological Insights: Another purpose of literature reviews is that it allows researchers to learn about the methodologies employed in previous studies. This can help in choosing appropriate research methods for the current study and avoiding pitfalls that others may have encountered. 
• Establishing Credibility: A well-conducted literature review demonstrates the researcher’s familiarity with existing scholarship, establishing their credibility and expertise in the field. It also helps in building a solid foundation for the new research. 
• Informing Hypotheses or Research Questions: The literature review guides the formulation of hypotheses or research questions by highlighting relevant findings and areas of uncertainty in existing literature. 

Literature review example

Let’s delve deeper with a literature review example: Let’s say your literature review is about the impact of climate change on biodiversity. You might format your literature review into sections such as the effects of climate change on habitat loss and species extinction, phenological changes, and marine biodiversity. Each section would then summarize and analyze relevant studies in those areas, highlighting key findings and identifying gaps in the research. The review would conclude by emphasizing the need for further research on specific aspects of the relationship between climate change and biodiversity. The following literature review template provides a glimpse into the recommended literature review structure and content, demonstrating how research findings are organized around specific themes within a broader topic. 

Literature Review on Climate Change Impacts on Biodiversity:

Climate change is a global phenomenon with far-reaching consequences, including significant impacts on biodiversity. This literature review synthesizes key findings from various studies: 

a. Habitat Loss and Species Extinction:

Climate change-induced alterations in temperature and precipitation patterns contribute to habitat loss, affecting numerous species (Thomas et al., 2004). The review discusses how these changes increase the risk of extinction, particularly for species with specific habitat requirements. 

b. Range Shifts and Phenological Changes:

Observations of range shifts and changes in the timing of biological events (phenology) are documented in response to changing climatic conditions (Parmesan & Yohe, 2003). These shifts affect ecosystems and may lead to mismatches between species and their resources. 

c. Ocean Acidification and Coral Reefs:

The review explores the impact of climate change on marine biodiversity, emphasizing ocean acidification’s threat to coral reefs (Hoegh-Guldberg et al., 2007). Changes in pH levels negatively affect coral calcification, disrupting the delicate balance of marine ecosystems. 

d. Adaptive Strategies and Conservation Efforts:

Recognizing the urgency of the situation, the literature review discusses various adaptive strategies adopted by species and conservation efforts aimed at mitigating the impacts of climate change on biodiversity (Hannah et al., 2007). It emphasizes the importance of interdisciplinary approaches for effective conservation planning. 

How to write a good literature review

Writing a literature review involves summarizing and synthesizing existing research on a particular topic. A good literature review format should include the following elements. 

Introduction: The introduction sets the stage for your literature review, providing context and introducing the main focus of your review. 

• Opening Statement: Begin with a general statement about the broader topic and its significance in the field. 
• Scope and Purpose: Clearly define the scope of your literature review. Explain the specific research question or objective you aim to address. 
• Organizational Framework: Briefly outline the structure of your literature review, indicating how you will categorize and discuss the existing research. 
• Significance of the Study: Highlight why your literature review is important and how it contributes to the understanding of the chosen topic. 
• Thesis Statement: Conclude the introduction with a concise thesis statement that outlines the main argument or perspective you will develop in the body of the literature review. 

Body: The body of the literature review is where you provide a comprehensive analysis of existing literature, grouping studies based on themes, methodologies, or other relevant criteria. 

• Organize by Theme or Concept: Group studies that share common themes, concepts, or methodologies. Discuss each theme or concept in detail, summarizing key findings and identifying gaps or areas of disagreement. 
• Critical Analysis: Evaluate the strengths and weaknesses of each study. Discuss the methodologies used, the quality of evidence, and the overall contribution of each work to the understanding of the topic. 
• Synthesis of Findings: Synthesize the information from different studies to highlight trends, patterns, or areas of consensus in the literature. 
• Identification of Gaps: Discuss any gaps or limitations in the existing research and explain how your review contributes to filling these gaps. 
• Transition between Sections: Provide smooth transitions between different themes or concepts to maintain the flow of your literature review. 

Conclusion: The conclusion of your literature review should summarize the main findings, highlight the contributions of the review, and suggest avenues for future research. 

• Summary of Key Findings: Recap the main findings from the literature and restate how they contribute to your research question or objective. 
• Contributions to the Field: Discuss the overall contribution of your literature review to the existing knowledge in the field. 
• Implications and Applications: Explore the practical implications of the findings and suggest how they might impact future research or practice. 
• Recommendations for Future Research: Identify areas that require further investigation and propose potential directions for future research in the field. 
• Final Thoughts: Conclude with a final reflection on the importance of your literature review and its relevance to the broader academic community. 

Conducting a literature review

Conducting a literature review is an essential step in research that involves reviewing and analyzing existing literature on a specific topic. It’s important to know how to do a literature review effectively, so here are the steps to follow: 1  

Choose a Topic and Define the Research Question:

• Select a topic that is relevant to your field of study. 
• Clearly define your research question or objective. Determine what specific aspect of the topic do you want to explore? 

Decide on the Scope of Your Review:

• Determine the timeframe for your literature review. Are you focusing on recent developments, or do you want a historical overview? 
• Consider the geographical scope. Is your review global, or are you focusing on a specific region? 
• Define the inclusion and exclusion criteria. What types of sources will you include? Are there specific types of studies or publications you will exclude? 

Select Databases for Searches:

• Identify relevant databases for your field. Examples include PubMed, IEEE Xplore, Scopus, Web of Science, and Google Scholar. 
• Consider searching in library catalogs, institutional repositories, and specialized databases related to your topic. 

Conduct Searches and Keep Track:

• Develop a systematic search strategy using keywords, Boolean operators (AND, OR, NOT), and other search techniques. 
• Record and document your search strategy for transparency and replicability. 
• Keep track of the articles, including publication details, abstracts, and links. Use citation management tools like EndNote, Zotero, or Mendeley to organize your references. 

Review the Literature:

• Evaluate the relevance and quality of each source. Consider the methodology, sample size, and results of studies. 
• Organize the literature by themes or key concepts. Identify patterns, trends, and gaps in the existing research. 
• Summarize key findings and arguments from each source. Compare and contrast different perspectives. 
• Identify areas where there is a consensus in the literature and where there are conflicting opinions. 
• Provide critical analysis and synthesis of the literature. What are the strengths and weaknesses of existing research? 

Organize and Write Your Literature Review:

• Literature review outline should be based on themes, chronological order, or methodological approaches. 
• Write a clear and coherent narrative that synthesizes the information gathered. 
• Use proper citations for each source and ensure consistency in your citation style (APA, MLA, Chicago, etc.). 
• Conclude your literature review by summarizing key findings, identifying gaps, and suggesting areas for future research. 

The literature review sample and detailed advice on writing and conducting a review will help you produce a well-structured report. But remember that a literature review is an ongoing process, and it may be necessary to revisit and update it as your research progresses. 

Frequently asked questions

A literature review is a critical and comprehensive analysis of existing literature (published and unpublished works) on a specific topic or research question and provides a synthesis of the current state of knowledge in a particular field. A well-conducted literature review is crucial for researchers to build upon existing knowledge, avoid duplication of efforts, and contribute to the advancement of their field. It also helps researchers situate their work within a broader context and facilitates the development of a sound theoretical and conceptual framework for their studies.

Literature review is a crucial component of research writing, providing a solid background for a research paper’s investigation. The aim is to keep professionals up to date by providing an understanding of ongoing developments within a specific field, including research methods, and experimental techniques used in that field, and present that knowledge in the form of a written report. Also, the depth and breadth of the literature review emphasizes the credibility of the scholar in his or her field.  

Before writing a literature review, it’s essential to undertake several preparatory steps to ensure that your review is well-researched, organized, and focused. This includes choosing a topic of general interest to you and doing exploratory research on that topic, writing an annotated bibliography, and noting major points, especially those that relate to the position you have taken on the topic. 

Literature reviews and academic research papers are essential components of scholarly work but serve different purposes within the academic realm. 3 A literature review aims to provide a foundation for understanding the current state of research on a particular topic, identify gaps or controversies, and lay the groundwork for future research. Therefore, it draws heavily from existing academic sources, including books, journal articles, and other scholarly publications. In contrast, an academic research paper aims to present new knowledge, contribute to the academic discourse, and advance the understanding of a specific research question. Therefore, it involves a mix of existing literature (in the introduction and literature review sections) and original data or findings obtained through research methods. 

Literature reviews are essential components of academic and research papers, and various strategies can be employed to conduct them effectively. If you want to know how to write a literature review for a research paper, here are four common approaches that are often used by researchers.  Chronological Review: This strategy involves organizing the literature based on the chronological order of publication. It helps to trace the development of a topic over time, showing how ideas, theories, and research have evolved.  Thematic Review: Thematic reviews focus on identifying and analyzing themes or topics that cut across different studies. Instead of organizing the literature chronologically, it is grouped by key themes or concepts, allowing for a comprehensive exploration of various aspects of the topic.  Methodological Review: This strategy involves organizing the literature based on the research methods employed in different studies. It helps to highlight the strengths and weaknesses of various methodologies and allows the reader to evaluate the reliability and validity of the research findings.  Theoretical Review: A theoretical review examines the literature based on the theoretical frameworks used in different studies. This approach helps to identify the key theories that have been applied to the topic and assess their contributions to the understanding of the subject.  It’s important to note that these strategies are not mutually exclusive, and a literature review may combine elements of more than one approach. The choice of strategy depends on the research question, the nature of the literature available, and the goals of the review. Additionally, other strategies, such as integrative reviews or systematic reviews, may be employed depending on the specific requirements of the research.

The literature review format can vary depending on the specific publication guidelines. However, there are some common elements and structures that are often followed. Here is a general guideline for the format of a literature review:  Introduction:   Provide an overview of the topic.  Define the scope and purpose of the literature review.  State the research question or objective.  Body:   Organize the literature by themes, concepts, or chronology.  Critically analyze and evaluate each source.  Discuss the strengths and weaknesses of the studies.  Highlight any methodological limitations or biases.  Identify patterns, connections, or contradictions in the existing research.  Conclusion:   Summarize the key points discussed in the literature review.  Highlight the research gap.  Address the research question or objective stated in the introduction.  Highlight the contributions of the review and suggest directions for future research.

Both annotated bibliographies and literature reviews involve the examination of scholarly sources. While annotated bibliographies focus on individual sources with brief annotations, literature reviews provide a more in-depth, integrated, and comprehensive analysis of existing literature on a specific topic. The key differences are as follows: 

References 

• Denney, A. S., & Tewksbury, R. (2013). How to write a literature review.  Journal of criminal justice education ,  24 (2), 218-234. 
• Pan, M. L. (2016).  Preparing literature reviews: Qualitative and quantitative approaches . Taylor & Francis. 
• Cantero, C. (2019). How to write a literature review.  San José State University Writing Center . 

Paperpal is an AI writing assistant that help academics write better, faster with real-time suggestions for in-depth language and grammar correction. Trained on millions of research manuscripts enhanced by professional academic editors, Paperpal delivers human precision at machine speed.  

