Squares of colored leather.

Leather Bindings: Conservation Research on Tanning

By Laura McNulty, Holly Herro and Kristi Wright ~

Have you noticed that books bound before the mid-19th century are in better condition than those bound after the mid-19th century? Seemingly simple questions have an interesting way of developing into larger, more complex questions. To answer a question like, “What do you think causes dyes to fade on leather bindings?” you must tease it out into a cascade of specific, testable questions about dyes, tanning processes, leather degradation, and time.

A leather sample in liquid in a glass beaker.
A Shrinkage Test on a Leather Sample

In 2016, the Leather Discussion Group formed to address these questions with the ultimate goal of determining the best products available to suit book conservation needs and to communicate the requirements to leather manufacturers. Currently there are four members of this group, two here at the National Library of Medicine (NLM): Holly Herro, Senior Conservator and Kristi Wright, contract book conservator; plus Katie Wagner, Book Conservator at the Smithsonian Libraries and William Minter, Senior Book Conservator at the Pennsylvania State University Libraries.  Laura McNulty, now in the conservation program at Winthertur, also contributed to the team’s goals while volunteering at NLM in preparation for her graduate studies.   We present regular updates at the American Institute for Conservation’s Annual Meeting, including in 2018 in Houston as part of a symposium titled, “The Current Use of Leather in Book Conservation” and a poster titled “A Group Effort to Understand the Material Properties of all Leathers Both Old and New” at the 2019 annual meeting in Connecticut.

A pie chart of qualities conservators look for in leather.
Qualities Conservators Look for in Leather

The initial focus of the group included both information gathering and laboratory testing. To gather information, we scoured published sources on both historic and modern leather tanning materials and processes. We also did a survey of book conservators and binders to discover what qualities they look for when buying leather and the techniques they use in their work. Questionnaires sent to tanners focused on the materials sourced for the tanning process. This survey gave us insight into how leather is created and used today. We received a variety of responses to the question, “What are the qualities you look for in leather.” As seen in this pie chart, based on current data most responders look for color, grain, thickness, and workability when choosing leather for projects; we are continuing to collect responses. Ongoing conversations with leather chemists, tanners, and conservators in other disciplines continues to provide valuable feedback. Additionally, we have recently established a relationship with leather researchers in the European Union to expand our study of leather and its aging properties.

Photogaphs of old and new leather in many colors.
Samples Collected by the Leather Discussion Group

Another source of data for the project comes from experiments designed to test the properties of variously treated leather products. In preparation for testing, we gathered samples of leather, both historic and modern. The initial tests we performed on the samples were to determine protein identification and proteomic analysis on skins produced before and after the Industrial Revolution. Potentially, this data could be used to find a correlation between the long-term stability of the leather and the materials used during the tanning process. We are replicating historically used tests—such as the Printing Industry Research Association (PIRA) test—to evaluate whether or not they still have potential for conservators and binders to use.

Squares of leather marked control (normal) and Sample (browned around the edges).
A PIRA Experiment on a Leather Sample

We’ve got a lot of other experiments designed: To evaluate storage environments; To determine the impact of treatment materials and methods; To understand the influence of mechanized tanning processes; To examine dye permanence and its effect on leather’s longevity; and an evaluation and identification of any associated health concerns when using dyes, consolidants, and other leather-related treatment materials.

These experiments will improve our knowledge about and ability to care for and preserve leather bound materials at the National Library of Medicine.

Read publications on conservation topics by Holly Herro and Kristi Wright in the National Library of Medicine’s free full-text archive of biomedical and life sciences journal literature PubMed Central.

Laura McNulty is a Graduate Fellow in the Class of 2021 Winterthur / University of Delaware Program in Art Conservation.

Holly Herro is Senior Conservator for the History of Medicine Division at the National Library of Medicine.

Kristi Wright is a contract conservator for the Conservation Program of the History of Medicine Division at the National Library of Medicine and principal of Wright Conservation and Framing.


  1. This is a fascinating topic and one of more than passing concern to me as I have worked with many 19th century books covered in red-rot. I would love to hear more about your research on the tanning methods back then.

    P.S. I noticed and odd word on your pie chart: “denisty”. Would this be “density”?

    1. Thanks so much for your comment. And for pointing out the typo. I’ll let the author know so she can make the correction.

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