Hi Martha,
As the acting C2C Care community monitor, I'd like to thank you for posting this question to the community!
Acid-free tissue or inert, thin foam sheeting are other options for protecting and supporting vertically stored plaques. Additionally, both materials could be used to fill out negative space and prevent shifting of plaques within the box. Recommendations for products suitable for storage may be found in the CCI's
Technical Bulletin 32 - Products Used in Preventive Conservation. Compiled Oddy Test results may be found at
Materials Testing Results - Case Constriction Materials.
While searching for other recommendations, I came across a 2000 posting to the Archives & Archivists listserv
http://www.ibiblio.org/archives-archivists/msg08987.html about storing plaques, which offers similar recommendations.
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Sharon Fickeissen
Connecting to Collections Care Monitor
Senior Library Conservation Technician
Hagley Museum & Library
Wilmington DE
sfickeissen@hagley.org------------------------------