Try it for free or upgrade to  Paperpal Prime , which unlocks unlimited access to premium features like academic translation, paraphrasing, contextual synonyms, consistency checks and more. It’s like always having a professional academic editor by your side! Go beyond limitations and experience the future of academic writing.  Get Paperpal Prime now at just US$19 a month!

Related Reads:

• Empirical Research: A Comprehensive Guide for Academics 
• How to Write a Scientific Paper in 10 Steps 
• Life Sciences Papers: 9 Tips for Authors Writing in Biological Sciences
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Systematic Reviews

• Types of Literature Reviews

What Makes a Systematic Review Different from Other Types of Reviews?

• Planning Your Systematic Review
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• Managing & Appraising Results
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Reproduced from Grant, M. J. and Booth, A. (2009), A typology of reviews: an analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26: 91–108. doi:10.1111/j.1471-1842.2009.00848.x

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Literature Reviews

• Getting Started

Selecting a Review Type

Defining the scope of your review, four common types of reviews.

• Developing a Research Question
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More Review Types

This article by Sutton & Booth (2019) explores 48 distinct types of Literature Reviews:

Which Review is Right for You?

The  Right Review tool  has questions about your lit review process and plans. It offers a qualitative and quantitative option. At completion, you are given a lit review type recommendation.

You'll want to think about the kind of review you are doing. Is it a selective or comprehensive review? Is the review part of a larger work or a stand-alone work ?

For example, if you're writing the Literature Review section of a journal article, that's a selective review which is part of a larger work. Alternatively, if you're writing a review article, that's a comprehensive review which is a stand-alone work. Thinking about this will help you develop the scope of the review.

This exercise will help define the scope of your Literature Review, setting the boundaries for which literature to include and which to exclude.

A FEW GENERAL CONSIDERATIONS WHEN DEFINING SCOPE

• Which populations to investigate — this can include gender, age, socio-economic status, race, geographic location, etc., if the research area includes humans.
• What years to include — if researching the legalization of medicinal cannabis, you might only look at the previous 20 years; but if researching dolphin mating practices, you might extend many more decades.
• Which subject areas — if researching artificial intelligence, subject areas could be computer science, robotics, or health sciences
• How many sources  — a selective review for a class assignment might only need ten, while a comprehensive review for a dissertation might include hundreds. There is no one right answer.
• There will be many other considerations that are more specific to your topic. 

Most databases will allow you to limit years and subject areas, so look for those tools while searching. See the Searching Tips tab for information on how use these tools.

LITERATURE REVIEW

• Often used as a generic term to describe any type of review
• More precise definition:  Published materials that provide an examination of published literature . Can cover wide range of subjects at various levels of comprehensiveness.
• Identifies gaps in research, explains importance of topic, hypothesizes future work, etc.
• Usually written as part of a larger work like a journal article or dissertation

SCOPING REVIEW

• Conducted to address broad research questions with the goal of understanding the extent of research that has been conducted.
• Provides a preliminary assessment of the potential size and scope of available research literature. It aims to identify the nature and extent of research evidence (usually including ongoing research) 
• Doesn't assess the quality of the literature gathered (i.e. presence of literature on a topic shouldn’t be conflated w/ the quality of that literature)

SYSTEMATIC REVIEW

• Common in the health sciences ( Taubman Health Sciences Library guide to Systematic Reviews )
• Goal: collect all literature that meets specific criteria (methodology, population, treatment, etc.) and then appraise its quality and synthesize it
• Follows strict protocol for literature collection, appraisal and synthesis
• Typically performed by research teams 
• Takes 12-18 months to complete
• Often written as a stand alone work

META-ANALYSIS

• Goes one step further than a systematic review by statistically combining the results of quantitative studies to provide a more precise effect of the results. 
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The Literature Review: A Foundation for High-Quality Medical Education Research

a  These are subscription resources. Researchers should check with their librarian to determine their access rights.

Despite a surge in published scholarship in medical education 1 and rapid growth in journals that publish educational research, manuscript acceptance rates continue to fall. 2 Failure to conduct a thorough, accurate, and up-to-date literature review identifying an important problem and placing the study in context is consistently identified as one of the top reasons for rejection. 3 , 4 The purpose of this editorial is to provide a road map and practical recommendations for planning a literature review. By understanding the goals of a literature review and following a few basic processes, authors can enhance both the quality of their educational research and the likelihood of publication in the Journal of Graduate Medical Education ( JGME ) and in other journals.

The Literature Review Defined

In medical education, no organization has articulated a formal definition of a literature review for a research paper; thus, a literature review can take a number of forms. Depending on the type of article, target journal, and specific topic, these forms will vary in methodology, rigor, and depth. Several organizations have published guidelines for conducting an intensive literature search intended for formal systematic reviews, both broadly (eg, PRISMA) 5 and within medical education, 6 and there are excellent commentaries to guide authors of systematic reviews. 7 , 8

• A literature review forms the basis for high-quality medical education research and helps maximize relevance, originality, generalizability, and impact.
• A literature review provides context, informs methodology, maximizes innovation, avoids duplicative research, and ensures that professional standards are met.
• Literature reviews take time, are iterative, and should continue throughout the research process.
• Researchers should maximize the use of human resources (librarians, colleagues), search tools (databases/search engines), and existing literature (related articles).
• Keeping organized is critical.

Such work is outside the scope of this article, which focuses on literature reviews to inform reports of original medical education research. We define such a literature review as a synthetic review and summary of what is known and unknown regarding the topic of a scholarly body of work, including the current work's place within the existing knowledge . While this type of literature review may not require the intensive search processes mandated by systematic reviews, it merits a thoughtful and rigorous approach.

Purpose and Importance of the Literature Review

An understanding of the current literature is critical for all phases of a research study. Lingard 9 recently invoked the “journal-as-conversation” metaphor as a way of understanding how one's research fits into the larger medical education conversation. As she described it: “Imagine yourself joining a conversation at a social event. After you hang about eavesdropping to get the drift of what's being said (the conversational equivalent of the literature review), you join the conversation with a contribution that signals your shared interest in the topic, your knowledge of what's already been said, and your intention.” 9

The literature review helps any researcher “join the conversation” by providing context, informing methodology, identifying innovation, minimizing duplicative research, and ensuring that professional standards are met. Understanding the current literature also promotes scholarship, as proposed by Boyer, 10 by contributing to 5 of the 6 standards by which scholarly work should be evaluated. 11 Specifically, the review helps the researcher (1) articulate clear goals, (2) show evidence of adequate preparation, (3) select appropriate methods, (4) communicate relevant results, and (5) engage in reflective critique.

Failure to conduct a high-quality literature review is associated with several problems identified in the medical education literature, including studies that are repetitive, not grounded in theory, methodologically weak, and fail to expand knowledge beyond a single setting. 12 Indeed, medical education scholars complain that many studies repeat work already published and contribute little new knowledge—a likely cause of which is failure to conduct a proper literature review. 3 , 4

Likewise, studies that lack theoretical grounding or a conceptual framework make study design and interpretation difficult. 13 When theory is used in medical education studies, it is often invoked at a superficial level. As Norman 14 noted, when theory is used appropriately, it helps articulate variables that might be linked together and why, and it allows the researcher to make hypotheses and define a study's context and scope. Ultimately, a proper literature review is a first critical step toward identifying relevant conceptual frameworks.

Another problem is that many medical education studies are methodologically weak. 12 Good research requires trained investigators who can articulate relevant research questions, operationally define variables of interest, and choose the best method for specific research questions. Conducting a proper literature review helps both novice and experienced researchers select rigorous research methodologies.

Finally, many studies in medical education are “one-offs,” that is, single studies undertaken because the opportunity presented itself locally. Such studies frequently are not oriented toward progressive knowledge building and generalization to other settings. A firm grasp of the literature can encourage a programmatic approach to research.

Approaching the Literature Review

Considering these issues, journals have a responsibility to demand from authors a thoughtful synthesis of their study's position within the field, and it is the authors' responsibility to provide such a synthesis, based on a literature review. The aforementioned purposes of the literature review mandate that the review occurs throughout all phases of a study, from conception and design, to implementation and analysis, to manuscript preparation and submission.

Planning the literature review requires understanding of journal requirements, which vary greatly by journal ( table 1 ). Authors are advised to take note of common problems with reporting results of the literature review. Table 2 lists the most common problems that we have encountered as authors, reviewers, and editors.

Sample of Journals' Author Instructions for Literature Reviews Conducted as Part of Original Research Article a

Common Problem Areas for Reporting Literature Reviews in the Context of Scholarly Articles

Locating and Organizing the Literature

Three resources may facilitate identifying relevant literature: human resources, search tools, and related literature. As the process requires time, it is important to begin searching for literature early in the process (ie, the study design phase). Identifying and understanding relevant studies will increase the likelihood of designing a relevant, adaptable, generalizable, and novel study that is based on educational or learning theory and can maximize impact.

Human Resources

A medical librarian can help translate research interests into an effective search strategy, familiarize researchers with available information resources, provide information on organizing information, and introduce strategies for keeping current with emerging research. Often, librarians are also aware of research across their institutions and may be able to connect researchers with similar interests. Reaching out to colleagues for suggestions may help researchers quickly locate resources that would not otherwise be on their radar.

During this process, researchers will likely identify other researchers writing on aspects of their topic. Researchers should consider searching for the publications of these relevant researchers (see table 3 for search strategies). Additionally, institutional websites may include curriculum vitae of such relevant faculty with access to their entire publication record, including difficult to locate publications, such as book chapters, dissertations, and technical reports.

Strategies for Finding Related Researcher Publications in Databases and Search Engines

Search Tools and Related Literature

Researchers will locate the majority of needed information using databases and search engines. Excellent resources are available to guide researchers in the mechanics of literature searches. 15 , 16

Because medical education research draws on a variety of disciplines, researchers should include search tools with coverage beyond medicine (eg, psychology, nursing, education, and anthropology) and that cover several publication types, such as reports, standards, conference abstracts, and book chapters (see the box for several information resources). Many search tools include options for viewing citations of selected articles. Examining cited references provides additional articles for review and a sense of the influence of the selected article on its field.

Box Information Resources

• Web of Science a
• Education Resource Information Center (ERIC)
• Cumulative Index of Nursing & Allied Health (CINAHL) a
• Google Scholar

Once relevant articles are located, it is useful to mine those articles for additional citations. One strategy is to examine references of key articles, especially review articles, for relevant citations.

Getting Organized

As the aforementioned resources will likely provide a tremendous amount of information, organization is crucial. Researchers should determine which details are most important to their study (eg, participants, setting, methods, and outcomes) and generate a strategy for keeping those details organized and accessible. Increasingly, researchers utilize digital tools, such as Evernote, to capture such information, which enables accessibility across digital workspaces and search capabilities. Use of citation managers can also be helpful as they store citations and, in some cases, can generate bibliographies ( table 4 ).

Citation Managers

Knowing When to Say When

Researchers often ask how to know when they have located enough citations. Unfortunately, there is no magic or ideal number of citations to collect. One strategy for checking coverage of the literature is to inspect references of relevant articles. As researchers review references they will start noticing a repetition of the same articles with few new articles appearing. This can indicate that the researcher has covered the literature base on a particular topic.

