Smoking is the process of subliming and depositing ammonium chloride or other white powder onto specimens, and is useful for enhancing specimen relief for photography. Ammonium chloride is acidic and highly soluble in water, and can etch delicate specimens in the presence of moisture. Though many methods exist for applying ammonium chloride to specimens, removing the coating is rarely discussed. To amend this, we performed an experiment smoking a series of invertebrate fossil specimens and cleaned them using eight different cleaning techniques. After undergoing the appropriate cleaning method, each specimen was then thoroughly rinsed in deionized water. Using a silver nitrate solution, which precipitates silver chloride in the presence of chloride ions, we tested the rinse water for remaining chloride contamination. Using this procedure, we found complete rinsing of the specimen to be the only method for removing contamination to a point below our detection limit, although various brushing techniques were moderately effective. Breathing on the specimen, a commonly used method, was ineffective, and likely exacerbates the problem of etching by dissolving remaining residue. We recommend a case-by-case approach to ammonium chloride residue removal, using one or more techniques, while making sure to record the smoking and cleaning procedures in your collection’s database.

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Author notes

Associate Editor.—Janet Waddington