The variety of calcium-bearing efflorescence phases – an explanation by crystal chemistry

Abstract

Cultural heritage objects are affected by various corrosion processes during decades and centuries of storage in museums and collections. Atmospheric gases like CO2, moisture or -as wood emits significant amounts of formic and acetic acid 1 -the storage furniture itself can induce corrosion. Calcareous heritage objects like historic Mollusca shells 2 , eggs 3 , ancient pottery (Figure 1, a) or marble reliefs (Figure 1, c, d) are very sensitive to acetic and formic acid vapours. The corrosion process leads to the formation of efflorescence crystals sometimes crystallizing in pores and cracks, which can cause severe damage to the artifacts. This phenomenon has been known as "Byne's disease" since the end of the 19 th century. 4 Both, simple salts like Ca(CH3COO)2•H2O 5 or Ca(CH3COO)2•½H2O 6 and complex compounds like calclacite (Ca(CH3COO)Cl•5H2O) 7 or thecotrichite (Ca3(CH3COO)3Cl(NO3)2•6H2O) [8][9] were found as corrosion phases on calcareous historic objects. Many of these efflorescence phases, however, still remain poorly characterized due to their microcrystalline character and the occurrence of polyphase mixtures.