Putting It All Together

In preparing to write a research paper, it is important to consider which citations to include and how they will inform the introduction and discussion sections. The “Instructions to Authors” for the targeted journal will often provide guidance on structuring the literature review (or introduction) and the number of total citations permitted for each article category. Reviewing articles of similar type published in the targeted journal can also provide guidance regarding structure and average lengths of the introduction and discussion sections.

When selecting references for the introduction consider those that illustrate core background theoretical and methodological concepts, as well as recent relevant studies. The introduction should be brief and present references not as a laundry list or narrative of available literature, but rather as a synthesized summary to provide context for the current study and to identify the gap in the literature that the study intends to fill. For the discussion, citations should be thoughtfully selected to compare and contrast the present study's findings with the current literature and to indicate how the present study moves the field forward.

To facilitate writing a literature review, journals are increasingly providing helpful features to guide authors. For example, the resources available through JGME include several articles on writing. 17 The journal Perspectives on Medical Education recently launched “The Writer's Craft,” which is intended to help medical educators improve their writing. Additionally, many institutions have writing centers that provide web-based materials on writing a literature review, and some even have writing coaches.

The literature review is a vital part of medical education research and should occur throughout the research process to help researchers design a strong study and effectively communicate study results and importance. To achieve these goals, researchers are advised to plan and execute the literature review carefully. The guidance in this editorial provides considerations and recommendations that may improve the quality of literature reviews.

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• Published: 20 May 2023

A review of glass corrosion: the unique contribution of studying ancient glass to validate glass alteration models

• Roberta Zanini   ORCID: orcid.org/0000-0002-3190-9191 1 , 2   na1 ,
• Giulia Franceschin   ORCID: orcid.org/0000-0003-1817-2962 1   na1 ,
• Elti Cattaruzza   ORCID: orcid.org/0000-0003-0643-0266 2 &
• Arianna Traviglia   ORCID: orcid.org/0000-0002-4508-1540 1  

npj Materials Degradation volume  7 , Article number:  38 ( 2023 ) Cite this article

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• Structure of solids and liquids

Glass has been used in widespread applications within several sectors since ancient times and it has been systematically studied under different perspectives. However, its thermodynamic properties and the variety of its compositions, several aspects related to its durability and its alteration mechanisms remain still open to debate. This literature review presents an overview of the most relevant studies on glass corrosion and the interaction between glass and the environment. The review aims to achieve two objectives. On one hand, it aims to highlight how far research on glass corrosion has come by studying model systems created in the laboratory to simulate different alteration conditions and glass compositions. On the other, it seeks to point out what are the critical aspects that still need to be investigated and how the study of ancient, altered glass can add to the results obtained in laboratory models. The review intends also to demonstrate how advanced analytical techniques commonly used to study modern and technical glass can be applied to investigate corrosion marks on ancient samples.

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Introduction

Throughout history, glass has played a central role in the production of goods for widespread use, making it a material that has been extensively studied from various perspectives. However, due to its thermodynamic properties and the vast range of existing compositions, there are still several aspects of its nature that are not fully understood. For instance, evaluating glass durability and alteration mechanisms remains a challenge, as it requires taking into account numerous factors, some of which are difficult to replicate in laboratory experiments. One of the most prominent challenges is the time required for the transformation of the glass structure, which is directly responsible for the kinetics and dynamics of the processes involved.

Technological innovation has recently introduced new tools for studying the durability of glass and shedding light on the deep connections in extremely complex materials between compositions, structure, and the surrounding environment. As an example, a recent review 1 reported about the possibility of employing mechanistic models to study aqueous glass alteration. Thanks to the availability of such novel techniques, glass alteration mechanisms and kinetics can be hypothesised using non-conventional experimental methods (simulations) and without direct experimental validations. In parallel, the long-term perspective provided by studying ancient vitreous objects can inform and underpin the predictions obtained through simulation and laboratory methods 2 , 3 , 4 , 5 , 6 .

Most of the studies performed to understand glass corrosion are based on artificial ageing experiments that are designed to monitor, step by step, the process of glass alteration and the physicochemical evolution of the glass structure 7 , 8 , 9 , 10 . Because of the great variability of the experimental conditions involved in these studies, however, results are not yet conclusive, and the glass transformation process is still only partially described in literature.

Standard approaches to the study of glass corrosion are limited in that they are valid in some specific experimental conditions, but not in others. Indeed, most of the available research focuses on the corrosion mechanisms of glass with specific compositions (lead-rich or borosilicate families), or under conditions of saturated humidity (relative humidity, RH ≥ 100%) 11 , 12 , 13 . In both cases, the atomic diffusion occurring in the material alteration processes is assisted by two key parameters, which are the presence of ionic species with high mobility and the presence of liquid water on the surface, respectively. Only an handful of published works 2 , 7 , 8 –focused on ageing under conditions of RH < 100% and on a wider variety of glass compositions, like those of the silica-soda-lime (SSL) glass family– are available nowadays in literature: they represent valuable experimental approaches to determine and separate the relative influence of different parameters on the chemical and environmental durability of glass.

In view of the above, this review focuses on the literature available on glass alteration and related structural investigations that have contributed to the comprehension of the modifications occurring in the glass network when glass degrades. In addition, an examination of the existing –but limited– literature on the analysis of ancient glass will bring to the attention of the chemistry community a vast amount of unexplored information that is relevant also for the study of contemporary glass types, to validate the prediction of glass behaviour obtained through laboratory experiments. The most advanced techniques to study alteration on heritage glass will be described with the aim of highlighting how deepening the understanding of altered ancient glass properties is key for a full understanding of the phenomenon of natural ageing of both modern and ancient glass objects.

For better clarity, the term a rchaeological glass will be used to refer to glass specimens that have aged in soil or marine environments for centuries, whereas the term historical glass will be used when speaking about glass aged under the effect of environmental moisture fluctuations (in museums or private collections). The term ancient glass is more generally used to refer to both the above mentioned cathegories.

By using a combination of comprehensive characterization of ancient samples and laboratory-based ageing experiments, researchers can obtain valuable information about glass alterations that have occurred over hundreds of years while also monitoring the alteration process step-by-step. This integrated approach offers a unique opportunity to consolidate and supplement current theories of glass corrosion and validate predictive models using real-world examples of long-term alteration. Overall, this approach provides a more complete understanding of the complex process of glass corrosion and can help guide the development of more durable glass materials.

Glass structure and its dissolution prediction: key highlights

Glass is an amorphous material that shares structural features at the atomic level with a supercooled liquid, while exhibiting mechanical properties typical of solids.

Zachariasen 14 was the first to describe the amorphous structure of glass in 1932. His studies led him to conclude that there is no long-range atomic periodicity in glass structure. He also defined the requirements for a particular oxide to exist in the vitreous state. Silicon dioxide, also known as silica, is the primary network former in both ancient as well as in modern glass.

In silica-based glass, the bonding forces involved in the formation of the glass network are those of crystalline SiO 2 , which has a tetrahedral structure with 4 oxygen atoms located at the corners of a tetrahedron and a Si atom placed in the middle of the structure 15 (Fig. 1 ). The glass network can be figured as a built-up of such tetrahedral elements, which share each corner with neighbouring tetrahedra, one per corner. The remaining corners are available to form other chemical bonds. SiO 2 is considered a primary network former, but other oxide types exist with these same properties, such as B 2 O 3 or P 2 O 5 . The addition of alkali oxides, such as Na 2 O, K 2 O, Li 2 O, to the glass network former is responsible for breaking some of the Si-O-Si bonds bridging silicon atoms and for including the alkaline cations into the glass structure. These network modifiers generate the formation of non-bridging oxygens (NBOs) in addition to the bridging oxygens (BOs) of the silica network. NBOs hold a negative charge that is locally compensated by one highly mobile alkali cation with a positive charge (i.e., Na + ). The main effect of such network modifiers is to decrease the viscosity of the glass melt, thus facilitating its workability at lower temperatures. Alkaline earth oxides like CaO are added to the glass batch as network stabilisers 16 . They connect to two NBOs and are commonly considered as lower mobility ions (i.e., Ca 2+ ) that are possibly effective in inhibiting the diffusion of other cations across the silica network, hence improving the chemical resistance of the glass (Fig. 2 ). It is now clear why 90% of commercial glass is made of a mixture of silica (as network former), sodium and calcium oxide (as network modifiers). Nevertheless, the positive influence of alkaline earths in increasing glass stability against alteration has not been fully demonstrated yet, and several works indicated that Ca 2+ and Mg 2+ ions are at least as mobile as alkalis within the hydrated layer in atmospheric conditions 17 , 18 .

Schematic representation of a tetrahedral silica unit (not to scale).

For clarity’s sake, only the BOs and NBOs helping to identify the Q n configuration in the silica network are coloured in red for BO and in yellow for NBO.

The structure of glass is not in thermodynamic equilibrium. The durability of glass depends on both the kinetic and thermodynamic stability of its oxide components. In a state of thermodynamic equilibrium, the chemical potential of the species on the glass surface and those in solution are equal, and as a result, no net mass transfer occurs. However, glass does not exist in a state of thermodynamic equilibrium and is therefore prone to undergo chemical reactions that can result in degradation over time. The understanding of the kinetic and thermodynamic stability of glass components is crucial for developing more durable and long-lasting glass materials.

An essential factor to be taken into consideration to predict glass dissolution is the knowledge of the relative concentration of bridging and non-bridging oxygen atoms 19 . The latter are bonded to only one silicon atom and their quantity within the glass network is proportional to the concentration of modifier ions.

Considering the two possible configurations for an oxygen atom (BO and NBO), the silicon atom may be found in five different tetrahedral arrangements: Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 , where the subscript indicates the number of bridging oxygens (Fig. 2 ). The structure of the glass network is the result of the distribution of the rings and voids regulated by the interconnection between these different silicate tetrahedra. The size of the voids in the network controls the rate of water diffusion, which is kinetically favoured when the dimension of the voids is comparable to the diameter of the water molecule (0.28 nm). In complex glass (mixed alkali glass), modifier cations can totally or partially fill the voids, but when the material is exposed to high relative humidity conditions, these alkali ions are leached from the glass surface and replaced by hydrogen ions as part of molecular water. The ion-exchange reaction drives the hydrolysis of the glass network with kinetics depending on both the distribution of local structural units (Q n ) and the modifier content. In addition, the exchange of high-radius cations as K + from the bulk is considered to leave a bigger void in the glass network compared to the exchange of smaller cation as Na + , thus facilitating the entrance of water molecules into the deeper areas. In general, it can be said that the higher the concentration of NBO, the higher the number of ion exchange sites available and the rate of ion-exchange and network hydrolysis, following the reaction trend: Q 1  > Q 2  > Q 3  > Q 4 19 , 20 . For this reason, in the discussion about the kinetics of the processes of ionic exchange and hydrolysis reactions it is essential to take into consideration the chemical composition of glass.

