Dana’s talk – the last of the Objects session talks given at AIC’s 42nd Annual Conference in San Francisco – presented the results of multiple tests conducted by the National Park Service (NPS) Intermountain Region Museum Services Program, evaluating several readily available materials for their ability to protect sensitive objects from less-than-ideal climate conditions. Tests sought to identify the best methods for long-term storage of two distinctly different categories of collections materials: archaeological metals and historic plastics. The ideal solutions would be cost effective and easy-to-use, would allow for easy monitoring and access, and could be consistently applied across all NPS sites.
Archaeological Metals
Various storage systems for archaeological metals have been employed at NPS sites in the past, including Stewart boxes, 2-4 mil polyethylene (PE) bags with twist ties, and heat-sealed Marvelseal enclosures in combination with desiccants and/or scavengers.
Following the work done by JP Brown (2010) and Alice Paterakis (2011), Dana did some short experiments to confirm that resealable PE containers with silicone gaskets held a microclimate better than similar containers without gasketing. This being established, she added data loggers and twice the calculated amount of desiccant recommended for the container’s volume, and conducted a longer test in three locations – Arizona, where storage conditions were generally dry (about 35% RH); Montana, where conditions ranged from 25-45% RH; and Texas, where conditions fluctuated around 50% RH. Even under the most humid conditions in TX, the worst-performing silicone-gasketed PE boxes only allowed an increase of 2% RH over the course of the year-long test period. Measurement of the TX test box after a second year registered only another 2.7% increase in RH. Based on this set of experiments, Dana calculates that this particular setup would only require recharging with desiccant every 5 years if a change of less than 15% RH was desired. In addition, Oddy testing of the materials involved in the system confirmed that there was nothing harmful being off-gassed.
Historic Plastics
To find an ideal solution for historic plastics, Dana started by consulting Yvonne Shashoua’s 2008 publication, Conservation of Plastics. She learned that different polymers have wildly varying requirements for safe storage: cellulose nitrate (CN) and cellulose acetate (CA) need ventilated or scavenged environments to slow deterioration, while natural rubber fares better in anoxic environments, and polyvinyl chloride (PVC) calls for only non-absorbent glass or Mylar fabrication materials. The deep trays with Tyvek covers that had been in use at NPS since the 90s were not effective, so alternatives were sought.
For use with CN and CA, several box designs were evaluated for their ability to ventilate the space and prevent dust accumulation inside the box. Acid-free board boxes were made with slatted or screened walls and were tested without lids, with Hollytex lids, or with acid-free board lids. Inside the prototype boxes, Dana placed a deteriorating CA shower curtain ring along with a sticky surface to determine how much dust found its way in, and A-D Strips to monitor for the buildup of harmful acetic acid vapors. Findings supported the use of a blue-board lid, as it prevented the most dust over a month-long period. Though screened and slatted boxes did equally well, the screened boxes were found to be easier to construct. No harmful buildup was detected by the A-D strips in any of the boxes.
As a space-saving alternative to ventilated storage, Dana next evaluated several common scavengers in conjunction with the previously described resealable PE containers. In order of their performance (worst to best), they were Kodak Molecular Sieves, a single MicroChamber board, Getter Pak, two MicroChamber boards, and Zorflex. The most effective adsorbent, Zorflex, prevented acetic acid build-up in the container for 18 days. Interestingly, each scavenger tested lowered the humidity in the box to some extent initially, though it did eventually rebound. A second round of tests that doubled the amount of scavenger found no benefit to doing so. Future work will include evaluating activated carbon cloth, and rerunning the above described scavenger tests on boxes containing a larger amount of deteriorating CA to see if the products can keep up with increased volumes of off-gassed acetic acid. She would also like to test resealable glass containers in this capacity.
Although storage for PVC objects was not extensively discussed, Dana mentioned that she has been using and is pleased with heat-sealed 1 mil Mylar enclosures. She prefers the use of a single-impulse heat sealer to a double-impulse model to make the enclosures.
Finally, Dana made a point to address a few limitations of the systems she evaluated for this paper, including size (the largest silicone-gasketed PE containers she’s been able to find only measure 9 x 12 x 6 inches) and the need for a robust monitoring/maintenance plan.
Questions/comments after the talk:
- One talk attendee mentioned that she had found much larger gasketed PE boxes, but that they didn’t necessarily seal well because of the distance between the clamps, especially on the long edges. She recommended testing the container with water – if it’s not watertight, it won’t be airtight either!
- A question was posed about the efficacy of these systems compared to non-gasketed PE boxes that were sealed with aluminum tape instead. Dana stated that aluminum tape was not evaluated in this round of testing.
- Dana, though she didn’t mention brand names, cautioned that some brands of containers definitely held a better seal than others. Test your enclosures first!!!
I really enjoy hearing about research with practical applications. Thank you, Dana, for a well presented and interesting talk!!