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Tuesday, May 30 • 2:00pm - 2:30pm
(Collection Care) Unhappy couples: degradation of microscope slides due to their mounting media

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Natural history collections are known to have used diverse forms of coatings due to the varied nature of their specimens. In the case of micro-scale organisms and histologic samples, organic coatings have been employed to mount specimens to observe them under microscopes since the first half of the nineteenth century. Over time, various materials have been used to improve either the stability of the mount or the clarity of the specimens for observation. Consequently, there are numerous formulas for every substance employed in microscope preparations. Some of these materials have begun to degrade, leading to the loss of very important specimens. In some cases, the salvage of degrading specimens involves remounting the specimen. This poses problems because many of these mounting media are of unknown composition, making it difficult to choose a solvent that would remove it without damaging the specimen. A major survey is underway at the Smithsonian's National Museum of Natural History (NMNH) to identify mounting media that have held up well over time, or media that are degrading in order to determine the most appropriate treatment options. Meetings with the collections managers and curators have determined the collections to be surveyed within the museum. During examination, important features like crystallization of the mounting medium and the level of degradation are being documented. To date, over 300,000 slides have been surveyed from the Botany, Amphibians and Reptiles, Fishes, Birds, and Entomology collections. Of the total, less than 20% show signs of degradation. However, these numbers do not reflect the real conservation issues. From the nearly 28,000 slides observed in the Botany collection, only 3% show deterioration of the mounting medium, if yellowing of Canada Balsam is not considered, given that it does not affect observation under a microscope. Of the over 96,000 slides examined in Amphibians and Reptiles, around 6% show signs of deterioration. Most of these deteriorating slides are uncatalogued, meaning that the majority of the cataloged specimens show no deterioration. In the Fish collection, of the close to 27,000 slides reviewed, around 65% have issues. Some of the slides that are in perfect condition are a result of remounting efforts, meaning that they had badly deteriorated, suggesting that the actual numbers are even higher. The collection with the least deterioration is Birds, with only 9 crystallized slides out of 3,244. The Entomology collection has been partially surveyed. Of the over 139,000 slides observed, only 7% show deterioration. The most problematic mounting media for all the collections so far have been paraffin, Permount and Hoyer's. In addition to observation of the slides, the mounting medium preferred in the Botany Department, Thermo Scientific™ Shandon™ Synthetic Mountant was tested at the Smithsonian Museum Conservation Institute using ATR-FTIR. It provided a very good match to a copolymer of methyl methacrylate and butyl acrylate and showed additional peaks that may be attributed to a butyl phthalate. This mountant is extremely similar to Acryloid/Paraloid B-48, which would explain why after 40 years, the slides mounted with it show no deterioration.

avatar for Mariana Di Giacomo

Mariana Di Giacomo

Conservation Fellow, Smithsonian National Museum of Natural History
Mariana Di Giacomo is a paleontologist with special interest in fossil preservation. She graduated in 2012 with a Master in Zoology from the PEDECIBA at Universidad de la República, Montevideo, Uruguay. In 2006 and 2007 she received tutoring from fossil preparators at the Museo de... Read More →

Tuesday May 30, 2017 2:00pm - 2:30pm CDT
Plaza Ballroom Lobby Level, East Tower