Taking into account what has been said above, knowing the chemical composition of complex glass and its Q n concentration and distribution is fundamental for the appreciation of its chemical stability and leaching resistance, so as to adopt an adequate preventive conservation strategy. Nevertheless, the glass reactivity does not depend on Q n species only. Understanding the correlation between local structural features of the glass and the activation energies of individual bonds is also crucial to predict the dissolution mechanism of the glass network. Potential Mean Force (PMF) calculations estimated the activation barrier for Si dissolution in presence of aluminium (Al). They revealed that Al is easily dissociated from glass network, but Si dissociation is hindered when Al is present as a second neighbour 21 . As a result, Al causes opposing effects on glass durability if added at low and high concentrations: the addition of Al in small concentration increases the durability by reinforcing the strength of Si and increasing the polymerisation of the glass network, while at high Al concentration, the preferential release of Al results in the weakening of the silicate network. This predicting method can be extended to understand the role of Na, Mg, or B in more complex glass compositions.

Since the prediction of dissolution of glass network is a complicated topic of research, many recent works 22 , 23 , 24 reported the use of machine learning based approaches to account for the percentage of bridging oxygen species, network connectivity, average ring size, as well as the composition modification due to the preferential release of modifier cations during the incongruent dissolution.

Several glass studies demonstrated the usefulness of Raman spectroscopy as an analytical technique to discriminate the characteristic vibrational modes of each Q n configuration 25 , 26 , 27 , 28 . Through the deconvolution of the Raman bands typically associated to the glass network it is possible to determine the single Q n distribution and associate the variation of the area of the related Q n band with the chemical composition of the sample analysed 29 , 30 . When using this analytical approach, Raman spectroscopy can be adopted as a technique to distinguish a stable glass from an unstable one by means of the rigorous deconvolution of the vibrational bands of the glass network 29 , 31 , 32 .

Nowadays, an analytical protocol that combines the potentialities of Raman spectroscopy described above with the advantages of using portable instrumentation to evaluate the chemical stability of glass, to predict the glass network dissolution for preventive conservation purposes, or even to establish the suitability of glass as storage material for nuclear waste is not available. Moreover, the in-depth spatial resolution of Raman spectroscopy is inadequate if one wants to determine the layered structures on an altered surface, whose features, in fact, sometimes vary on the nanometre scale. The association of other complementary analytical techniques would be ideal to optimise the reliability of Raman spectroscopy results.

X-ray absorption near edge structure (XANES) spectroscopy at the Si K-edge is another interesting technique that has been used to study the polymerisation degree of SiO 4 tetrahedra silica glass. In XANES spectra, position and structure of the absorption edge are largely determined by the charge of the absorber atom and by geometry of the first coordination shell, which depends on the coordination of the nearest neighbour atoms, the degree of polymerisation, and the presence of network modifiers and network substitutes 33 . A study conducted on silicate glass reported that, with the increase in the polymerisation degree of the silica tetrahedra, the Si K-edge shifts towards higher energy, while it shifts towards lower energy when Si is substituted for another network former (Al) 34 .

The chemical changes occurring at the surface of a corroding glass often cause an alteration of the local environment of metal atoms, especially of the metal-oxygen pair distribution. These characteristics have been measured using conventional and glancing angle extended X-ray absorption fine structure (EXAFS) techniques, able to give information about number and distance of the atoms surrounding the absorber one. Examining the modifiers distribution in the vicinity of the surface gives important hints on how the surface is modified as corrosion advances 35 .

As transition metal cations are generally of interest for the application of X-ray absorption techniques, their study can be easily exploited to monitor the decay of ancient glass. The modifications of the chemical environment of chromophore species (i.e., transition metal cations) can be recorded using both XANES and EXAFS techniques by monitoring the spectra of a selected metal species in an altered glass sample. By means of this method, it was possible to establish a relationship between the oxidation state of Fe and Cu cations during glass decay. Whereas, the Mn oxidation state was not directly correlated with the glass decay of the samples studied 36 .

XANES and EXAFS analysis are not common and straightforward techniques, since they require the preparation of tailored samples from analysed objects (which is not always possible when dealing with cultural items) and the access to a synchrotron facility. However, this type of analysis provides important information to help the identification of structural and chemical changes in altered glass samples when investigating the molecular changes around cations with high field strength and well-defined short-range order.

In the following section, the main models of glass corrosion will be explored: they are based on the observation of the interactions between glass structure and external environment and on the different mechanisms in place during the process of glass alteration.

Exploring the mechanisms of glass alteration through the different existing models

The terminology used in the published works is imprecise. The terms corrosion , alteration , degradation , and deterioration are often found to be frequently used interchangeably as synonyms, despite having different shades of meaning. For the sake of clarity, all these terms will be used here as synonyms when talking about the phenomena that induce a change in glass physicochemical properties, regardless of the intrinsic or environmental factors that have determined such change. The term dissolution will be, instead, used to refer only to the rupture of the Si-O covalent bonds and the breakup of the structural silica network, and the term leaching will be used to identify the step of degradation that consists in the loss of alkali and alkaline earth metals ionically bonded to the silica network that precedes the network dissolution 37 . Even if leaching is often associated with the initial steps of glass degradation, the loss of alkalis can be a secondary phenomenon in specific conditions of high temperature (RH < 100%) and unstable glass compositions 16 . In other particular conditions, the degradation process occurs without any loss of alkalis, because the water penetrated into the glass network is unable to solvate them 8 , 9 , 18 .

The interaction between water and glass can activate two different degradation phenomena, i.e., leaching and network brakeage, mainly depending on the pH of the solution in contact with the surface. During leaching, the aqueous solution in contact with the surface typically has pH<9 38 , 39 . In this condition (pH<9), ion exchange that involves alkalis (Na, K) and alkaline earth metals (Ca, Mg) occurs, forming ionic bonds with the oxygen of the glass network and the H + ion from the aqueous solution 16 . This is a diffusive phenomenon and the thickness of the glass region involved in the reaction (indicatively a few microns) depends on the glass composition and on the time and temperature of exposition. The altered layers that form on the surface can act as a diffusion barrier to further extraction, even if hazardous cracks that allow the penetration of water molecules into the pristine glass may form. The alteration due to leaching does not affect the Si atoms: the network distribution does not change, only the Si-O-M bonds do.

On the other hand, the ions interdiffusion during leaching leads to a pH increase (eventually above 9) due to the formation of Na + OH - species in solution and Si-OH acid from the reaction between Si-O-Na and H 2 O. An alkaline environment results in more aggressive attacks to the glass network, since it promotes the dissolution of the Si–O bonds 40 . The reaction with the hydroxyl ions (OH – ) breaks the Si-O–Si bonds and silanol groups Si–OH are formed.

The sites left free by the leaching of cations from the glass surface can be easily filled by hydrogen ions, which have small ionic radius. The hydrolysis process induces the introduction of H 2 O molecules and OH - ions into the opened silica structure, thus increasing the rate of the hydration process and the ion exchange.

The interactions and reactions that occur between aqueous solution and glass have been over the years the subject matter of extensive research focusing on the mechanism involved in this process at different scales of observation, research that has been key to support the formulation of the classic theory of alteration and Interfacial Dissolution Precipitation model for dissolution of vitreous materials.

The Classic Inter-Diffusion (CID) model of glass corrosion is based on diffusion-controlled hydrolysis and ion exchange reactions 41 , which lead to the formation of structurally and chemically distinct zones (Fig. 3 ).

The latter consists of a hydrated, cation-depleted layer resulting from a selective cation release. Courtesy of Gin et al. 48 .

The classic theory of glass corrosion is underpinned by different formulated models of ion-exchange that take into consideration the effects of the preferential dissolution of more soluble cations during the initial part of the leaching process 42 , 43 .

Many experimental and theoretical results obtained from observation in liquid conditions (and not in unsaturated humidity ones) have reported that the leaching mechanism involves the preferential release of alkali and alkali-earth ions rather than that of network formers, such as Si or Al ions, with a consequent formation of alumina/silicate-rich layer on the glass surface 38 , 43 .

In general, the concept of preferential leaching is based on the thermodynamic and kinetic stability of the different glass modifiers. At lower temperatures and for ions with the same charge, the diffusion of larger ions (for example Ba 2+ or Ca 2+ ) becomes energetically unfavoured, while the smaller ions (for example Mg 2+ ) can move more easily through the glass network 44 . In any case, double-charged ions usually show less diffusivity in SSL glass than single-charged ones, mainly because of the marked effect of the very intense local electric fields acting on them. If the leaching mechanism proceeds, the solution becomes richer and richer in OH – and its pH increases favouring the dissolution of silica through the break of O-Si-O network. Preferential leaching supports the selective removal of specific cations (non-stoichiometric release) and designs a theory about incongruent dissolution of glass that explains the formation of altered surface layer.

The technological developments of the last decades have made it possible to reveal increasingly more detailed evidence on the process that controls glass corrosion 11 . The chemical reactions proposed in the general mechanism of glass corrosion (hydration, hydrolysis, and interdiffusion) are still considered valid in the most recent studies. However, over the last years the attention of the scientific community has been focusing more on understanding how these reactions evolve kinetically and thermodynamically during the alteration process, and how they influence the structural and microstructural properties of the alteration layer at the atomic scale 1 , 45 .

In 2015, a nanometre-scale study 46 of glass corrosion was performed using a combination of high mass and spatial resolution techniques, proposing a revised theory of glass corrosion called the Interfacial Dissolution-Reprecipitation (IDP) model. The IDP model is based on the congruent dissolution of silicate glass coupled in space and time with the reprecipitation of amorphous spherical silica aggregates of variable size. In opposition to the traditional glass alteration model, this recent theory supports the stoichiometric dissolution of glass without interdiffusion-controlled ion-exchange mechanisms at the glass reaction front 46 .

Hellmann et al. validated this model through the study of artificially aged borosilicate glass altered at 50 °C in deionised water, using a unique combination of techniques with high spatial and mass resolution 46 . By following the mobility of the major constituent elements of complex borosilicate glass, an identical release behaviour was noticed for modifier and former ions, regardless of their charge. These results validate two processes at the basis of this novel corrosion mechanism, i.e., the stoichiometric release of all the glass elements and the precipitation of amorphous silica with the formation of an altered surface layer. Furthermore, the interface between pristine glass and altered zone was demonstrated to be chemically and structurally well defined, with elemental gradients in the nano- to sub-nano-metric range.

A schematic representation of the IDP model is presented in Fig. 4 according to the results of oxygen and silicon isotope tracer experiments in ternary borosilicate glass 47 .

a Initial congruent dissolution of glass is the first step occurring at the glass-water interface. This stage continues until the amorphous silica solution is supersaturated and etching pits are formed on the surface; ( b ) Si-rich interfacial solution layer is formed depending on the ratio of silica in solution to silica released during glass dissolution. Under this condition, the localised saturation of silica in solution promotes condensation and nucleation reactions that lead to the polymerisation of monomeric silica to form dimers and oligomers; ( c ) Precipitation of silica in the form of spheres on the dissolving glass surface occurs after silica supersaturation and nucleation in the solution; ( d ) Formation of SAL (Surface Alteration Layer) composed of altered amorphous silica proceeds along with congruent glass dissolution and further diffusion of dissolved species through the developing SAL; ( e ) Diffusive transport of water and dissolved species through the corrosion rim continues depending on the porosity of the SAL, i.e., higher porosity values correspond to higher diffusion rates; ( f ) Precipitation of secondary minerals like zeolites and clays sometimes occurs within and at the silica surface.

