An unusual efflorescence on Greek ceramics

Salt efflorescences were found on four Greek ceramics on exhibition in cherrywood display cases at the Metropolitan Museum of Art. These salts were thought to be either a common archaeological salt, such as halite, or calclacite—a salt which results from the interaction of acetic acid vapors produced by the cherrywood with calcium and chloride ions already present in pores of the ceramic. X-ray diffraction, infrared spectroscopy and elemental analyses determined that one of the salts was, in fact, calclacite (CH3CO2CaCl·5H2O) while the other three were a relatively uncommon salt, ammonium carnallite (NH4MgC13·6H2O). It is suggested that this salt results from a two-step cleaning procedure which at times in the past has been employed for ceramics: hydrochloric acid followed by ammonia.