Salt efflorescence on pottery in the Athenian Agora: A closer look

Abstract

Salt contamination in the ceramic collection of the Athenian Agora, American School of Classical Studies, Greece, was confronted for the first time by the fulltime conservation staff that was appointed in 1979. A review is presented of the examination and analysis of the salt contaminants for their identification and of simulation tests to determine their source(s) and mode of formation since 1990. In the analytical campaigns of 1992 and 1994, calclacite (Ca(CH3COO)Cl·5H2O) (ICCD 12-0869) and calcium acetate hemihydrate (Ca(CH3COO)2·0.5H2O) (ICCD 19-0199) were identified with X-ray diffraction and polarized light microscopy and examined with the environmental scanning electron microscope. The various means with which to mitigate the contamination and the crystallization-deliquescence cycles, e.g. consolidation, desalination, and climate control, were also studied. The article focuses on the processes involved in the formation of acetate salt compounds with the publication of the first solubility and deliquescence diagrams of the Ca(CH3COO)2-CaCl2-H2O system at 25°C. Comparisons are drawn with recent work carried out on thecotrichite (Ca3(CH3COO)3Cl(NO3)2·7H2O). The course of action adopted by the conservation staff of the Athenian Agora to combat efflorescence has been termination of cleaning with hydrochloric acid and desalination as a remedial measure in lieu of the costly replacement of wooden cabinets with enameled steel storage cases for the extensive ceramic collection (52,000 catalogued ceramics). Volatile acetic acid concentrations in wooden storage cases in the collection have been found to range from 1039 ± 20 to 1267 ± 20 μg/m3 (400 to 500 ppb) at 25°C. Since 1979, all newly excavated ceramics and all ceramics exhibiting efflorescence in the collection have been routinely desalinated. No efflorescence has formed on the desalinated objects. Various methods of mitigation such as controlling the RH are also discussed.