As described above, the IDP model entails the formation of alteration layer through the precipitation of hydrated species from the thin film of water to the hydrolysis front, with high degree of liberty to reorganise (Fig. 5 ). This leads to a sharp concentration profile of highly soluble cations and an interstitial water layer that would allow an easy separation of the altered layer from the pristine glass 48 .

Courtesy of Gin et al. 48 .

The rate of the reactions involved in the glass corrosion (ion-exchange, hydration, and dissolution) depends on factors such as glass composition, temperature, and pH. All of them may occur simultaneously during process of alteration in contact with liquid water and can be rate-limiting in function of the experimental conditions. In conditions of unsaturated humidity, studies demonstrating the existence of a rate drop followed by a residual regime are lacking. For these reasons, both the two models presented in literature (CID and IDP) are not able to describe universally the distribution of mobile ions and hydrous species inside the alteration layer.

A work published in 2017 49 reported the in-depth characterisation using atom probe tomography (ATP), transmission electron microscopy (TEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) of the alteration layer formed under close-to-saturation conditions. The results revealed the presence of an alteration layer with a more complex structure, made of three different sub-layers (Fig. 6 ): (i) close to the pristine glass, a thin hydrated layer containing all the glass components, (ii) moving towards the surface, a passivating layer with constant concentration of glass formers (Si, Al) and decreasing concentration of modifiers (Na, Ca) which is delimitated by a rough interface where alkaline and alkaline earths are preferentially leached out, and (iii) an external nanometric layer where Si undergoes hydrolysis and condensation reactions 49 . These results contradict the IDP model of glass corrosion recently designed, which highlights that many gaps are still present in the explanation of the mechanism of glass alteration.

Courtesy of Gin et al. 49 .

The authors of this review would like to emphasize that the universal application of the IDP model and other intermediate models to all silicate glass types is still an open question. This is because most recent research has focused solely on the artificial alteration of borosilicate glass, and the applicability of these models to other types of silicate glass remains uncertain. The characterisation of synthetic and geological glass altered over long time periods can be key to understand both the complex mechanisms responsible of the long-term transformation of glass network and the physicochemical features of the material transformed by the alteration. Multi analytical and high spatial and mass resolution investigation performed on surface of ancient glass provide clues about the characteristics of the altered layers and about how they reorganised their internal structure during long time periods. Instead, looking at the interface between the alteration layer and the pristine glass provides information about the mechanisms of glass alteration 2 . In this way, this approach promises to be efficient in validating the above mentioned debated kinetic models. The knowledge of the long-term behaviour of glass structure is also pivotal to predict the network dissolution in burial conditions in nuclear waste management studies 4 , 50 .

Evidence of degradation on ancient glass

Archaeological glass shows multiple and clearly visible symptoms of deterioration that can help identify well-distinguished classes of glass alteration 51 depending on the conditions it aged in (i.e., in soil, underwater, under extreme environmental conditions) for several centuries. Dulling, iridescence, weeping, pitting, discolouration and cracking of the surface affecting the specimens are common phenomena that can be observed on glass that has been recovered from archaeological excavation 52 . The formation of one or the other of them depends both on the physicochemical properties of the glass and on the environmental factors it has been exposed to. More than one of the manifestations of alteration can be found in a single glass object, thus making it sometimes difficult to identify the most appropriate strategy for conservation and/or stabilisation.

The atmospheric deterioration of glass appears markedly different from the degradation that occurs when glass objects have been buried for centuries, for example under soil, and it is rarely observed on this type of glass that has remained interred up to discovery and recovery. Crizzling , also known as glass sickness or glass disease, has been identified as the major alteration symptom for glass objects stored in museums and in private collections.

Alteration of ancient glass in archaeological stratigraphic contexts

The term dulling is used to refer to the loss of clarity and transparency typically observed in ancient glass and caused by the formation of layers of alteration products on the glass surface 52 .

As discussed above in the section dedicated to the general mechanism of glass corrosion (Section “Exploring the mechanisms of glass alteration through the different existing models”), in presence of neutral or acidic conditions, elements like alkalis are typically leached out from the first glass layers onto the surface. These leached species, reacting with humidity and moisture of the environment, tend to form corrosion products (like salts) that build up on the object’s surface and determine at first the loss of the original clarity 53 . In addition, at advanced stage of alteration, also hydrated silica (silica-gel) particles can also reprecipitate on the glass surface, leading to the formation of thicker alteration layers and causing an additional loss of glass transparency and the appearance of translucency 54 . This phenomenon is due to a combination of effects occurring between the local presence of water and the composition of glass, which determines the diffusion of ionic species from the first atomic layers under the surface to the environment and the consequent reprecipitation of hydrated silica and other alkali-derived compounds. The extent of the visual effect is much more considerable as the ion exchange proceeds and the thickness of the deposited layers on surface increases.

In more advanced stages, dulling can lead to the formation of iridescence patinas (Fig. 7 ) that may eventually detach in the form of crusts from the original glass substrate. In 1863 Brewster 55 demonstrated that this iridescent effect is due to the diffraction of incident light from layers of weathering products containing metal oxides formed after ion leaching. The rays of light are reflected from thin alternating layers of air and weathered glass crusts. These densely overlapping layers gradually penetrate deeper into the glass and they eventually change in colour towards darker hues 56 . The cationic species leached from the glass are often prone to reacting with the anionic species derived from reaction between OH - (especially in basic conditions) and atmospheric acid gases, thus forming salts with hygroscopic properties on the surface. This generates a phenomenon called weeping , which was first described by Organ in 1956. Weeping can lead to the formation of crystals or solutions of salts, depending on their deliquescence relative humidity 57 .

Images collected with Olympus BX43F optical microscope, x10 magnification. A detail of the indented rim where the overlapping of thin layers of altered patina is clearly visible (right panel).

The alteration phenomenon called pitting , is described as micro, small or large based on pit size, and it can occur simultaneously at different individual sites that later merge into interconnected complex structures producing an altered top layer, which causes the loss of glass transparency 52 .

Contrary to dulling, pitting is a visible mark of the weathering process, which occurs in alkaline solution 58 and commonly found on excavated glass. As described in Section “Exploring the mechanisms of glass alteration through the different existing models”, during the alteration process in alkaline solution, the prevailing deterioration mechanism is the dissolution of the silica network through the breaking of Si-O-Si bonds. Subsequently, the prolonged presence of a layer of moisture on the surface of glass causes an increase in the pH of the attacking solution, and ultimately pits are formed as a result of local dissolution of the silica network 52 .

A model that explains the formation of altered pits was recently developed by observing the decay process of different silicate glasses in river and marine aquatic environments 59 . The experimental results showed that the alteration of SSL glass is characterised by a two-step mechanism. The first step, called “hydration period” , is short and causes the formation of isolate fissures, while the second step, called “pit development period” , involves the creation of basic species (OH - ) during the dealkalinisation process that progressively break the silica network, thus widening the fissures to form pits. According to the results of this work 59 , the formation of pits is correlated to a dynamic loss of mass, i.e., the slow rate of the first step of hydration allows the diffusion of solution and the consequent basic attack inside the fissures, causing local network dissolution.

Figure 8 shows the surface of a Roman archaeological sample affected by pitting.

Image collected with Olympus BX43F optical microscope, x10 magnification.

Discolouration can be often seen on archaeological glass surface in combination with other types of weathering phenomena described above. It is closely related to darkening, which occurs when the oxidation of specific leached ions, such as iron, manganese, and copper, changes the colour of the weathering crusts, or to the production of hydrogen sulphide by sulphur-reducing bacteria in anaerobic environment and the formation of lead sulphide 53 . The latter case occurs only when the glass contains a high concentration of Pb oxide, and it is buried under anaerobic condition. In other cases, the presence of manganese and iron causes the darkening of glass with the formation of brownish pits 60 , as those visible in Fig. 9 . Ancient glass contains these elements in the form of impurities present in raw materials (sand and wood ash) or as a result of their deliberate addition as chromophores or/and decolourant agents in the form of minerals (i.e., pyrolusite) 61 .

In the case of archaeological glass that has been interred for centuries, cracks may be present on the surface of the glass fragments: these are caused by the shrinkage of the alkali-deficient layer due to temperature and humidity changes. Typically, these cracks are filled with mineralised material, likely originating from the soil in which it was buried. The mechanism of their formation is currently being studied since it has not yet been fully understood.

Alteration of glass in atmospheric conditions

Crizzling has been identified as the appearance of minute cracks on the surface of glass (Fig. 10 ) developing over time. These cracks penetrate deeper in the body of the object, ultimately resulting in its physical collapse. This phenomenon is due to two main factors: the unstable composition and the storage in fluctuating humidity environments 62 . Better storage conditions can slow down, but not stop the deterioration, because the role of glass composition remains a key factor in the evolution of the crizzling alteration 63 .

Object exposed at the Museo Civico di Modena (Italy). Picture by Renaud Bernadet.

Early publications reported the chemical effects causing crizzling through experiments that reproduced the condition of glass alteration in the laboratory. The results showed that crizzling is mostly associated with glass compositions characterised by high alkali and low CaO contents, and/or a high K/Ca ratio 64 .

In 1975 Brill 63 first used the term crizzled to describe glass with a decrease in its transparency due to the formation of fine cracks on the surface. He noted that certain glass objects that were in contact with water for centuries do not exhibit a high degree of degradation; however, once they are exposed to museum storage conditions (light, low RH and temperature), they display the formation of crizzling. This alteration mechanism is due to the dehydration of the glass surface, i.e., the low RH in museum display cases (15-20 %) causes a loss (up to twenty percent in weight) of the water that penetrated the gel layer of altered glass, bringing on a significant loss of volume in the gel layer itself which ultimately results in the cracking of the glass surface 62 , 63 . In general, the cracking of the hydrated gel layer formed on the surface of unstable glass can be attributed to several factors, including the dehydration of the gel layer itself - as mentioned above - the leaching process, which can lead to network contraction after the replacement of larger alkali ions (Na + ) by smaller hydrogen ions (H + ), and the different coefficient of expansion of the bulk glass and the gel layer 16 .

The guidelines of the Corning Museum of Glass 52 describes the process of crizzling indicating five stages. In the first stage (Initial stage) the glass has a blurred appearance due to the presence of leached alkali on the surface. During this phase, it is still possible to wash the surface and the glass can return to its original appearance. Conversely, in the second stage (Incipient crizzling) the haziness remains also after washing and the glass surface exhibits fine cracks like tiny silvery lines. Cracking progresses in stages three (Full-blow crizzling) and four (Advanced crizzling) until it gets to the deepest regions, leading to the loss of small fragments. Eventually, crizzling is so deep that the glass loses its structural integrity, even without any external contribution (Fragmentation stage).

Often crizzled glass has a pinkish hue. When alkaline leaching occurs and the glass structure is open, the manganese ions present in the surface cracks oxidise, yielding a pink colour 62 . This phenomenon is more evident in ancient glass which contains manganese as a decolourant.

To limit the evolution of the crizzling process, preventive conservation is an essential strategy for the safety of museum glass objects. The Corning guidelines set the optimal RH range for glass conservation between 35 to 65%, however crizzled glass or glass with a particularly fragile composition require specific individually controlled cases with a stable RH condition in the range of 40–50% inside 65 .

