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Recently Published: A Final Report on Heat- and Solvent-Set Repair Tissues

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This post was authored by Katherine Kelly. Katherine is a Senior Conservator at the Library of Congress where she works primarily on repairing and rebinding books from the Geography and Map Division and the Music Division.

In December of 2016, a group of conservators and conservation scientists from the Library of Congress (LC) and the National Archives and Records Administration (NARA) gathered to discuss challenges that both institutions were experiencing with heat- and solvent-set repair tissues. From these discussions emerged a research project to investigate different adhesives and methods, with the aim of identifying some well-tested and successful options. The final report of this research was recently published by the Journal of the American Institute for Conservation.

Pictured from left to right. Top row: Lauren Varga, Andrew Davis, Katharine Morrison Danzis, Yasmeen Khan. Middle row: Michele H. Youket, Steven Loew, Anne Witty, Jennifer K. Herrmann, Tamara Ohanyan, Alisha Chipman. Front Row: Katherine Kelly. Photographer: Leah Gibbs.

Precoated heat- and solvent-set tissues have been used for decades to mend library and archival materials. Because they can be used without introducing moisture, they are useful for mending tracing paper, brittle newspaper, mold-damaged paper, and books and papers with water-sensitive media. A variety of adhesives have been used over the years, but the LC-NARA research team had begun to question the quality of some of the adhesive mixtures currently in use.

LC and NARA both have robust digitization programs that rely on the use of heat- and solvent-set tissues by conservators to quickly stabilize a vast amount of collection materials in preparation for scanning. As a result, conservators at both institutions have broad experience with making, using, and evaluating repair materials. In addition, scientists at NARA and LC regularly conduct quality assurance testing and materials analysis. By bringing together this scientific and conservation expertise from two large federal institutions, this project was able to examine a wide range of materials from many angles.

Before and after treatment images showing how precoated mending tissues can be used to mend fragile originals (1920s newspaper from sample collection).

The team selected eleven adhesive mixtures that were either commonly in use for this type of work or which matched the same criteria for preparation, use, and apparent effectiveness. Each mixture underwent a variety of tests, including FTIR, glass transition temperature, cross sectional imaging, the Photographic Activity Test, color change after artificial aging, reversibility after artificial aging, and blocking during natural aging.

Of the eleven mixtures, six passed the testing criteria. The tissues prepared with these adhesive mixtures can be used to mend tears in a wide variety of papers, are translucent enough to read text through, and can be easily prepared and used. The testing predicts that they will not yellow or become brittle over time or cause chemical damage to the paper underneath. The published research paper gives instructions for how to prepare and use all of the successfully tested mixtures.

Some of the adhesive mixtures did not meet the testing criteria, and these failures are an equally important result of the research project. Some testing revealed the need to balance physical properties of different adhesives – mending tissues needed to stick well over time, but be easily removable when desired. Success or failure depended on the proportions of different adhesives in the mixture. Interestingly, the method of application – heat or solvent – did not affect aging or testing results.

Other tests revealed adhesives that did not perform consistently, and called into question previous test results that had supported their use. In 2013, a certain adhesive mixture was artificially aged and did not change color. In 2018, that same mixture was artificially aged and became unacceptably yellow and brittle. Chemical analyses revealed that this may have been due to formulation changes in the commercial product over the years that passed since previous testing. Conservators need to be able to rely on the effectiveness and stability of the materials that they use, and not all commercially available adhesives meet this standard. These results highlight the importance of technical reference datasets and sample collections maintained by scientists at NARA and LC.

Testing revealed that this adhesive mixture became yellow and brittle over time. As a result, it is no longer used by the Library to mend collection materials.

It is important to share both positive and negative results. Most libraries and archives are not able to perform technical research and quality assurance testing, and they rely on labs such as ours to share results. The research team presented their findings at a national conference, disseminated practical instructions in online forums, and submitted the paper for peer-review. The publication, which would normally only be available through subscription, is accessible to all, thanks to generous open access funding from the two agencies.

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