How permanent are mountants? An overview of the conservation state of the G. Deflandre microscope slides collection

The French National Museum of Natural History (MNHN, Paris) hosts Georges Deflandre’s valuable collection, assembled from 1920, and composed of about 13,000 microscope slides. The present condition report of the collection was performed through a detailed inventory of 885 slides and highlighted the mounting techniques used by Deflandre (medium, sealant, dyes). First, the recipes were documented using the available literature. Then, an inventory was undertaken using a spreadsheet with pre-defined entries, designed to facilitate analysis of collected data. Well-documented labels and some chemical analyses were used to fill in the fields. More than 50 combinations of dyes, 19 mounting media and three sealants were used. Regarding the media condition, attention was focused on three major points affecting the observation of the specimens and the long-term conservation of the slides: the color, the mechanical degradation, and the presence of air. Most widespread media in Deflandre’s collection are Canada balsam, coumarone resin and kumadax, all exhibiting yellow or orange tints due to oxidation, but not impacting the condition of specimens. Glycerin jelly was the least stable mounting media with evidence of darkening and shrinking. In general, no relationship between yellowing and mechanical degradation was noticed, nor was a link between the macroscopic sealant degradation and the drying of the mounting medium. Cross-referencing information between the literature and the collection allowed a better understanding of the mounting practices. Additionally, observing these 50–100 year old slides was one of the best ways to assess the durability of mounting media through time.