In addition to preventive conservation, the development of an innovative and compatible consolidation treatment for the protection or repair of cracked glass is an open challenge for the scientific community of cultural heritage conservation.

Intrinsic and extrinsic parameters influencing glass deterioration

As already mentioned, atmospheric conditions, such as temperature, the pH of the environment, salts and ions concentration, relative pressure under burial or marine conditions, and the presence of water in liquid form (RH ≥ 100%) or vapour (RH < 100%), strongly influence the kinetic of the glass surface alteration and its chemical transformation.

Many published papers of archaeological interest use the term weathering to refer to the typical degradation process that affects archaeological glass that has been exposed to particularly unfavourable environmental conditions (especially in burial and underwater contexts) 52 . Weathering is a degradation process occurring through contact with water in the environment, both in the vapour and in the liquid state, whereas the term atmospheric deterioration is used to describe glass that aged under the effect of water in the form of moisture (especially in protected environments like museum display cases), which means in the form of vapour state interaction.

The study of extrinsic factors, that are closely linked to the environmental conditions acting during the alteration process over centuries requires the development of appropriate artificial ageing protocols, which allow the modelling of the phenomenon as a function of these parameters. Real cases of glass degradation are the result of the combined actions of the intrinsic and extrinsic factors mentioned above, which generate entangled mechanisms of ionic interdiffusion from the glass network to the environment and vice versa. From an experimental point of view, studying this interconnected process of ionic interdiffusion and the formation and growth of novel phases implies the need of relying on simplified model systems to investigate the effect of specific variables to the detriment of others, which are kept constant.

The study of the effect of the various factors on glass degradation has attracted the curiosity of researchers since the beginning of the XX century. Already in 1925, G. W. Morey 66 stated that the subject was still in an empirical state, despite the considerable number of works carried out. At that time, to understand the effect of water on the alteration process many experiments were conducted by varying temperature, the pH of the environment and glass composition.

The following paragraphs describe the effect of various parameters that have a primary influence on the process of glass alteration. The studies there cited use ancient glass samples as evidence of the long-term effect of these parameters.

Effect of glass composition

Many ancient finds of SSL glass are macroscopically preserved intact in their shape, despite the physicochemical alterations caused by the burial environment over the centuries. Currently, 90% of globally manufactured glass is still based on the SSL composition, which has been kept largely consistent over the centuries except for a few modifications introduced at the beginning of the 20 th century to enhance chemical durability and resistance to devitrification 67 . Besides SSL glass, many other types of ancient glass exist, such as potash lime glass (K 2 O – CaO – SiO 2 ), lead silicate glass (PbO – SiO 2 ), or potash lead silica glass (K 2 O – PbO – SiO 2 ) 68 . The optimal preservation of certain ancient samples and the complete collapse of others is the result of a complex interplay between their intrinsic material properties and the extrinsic factors acting on them.

Intrinsically, glass physicochemical properties play a significant role in determining its degradation behaviour. Such properties typically correspond to the chemical composition of glass, the nature of its surface, the presence of impurities, inclusions, inhomogeneity, and phase separations. In particular, the concentration of silica, alkali (soda, potash), stabiliser (lime, lead), as well as the inclusions of trace elements and additives like metal oxides, added into glass as chromophores, opacifiers and decolourants all strongly affect material durability 53 . Small variations in the concentration of these components determine strong variations in glass durability. Diffusion through the leached layer is more likely to occur for smaller ions, such as Na, Mg, Li, rather than for larger ones, i.e., Ca or Ba. Although all types of alkaline silicate glass are susceptible to weathering degradation, from a thermodynamic point of view stability increases as in the following: K 2 SiO 3  < Na 2 SiO 3  < Li 2 SiO 3 38 .

Silica rich glass, such as Roman SSL glass, is more durable than poor silica glass, like medieval glass 69 . In addition, K + , which is contained in medieval glass as a monovalent cation modifier, is more susceptible to leaching out from the deeper region of the glass network during weathering alteration than bivalent cations such as Ca 2+ , which are present in Roman glass 70 , 71 . This behaviour is due to the bivalent cations forming stronger bonds with non-bridging oxygen, as previously mentioned. The preferential leaching of K + over Ca 2+ during the weathering process was also confirmed by an experimental study that investigated the weathering phenomena on naturally weathered potash-lime-silica-glass 72 . This behaviour of K + cations was also observed during leaching experiments in aqueous acidic solution.

The study of the varieties of composition of ancient glass and their resistance in RH > 100% have made it possible to identify the presence of compositions that are more chemically stable than others and to define general conditions to discriminate between stable and unstable glass 73 .

In 1978 74 , Hench systematically studied the surface of glass exposed to pure water and distinguished between 6 main types of surfaces with increasing inclination to deterioration in relation to their composition. The surface layers of the different glass types may have protective or non-protective properties for the glass substrate, depending on the capacity of reducing ion leaching and glass dissolution. In this work Hench distinguished stable and unstable glass based on the different compositions of the alteration products that form the first surface layers when glass is altered in liquid conditions. However, the protective character of hydrated layer has not been demonstrated in unsaturated humidity conditions yet; and, moreover, a work of Sessegolo et al . 75 studied medieval stained-glass windows in unsaturated conditions with isotopic water experiment and demonstrated that the alteration layer is not protective against vapour transport and interdiffusion.

A further way to discriminate a stable glass from an unstable one could be to use the ternary diagram of Fig. 11 , which was formulated in 1975 by Newton et al . 76 with a view to help predict the weathering behaviour of different types of glass. Plotting the concentration (mol. %) of network stabilisers (RO), network modifiers (R 2 O), and silica (SiO 2 ) determines the chemical stability of a given glass composition, i.e., highly durable glasses are placed near the centre. This diagnostic model may work well when binary or ternary glass is considered, however complications may occur when classifying ancient glass, which has a more complex composition, because it may be necessary to consider the combination of multiple formers (SiO 2 ad Al 2 O 3 ), monovalent oxides (R 2 O) and divalent oxides (RO).

R 2 O represent the content of network stabilisers (monovalent oxides), RO the content of network modifiers (divalent oxides), and SiO 2 the content of silica.

By using electron microprobe analysis (EMPA) and hydration-dehydration experiments on ancient glass Brill observed that a deficit of stabiliser (CaO content less than 4 wt%) combined with an excess of alkali (over 20 wt%) in SSL glass easily leads to an extensive surface deterioration known as crizzling 63 . He defined as unstable glass the one with total alkali oxide concentration over 20%; this composition range determines silica network configurations that are open enough to facilitate the migration of monovalent cations. In line with these findings, other studies reported that an increase in the CaO to SiO 2 ratio increases glass stability, while a concentration of calcium oxide over 15 wt% entails rapid glass instability 38 . Recently, a comparison in terms of glass chemical stability was made using artificial mock-ups with different glass compositions which were exposed to high humidity environment and different levels of formic acid 77 . The results showed that glass with a higher content of stabilisers exhibits greater stability, especially glass with calcium.

The presence of potash as a modifier and the low silica content in the composition make glass particularly fragile and susceptible to chemical alteration. Numerous works reported the considerable damage that affects medieval glass windows with Si-K-Ca-based composition, which is considered one of the most unstable 78 , 79 , 80 . Typical alteration marks are mainly pitting 81 or the formation of a corrosion crust 71 on the surface due to the combined attack of water and increased pollution in the air. The weathering crust is often very heterogeneous and fractured because of the wetting-drying cycle conditions and generally consists of calcite, gypsum and/or syngenite formed by the reaction between the alkaline and alkaline-earth elements released during ion-exchange and sulphur dioxide in the atmosphere 6 .

Effect of minor components in glass composition

In addition to the major elements discussed above, other minor elements may be present in the composition of ancient glass as decolorising, colorising or impurities of the raw materials used for their production. These minor elements can in turn contribute to the alteration process of the glass itself by giving rise to distinctive corrosion marks.

Several authors have remarked that manganese that accumulated between the altered layers may have originated from the burial environment 82 , 83 . Dark deposits have been detected inside the dealkalinisation layer on the surface of Roman glass samples 84 . Secondary electron images have uncovered that these deposits are formed from the interconnection of spherical particles of about 2 µm in size, with iron and manganese oxides as their major components. These studies clarify that during the leaching process Fe (II) and Mn (II) ions are hydrated and oxidised, giving rise to the formation of dark amorphous products that precipitate into the pores of the leached silica film.

It is well-known that glass technologists used to add Mn, as well as Sb, to the glass melt as a decolorising agent, but it is also known that, in this type of glass, the appearance of brownish areas on the altered surface is due to an oxidation process of Mn(II) to higher oxidation states. It is generally observed that in the dark areas Mn is present in +IV oxidation state 85 , 86 . Nevertheless, a recent work performed using synchrotron radiation X-ray absorption spectroscopy (XAS) analysis on historical stained-glass windows demonstrated that the most extended brown altered areas mainly contain Mn mainly under a +III oxidation state 87 .

Through the analysis of 14th–17th century window glass, Schalm et al. 82 concluded that the formation of Mn-rich inclusions takes place simultaneously with the growing of leached layers settling along their interfaces and their concentration is mainly caused by the environment (soil) in which the glass was buried for several centuries.

Although brown/black staining has mostly been attributed to manganese compounds, iron (and titanium) compounds were often reported in association with the manganese 53 . Other elements which are often present in historical glass network as impurities can improve the chemical stability of glass. For example, low amounts of ZrO 2 (2 %wt) increase the acid and alkaline durability of glass 88 , and a hydrated ZrO 2 surface can act as a barrier to further dissolution of other ionic species. More recently, it was demonstrated that the substituting an insoluble oxides like zirconia to a fraction of silica slows the glass dissolution kinetics, but prevents the alteration gel reorganisation inhibiting the pore closure mechanisms and leading to greater degree of corrosion 89 .

These results show that it is essential to know the exact composition of an ancient object to determine a range of the aging behaviour that it is prone to. To do that, it is important to also consider the presence of elements in lower or trace concentrations to fully describe the corrosion phenomenon and formulate models that are closer to real cases of ancient glass alteration.

Effect of liquid vs atmospheric water

Water was observed to be the primary environmental ageing factor that causes glass deterioration by Lavoisier already in the early 1770s. Lavoisier also indicated two different mechanisms of water penetration into the glass network, i.e., through network voids between oxygen atoms in molecular form, and by hydrolysis and condensation reactions with the metal-oxygen bonds 19 .

The alteration of silicate glass differs when it occurs in liquid-phase or in vapour-phase regime. In contact with water the glass surface undergoes chemical attack through ion-interdiffusion and hydrolysis of metal-oxygen bonds 6 . In vapour conditions (RH < 100%) the hydration process does not release elements into the fluid, but it involves a redistribution of elements in the alteration layer (expect for volatised elements) 90 . The formed hydrated layer has chemical composition and porosity that are different from those obtained in liquid conditions, thus resulting in the glass durability properties specific for liquid or vapour-phase regimes. Even if the molecular process is the same, the interplay between the intrinsic and extrinsic variable changes affecting the macroscopic transformation of the material 45 .

