The deterioration of Mollusca collections: identification of shell efflorescence

Shells from several Mollusca collections have been examined. Efflorescence—white or grey crystalline salts on the exterior or, to a lesser extent, the interior of the shells—is a widespread problem and occurs in many shell species, both land and sea shells. The efflorescence has been identified by means of several techniques: X-ray diffraction (XRD), infrared (IR) spectroscopy, thermogravimetric analysis (TGA) and nuclear magnetic resonance (NMR) spectroscopy. The efflorescence products consist of one or a mixture of Ca(CH3COO)2 ½H2O, Ca(CH3COO)2H2O and Ca(CH3COO)(CHOO)H2O. When this mixed acetateformate salt is dissolved from the shell and recrystallized, the product is almost invariably 3Ca(CH3COO)2 4Ca(CHOO)25H2O. Synthetic preparations of these novel calcium acetate-formate double salts aided in their identification. The nature and circumstances of the formation of efflorescence indicate acetic and formic acid vapours, liberated by wooden storage cabinets, primarily oak, as the cause of deterioration. For certain specimens, coatings applied to the shells prior to storage have given long-term protection. Other forms of shell deterioration are described briefly and some methods of conservation and preservation are discussed.