Protecting the Past: A study of Acetic Acid Offgassing
By Laura McNulty
Last week, Laura McNulty, NLM Pathways Student and Kristi Davenport, NLM contract conservator, shared their research findings at a meeting of The Washington Conservation Guild at the S. Dillon Ripley Center, Smithsonian Institution in a presentation titled “Materials and enclosure testing prior to rehousing a Nobel Laureate’s research”.
This summer I worked on a project in support of a national treasure held by the National Library of Medicine: the working copy of Marshall Nirenberg’s Genetic Chart, which is considered by some scientists to be the “Rosetta Stone” of modern science. The conservation staff of the History of Medicine Division recently met with Dr. Nirenberg’s former laboratory technician of forty years to discuss the history and significance of the Charts, but when they opened the box containing the chart they smelled acetic acid. This vinegar-like smell was cause for concern, because it is an indicator that the environment in the box may be harmful to the collection item.
Proper protective housing is the first line of defense in the preservation and sustainability of library and archive collections. For significant holdings, and especially those which are considered to be historical treasures, conservation staff sometimes conducts scientific testing to determine the best materials to use when creating a custom storage container. It is important that the materials be chemically stable because any chemicals released by the materials will be trapped in the container and may affect the collection item.
While attending the American Institute for Conservation Annual meeting in 2013 my supervisor Holly Herro, Conservation Librarian at NLM, discussed acetic acid offgassing in storage containers with Allison Brewer, an undergraduate intern at the University of Kansas, whose poster addressed the possibility of acidic offgassing in boards and adhesives. Following up on Ms. Brewer’s research I had the opportunity to investigate, with NLM staff, to try to determine whether the box currently housing the Nirenberg Genetic Chart is chemically stable. The Nirenberg chart is stored in a clamshell style box constructed in the early 1990s. Based on past written conservation documentation I determined that bookcloth, library binding board (known as Davey board), and the synthetic adhesive polyvinyl acetate (PVA- Jade 403) were used in the construction.
To test the chemical stability of each of these materials separately, I carefully sealed samples of each in individual pouches of Marvel Seal, a barrier film that resists the passage of vapor and gas, along with an A-D strip which is an indicator for atmospheric acetic acid (a.k.a. “vinegar syndrome” because of its smell). Similar to a litmus strip, A-D indicator strips are dye coated and turn from the original blue color to blue-green, green, yellow-green, and finally to bright yellow depending on the level of acid exposure. As a control, I also made a pouch containing only an A-D Strip to confirm that the pouch itself was inert. Then I placed the pouches in different temperature environments (73F, 64F, 60F, and 40F).
Recommended exposure periods for testing start at around four days to ensure adequate time for any color change to occur. None of the samples showed change within the four-day exposure period, so I resealed the pouches after inserting a fresh AD strip and left them in the same environments for two weeks. After two weeks, the AD strips remained blue for the board and cloth samples; however, the strips with the samples of the adhesive PVA-R shifted in the warmer environments showing evidence of off-gassing above certain temperatures. Additionally, I smelled a vinegar odor when the pouches containing these samples were opened.
I also tested other existing clamshell enclosures housing NLM collections. I placed A-D strips in boxes made in the 1950s using davey board and PVA Jade 403. Additionally, I placed a strip in a box constructed in 2001 using davey board and PVA-R. The A-D strips in the 1950s box turned green after four days all of the tested temperatures, but the color on the strip in the 2001 box remained unchanged regardless of the temperature, demonstrating that the PVA-R does not emit acetic acid to the degree that the PVA Jade 403 does.
The experiment showed that PVA Jade 403 in particular is not a good material for use in housing historical collections because the acid vapors released create an unacceptable microenvironment inside storage boxes and will contribute to the deterioration of collection items over time.
Today, countless library and archive collections are stored in protective enclosures of various shapes, sizes, and materials. As the enclosures constructed today naturally age, are they going to be chemically stable? The promise of answering this question lies ahead as the Nirenberg chart receives the evaluation, stabilization, and preservation it deserves as an important piece of our medical heritage from the early years of the genomic era.
Laura McNulty is a recent graduate of the University of Massachusetts Amherst. She was a summer Pathways Student in 2013. She is currently working as a conservation intern at the American Philosophical Society in Philadelphia, PA.