During the phenomenon of water diffusion, no matter in which RH condition, the glass surface is covered by different monolayers of water, depending on the water partial pressure in the atmosphere. As soon as the silica network of a freshly made glass encounters moisture from the environment the chemisorption of water molecules starts from the first layers of the surface, which is consequentially more subjected to physical and chemical transformation. Water is chemisorbed on the glass surface and a thin film of water of few molecular layers builds up onto the chemisorbed layer, depending on RH condition.

One of the first characterisations of altered glass surface was conducted by Hench in 1975 56 , using a combination of analytical techniques that enables to obtain physicochemical information from different depths of the altered glass surface. He systematically studied the glass surface altered in liquid water, using techniques like Auger electron spectroscopy (AES), infrared reflection spectroscopy (IRRS), and electron microprobe (EMP), and distinguished 6 main types of surfaces with increasing inclination to deterioration (Table 1 ). Of these, only Type I is stable and shows no main differences with bulk glass. The other types are considered unstable and prone to deterioration over time. Type II and type IV are of interest for historical deterioration. Type II is formed when the glass contains a high level of network formers, and it involves the formation of a silica-rich layer on the surface that acts as a protective film, preventing the leaching of alkali ions and the rupture of the glass network (Si-O-Si). On the other hand, no protective film is formed for Type IV and so the leaching of alkali can proceed. This latter situation is typical of unstable glass.

According to Hench’s classification, ancient glass has a surface that corresponds to Type II and Type IV surfaces, which are characterised respectively by the presence of silica-rich surface protective layers (when the concentration of network formers is high enough) and the presence of non-protective surface layers that allow the alkali leaching to proceed (when the soda to silica ratio is high enough) 56 .

In the case of discontinuous contact with aqueous media, which is typical of the atmospheric alteration, the formation of laminated structure on the altered glass surface has been explained by the intermittent water supply 6 . Through the observation of ancient glass surface by TEM, a sequence of bands of different thickness has been recently distinguished 91 , i.e., thinner bands called lamellae (20-50 nm) and thicker ones called laminations (0.1-4 µm), which consist in groups of laminae with the same orientation. The amorphous laminae, which are depleted of alkaline ions, are formed from the local rearrangement of glass elements resulting from the repetition of several cycles of interdiffusion and glass dissolution processes. Moreover, cracks are developed perpendicularly and parallelly to the laminae and laminations. Based on this study, two main processes take place within the cracks, i.e., the migration of atmospheric solutions deeper into the bulk glass, thus moving the alteration front further, and the precipitation of secondary mineral phases which favours the mechanic separation and the loss of glass fragments.

In 2016, a study of the altered layers observed in ancient glass proposed a model to explain the formation process of this laminated degradation 92 . Thanks to optical microscope, field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDX) analyses, it was possible to describe surface lamellae as an alternation of random amorphous silica nanoparticles with different packing densities and with thickness between 0.1 and 10 µm. Moreover, the growth of nanosized silica particles on the surface of the altered glass was observed to be one of the by-products of the leaching process of glass components that occurs under alkaline conditions.

While glass alteration in contact with water in a liquid state was widely studied during the 1900s, testing approaches in unsaturated atmosphere have been seldom explored until more recently 93 . In 2020, Majerus et al. 7 published an overview of the experimental protocols for glass alteration tests in unsaturated atmosphere. All the tests reported were conducted under RH < 100% condition and, when a saline solution was present, their configuration ensured that the samples were never in contact with it. These results obtained under RH < 100% suggested that the glass alteration mechanism was different from the one obtained in contact with liquid water (RH > 100%). In contrast to saturated humidity conditions, in unsaturated environments, the hierarchical order of glass alteration processes involves network hydrolysis first and interdiffusion second, particularly for glass with compositions classified as unstable.

Alloteau et al. recently demonstrated that in water vapour conditions, the process of glass hydration is independent from dealkalinisation: at higher temperature (80 °C) hydrolysis predominates over diffusion processes and solvation, whilst at lower temperature the two processes proceed together in parallel. To state that, they performed ageing test in static conditions (no T and RH cycling) at controlled temperature and humidity (RH 85%) on three different types of unstable historical glass (soda-lime silicate from antiquity, mixed alkali silicate from middle-age/renaissance and potassium silicate from XVI-XVIII century). The conditions were set to avoid any liquid water flow on the samples during tests.

Sessegolo et al. 75 evaluated the respective contribution of rain, wet periods and unsaturated humidity on the kinetics of formation of the alteration layer of potash-lime-silica glass. The results demonstrated that the fluctuation of dry and wet periods (non-static condition) leads to the formation of an altered layer rich of cracks, pits, and scales, which constitute a network of pores and fractures behaving as a major vector for interdiffusion of liquid and vapour water. The characterisation of this fracture network is particularly important in case of prolonged alteration in atmospheric conditions, where liquid and vapour water phases are systematically combined in the associated diffusion processes. In another subsequent work 94 the authors proposed a model to estimate the thickness of alteration of stained glass windows subjected to liquid water (such as rain), comparing the expected value of alteration thickness extrapolated linearly over 650 year with those observed on ancient stained glass. The values predicted by the developed model match well with the alteration layer thickness observed on real ancient medieval samples 6 , 95 , 96 .

Effect of saline and burial environments

In the last two decades, several works have been done to explore the effect of ageing in burial or marine environments on the alteration mechanisms of glass and on the formation of new alteration phenomena. A considerable number of works focused on the prediction of the durability of high-level nuclear waste glass and the migration of radioactive and non-radioactive elements into the burial soil 9 , 97 , 98 .

Verney-Carron et al. 5 studied ancient Roman glass blocks naturally aged in seawater obtaining the kinetic parameters to develop a geochemical model to simulate the alteration of archaeological glass. Comparing the results obtained from the characterisation of ancient glass with those from the computer simulation, this work demonstrated the importance of archaeological glass for validating the predictive capacity of geochemical long-term model, thus bridging the gap between the results obtained from short-term experiments and long-term alteration of complex systems.

Palomar devoted many studies to weathering and to the comprehension of the environmental effect on the stages of glass alteration 59 , 99 , 100 . In particular, she reported the effect of coastal atmosphere on glass degradation 101 , which is a scarcely investigated subject. The alteration of glass surface exposed to coastal environment is mainly caused by the high presence of liquid water that covers the glass surface, thus inducing a hydrolytic attack and the dealkalisation process, and by the high wind speed, which favours the transportation and deposition of sodium and chlorine ions on the glass surface. Marine aerosol in elevated concentrations represents a hazardous agent for the chemical stability of glass, since its action could increase the hygroscopicity of the glass surface and open the glass structure, allowing the alteration to proceed deeper into the glass.

Another visible mark of degradation observed on the surface of glass altered in saline environment is the formation of peculiar chemical ring-like patterns that have been explained according to the Liesegang theory (Fig. 12 ). Diffusion, reaction, nucleation and crystal growth are all phenomena that have been used to formulate models that explain the Liesegang rings formation 102 , 103 . Dal Bianco et al . observed these weathering rings present on glass fragments from the Roman ship Iulia Felix found on the Grado lagoon, in North-East Italy, and dated back to the 2 nd century AD 104 , 105 . Their characterisation study showed a maximum diameter of the rings of about 1 mm and the interconnection of interface lines during the simultaneous growth of adjacent rings. This study did not report an exhaustive theory about the formation process of rings, but the authors observed that the structure was similar to the descriptions of Liesegang kinetic evolution of precipitates in gel found in other research works 106 , 107 . This assumption is acceptable since hardly corroded glass structure can be assumed as a gel where weakly soluble salts periodically precipitate due to the reaction between two soluble substances, one of which is dissolved in the gel medium. The final appearance of the precipitates depends on their solubility and on the initial concentration ratio of the reagents, but generally their aspect is concentric around the centre in which precipitation starts 108 .

Image collected with an Olympus BX43F optical microscope, x10 magnification. Interconnection between adjacent rings can be observed.

Nowadays, the formation of this type of rings on archaeological glass has been observed on the surface of samples recovered in submarine environment 104 , 109 . The authors of this review suggest that a saline-rich environment, whether seawater or wet soil, play a role in the formation of the rings. The soluble substances present in these environments may react with soluble substances from the aged glass forming salt precipitates on the surface of glass that act as centre of nucleation for the growth of the concentric structure.

The mechanisms that generate the formation of rings on altered glass surface have not been analytically confirmed, and the study of the evolution of the kinetics that controls ring growth on archaeological glass remains incomplete, despite the availability of modern techniques. Although the layers observed and experimentally described by Schalm 110 can be compared to the bands structure obtained from Liesegang kinetics, there is a lack of systematic experimental work in the literature to confirm whether this theory can be applied to the dissolution-precipitation mechanisms of glass.

Chemical studies on the composition of the soil where archaeological glass aged for centuries might validate the hypothesis of ionic exchange between the elements of the soil and the elements of the glass network, providing the opportunity to understand how the interaction with the external environment can drive the process of alteration in different types of glass.

Many published research studies mention that also for glass samples that altered buried in soil the most common pathology observed is the formation of dealkalinisation layers 111 , 112 , 113 . One of these performed a stratigraphical analysis using a non-invasive technique (laser induced breakdown spectroscopy, LIBS) to observe the progressive dealkalinisation of glass bulk composition, reporting an increase in the calcium and sodium intensity signals on the glass surface 84 .

In the field of ancient glass, experimental studies simulating ageing in soil are less common 114 , 115 , 116 . In a recent work, Palomar et al . tried to replicate ageing in soil in a natural burial environment to understand the corrosion mechanisms acting on different ancient glass types (Roman, medieval, lead crystal glass, common window glass) 117 . The burial tests were set up to last 300 days and were carried out at 60 °C, to accelerate the alteration processes. The pH of the burial soil and the glass composition showed to have a key role in controlling the reactions between the constituent glass elements and those from the environment.

When considering burial artificial alteration of mixed alkali glass compositions with low silica concentration, acidic or neutral conditions (pH 6.5 – 7.5) lead to the formation of micro pits and cracks; an iridescent and translucent layer is formed, instead, on the surface of glass under alkaline soil condition (8.0 – 9.0) 118 . Silica-soda-lime glass shows a different behaviour. In fact, in acidic soil the formation of both isolated and interconnected fissures may be observed. However, under neutral and alkaline soil conditions an increase in the number and depth of pits, whose rate of accretion depends on the content of alkaline oxides in the glass, can be detected, with a considerable increase in the diffusion of surface degradation under the alkaline condition 117 . This experimental evidence is a clear example of the necessity to consider glass composition and environmental factors simultaneously when approaching the understanding of the glass corrosion mechanism.

Effect of the environmental pH

A pivotal factor in determining the rate of glass corrosion is the pH of the attacking solution both in case of vapour and liquid conditions 10 . Under conditions of low pH (acidic solution), the deterioration mechanism predominantly involves the ion-exchange process due to the abundance of hydronium ions in solution and the formation of silanol groups [Si-OH], generating a hydrated gel on the surface which slows down degradation 117 . Differently, at high pH, the interaction between the glass surface and the alkaline solution leads to the dissolution of the silica network through the rupture of the Si-O-Si bonds, which implicates a more aggressive condition.

In works performed in 2016 92 and 2021 110 , Schalm et al. observed that glass surface is transformed in both acid and alkaline condition, but different morphologies can be developed depending on the pH of the solution. At pH<7 the transformed glass has a homogeneous morphology determined by the dissolution of leached silicate network and precipitation of dissolved compounds as amorphous silica forming linear and randomly branched chains of alteration product that can be described as a silica gel. At pH between 7 and 10 of the local solution the transformation process leads to the formation of silica nanoparticles packed into alternating density lamellae. The authors successfully reproduced the lamination in laboratory experiment performed around pH 10, where silica solubility is highly variable and induces the cyclical dissolution-precipitation of silica. The mechanism of formation of consecutive lamellae with alternating packing density is proposed to be dependent on the pH oscillation in the local solution during the alteration process. In extreme alkaline condition (pH>10) the fast dissolution of silica network occurs without any material precipitation.

A recent work 94 highlighted how the pH-dependency is linked to the specific composition of glass and consequently to the solubility of different glass constituents. The results show that the pH-dependency of Ca-Mg-silicates highly differs from the behaviour of aluminosilicates, and that Si-K-Ca medieval glasses have a very low pH dependency at alkaline conditions.

Effect of pollutants

Air pollution was identified as a particularly dangerous agent that enables to speed up and enhance alteration processes. In the museum context, the presence of carboxylate acids pollutants is generally the main cause of glass corrosion and of the formation on the glass surface of efflorescence salts as deterioration products 119 . With regards to historical stained glass, an experimental work carried out over a six-year period to quantify the influence of various air pollutants from different local environments (Europe and North America) on the degradation of potash-lime-silica glass, which has a similar composition to that of medieval stained glass, showed the formation, after exposure to rain and solar radiation, of crystalline carbon-rich products unlike those of samples aged under sheltered conditions 120 .

Melcher and Steiner 120 performed a 6-year experiment to evaluate the effect of acid gas and pollutant on replicas of stained glass, comparing the leaching depth formed on potash-lime-silica glass replicas to former leaching studies performed on medieval stained glass. This work demonstrated that the hypothetic leaching depth cannot be directly related to pollution data because, while the leaching depth increases with time, the leaching rate decreases. Even a paper of Robinet et al . 30 highlighted the role of the organic pollutants formic acid, acetic acid and formaldehyde in the alteration of unstable soda silicate glass. He confirmed that museum wooden cabinets emitting organic pollutants must be avoided since they foster the progressive alteration with an estimated rate of ~2 nm/day.

Advancements in monitoring ancient glass conservation state

The kinetics of glass corrosion, as well as the sequence of events involved and the prevalence of one interconnected mechanism over another, are dependent on both the chemical composition and structure of the glassy material and the environmental conditions to which it is exposed. These conditions include the amount of water that reacts, the chemistry and pH of the solution, and the duration of exposure. Given the complexity of these factors, the alteration of glass is a complex phenomenon that requires careful study. Therefore, particular attention should be paid to the analysis of ancient glass samples that have undergone modifications over time scales that cannot be replicated in the laboratory. These samples provide valuable insights into the long-term alteration mechanisms of glass and can aid in the development of more accurate predictive models for understanding the complex behaviour of glass materials in various environmental conditions.

The characterisation of altered ancient glass provides concrete evidence of the transformation of the vitreous structure, the nature of the dissolution products, and the way in which a specific glass composition reacts to a particular environment 3 , 4 , 7 , 70 , 121 , 122 . Over the course of history, the evolution of glass manufacturing has led to the production of various types of glass, such as Roman SSL glass, medieval stained glass, or Venetian crystal glass ( Cristallo ). Artefacts (and their fragments) made from all of these types of glass inherited from the past represent an exceptional opportunity to better understand the processes involved in glass corrosion on a long-term scale. This is due to their specific chemical stability, resulting from their unique chemical composition, and their aging in soil, underwater, or confined spaces with specific microclimates for centuries.

The analysis of glass surfaces is currently considered the most effective scientific approach for studying chemical and physical variations that occur at the interface between the atmosphere and the glass surface. Advanced surface analysis techniques, such as X-ray photoelectron spectroscopy (XPS) and ToF and/or dynamic SIMS, provide high-resolution data that enable the investigation of the chemical composition of the glass surface’s first nanometres and facilitate monitoring of its modification during the alteration process 67 , 123 . SIMS is widely considered one of the most suitable techniques for studying corroded glass 124 , due to its ability to detect hydrogen and investigate the glass surface up to a depth of a few microns. Another technique commonly reported in the literature for studying the weathered surface of ancient glass is laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which allows for the acquisition of 2D and 3D elemental distribution with spatial resolution as high as the spot size used for the analysis. Only one work, published in 2013 125 , presents the procedure to obtain elemental maps to investigate surface layer phenomena on pitted ancient glass. The results of this application showed that the dealkalinisation of glass sample occurred on surface, as the direct or indirect consequence of the hydration process, resulting in the final formation of pits and of the so-called Liesegang rings on the surface of the sample. Despite the novelty of this research, the spot size of the laser beam used in this work (diameter of 80 µm) is higher than the average size of the corrosion marks present on ancient samples. The spatial resolution on the reconstructed maps can be improved by using smaller spot size of the laser beam (down to 10 µm) making it possible to obtain a resolution suitable for appreciating the chemical variability of extremely heterogeneous samples. This can potentially increase the understanding of the mechanisms of formation of these altered phases.

Critically speaking, the extensive study of glass surface composition using these advanced analytical techniques is still seldom considered when investigating the corrosion mechanism of glass both on ancient samples and through an artificial ageing approach in the laboratory 30 , 126 , 127 . Each surface technique has its own peculiar applications and limitations 128 , but a combination of different techniques allows a complementary and more comprehensive characterisation of corroded glass.

Collecting morphological, structural, and chemical information through a multi-scale analytical approach is necessary to gain a comprehensive understanding of the evolution and kinetics of glass alteration, including the modification of glass composition. Well-designed experiments are crucial due to the co-operation and mutual influence of different mechanisms involved in glass corrosion. This approach allows for investigation from a macroscopic to a nanometre scale observation. The information gathered can help determine how and when to intervene for conservation and preservation of glass objects, whether they are of industrial or historical significance.

The evaluation of storage parameters such as relative humidity, temperature, and light, as well as the condition of the glass, are crucial for preventing degradation of glass objects in museum collections or other sites of cultural interest. This approach is particularly important for understanding the influence of intrinsic and extrinsic factors in the processes of alteration and finding the best conditions for conservation. Surveys 129 , 130 , 131 conducted in various museums in Europe have shown that a significant number of glass objects are in critical condition due to extensive degradation, highlighting the need for preventive actions. For example, at the Victoria and Albert Museum, out of 6,500 glass vessels optically investigated, more than 400 showed clear signs of glass deterioration 54 . Similarly, the glass storage conditions of the Royal Palace of Madrid and the Technological Museum of Glass (Segovia) were evaluated during the period from September 2019 to November 2020, detecting a high concentration of formic acid in the display cabinets and wardrobes, mainly due to the presence of wood, which results in a higher glass surface hygroscopicity (pH=8) 132 .

Several methods have been used to categorise unstable glass from museum collections based on its appearance 65 or by analytical techniques –like X-ray fluorescence (XRF) 133 , ion beam techniques (particle -induced X-ray emission, PIXE, and particle-induced gamma ray emission, PIGE) 134 or spectroscopic techniques (Raman and Fourier-transform infrared, FTIR) 135 , 136 – in order to determine glass composition. Using these techniques to analyse cultural heritage objects can be complex due to many operative limitations, as for instance the need of carrying out micro-sampling (which is often not possible) or moving the objects to the specialised laboratories where the instruments are hosted. With the aim to distinguish stable from unstable glass in museum collection, the ideal analytical technique should provide the chemical composition of the sample, be highly sensitive, and have a very fast time of analysis to characterise as many samples as possible in a short period of time, while operating in a non-invasive way. Consequently, it can be safely stated that no scientific and straightforward approach for understanding the chemical nature and composition of unstable glass in a non-invasive way and for large museum collections of glass objects has been developed yet.

In the light of the above, it is evident that the most effective approach to study glass degradation involves the integration of two main distinct but interconnected methods:

the evaluation of the surface of ancient glass artefacts;

the evaluation of intrinsic and extrinsic causes for glass alteration.

Advanced analytical techniques can be used to characterize ancient glass surfaces and observe the products of the long-term transformation of the glass structure resulting from the alteration process. In parallel, laboratory-based aging experiments can be used to evaluate intrinsic and extrinsic causes of glass alteration, distinguishing the effects of different parameters and identifying the most relevant factors that influence glass alteration kinetics. Parameters such as the content of alkalis and stabilizers in the glass composition, as well as temperature and humidity, strongly impact the chemical durability of glass by affecting the concentration of alkalis and hydroxyl or non-bridging oxygens in the hydrated layer. It is difficult to determine which factor has the most significant influence on the process of glass alteration because the final degradation symptoms are the result of a mutual effect between all the aforementioned.

As evidenced, over the years many experiments aimed at understanding the phenomenon have been reported in literature and almost as many methods have been proposed. This abundance of results represents an outstanding opportunity for future research studies in this field, which will be able to rely on advanced analytical approaches and more accessible high-resolution techniques even for the domain of cultural heritage science. The results obtained from the study of ancient materials are pivotal to validate the long-term capacity of kinetic models -which are based on experimental data only - and to strengthen current theories of glass corrosion.

Moreover, comparing the characteristics of artificial replicas with those of ancient glass enables a better understanding of the glass corrosion phenomena, that in turn can underpin both the formulation of new protective solutions to preserve and protect glass artefacts in the long-term and the design of technologies exploiting glass properties in a variety of applications.

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The authors would like to express their gratitude to the anonymous reviewers: their insightful comments and critiques helped to significantly improve the quality and clarity of this paper. The authors are deeply grateful for their time, effort, and dedication.

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Zanini, R., Franceschin, G., Cattaruzza, E. et al. A review of glass corrosion: the unique contribution of studying ancient glass to validate glass alteration models. npj Mater Degrad 7 , 38 (2023). https://doi.org/10.1038/s41529-023-00355-4

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BENEFITS OF PALLIATIVE CARE IN ADULTS WITH A DIAGNOSIS OF HEART FAILURE: AN EXPLORATORY LITERATURE REVIEW

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Introduction: Heart Failure is a clinical syndrome characterized by a series of symptoms such as dyspnea, orthopnea and edema in the lower limbs. This pathology continues to have a high prevalence despite advances in pharmacotherapy and device therapy and given that it is a pathology that significantly impairs the quality of life of patients, the implementation of care is of vital importance. However, these are underused due to lack of knowledge on the part of health personnel and also due to poor implementation in the different health providers. Objective: An exploratory review of the literature was carried out regarding the benefits of palliative care in patients with advanced heart failure, in order to synthesize the available and updated evidence. Methodology: Searched for articles published from 2017 to 2022 related to palliative care in patients with heart failure and using the PRISMA 2020 methodology for this study. This inquiry of articles was carried out in the following databases: UpToDate, PubMed, MESH, PMC (US National Library of Medicine National Institutes of health). Results: A total of 5 articles were obtained, from which they concluded that palliative care has a positive impact on the quality of life of patients with heart failure, there was a lower rate of hospital readmissions, improvements in physical, psychological and existential.

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