The Influence of Conservation Treatments and Environmental Storage Factors on Corrosion of Copper Alloys in the Ancient Athenian Agora

Abstract

The Agora collection of copper alloys consists of mirrors, pins and needles, lamps, vases, tools, nails and other hardware, official weights, voting ballots, sculpture, jewelry, medical instruments, weapons, and coins excavated on the North Slope of the Acropolis and in the Ancient Agora of Athens, and is housed in the Stoa of Attalos on-site. Blue, turquoise blue, dark brown, and white corrosion products on these copper alloys have been identified as a sodium copper carbonate actetate, copper (II) hydroxide, copper and tin oxide, and sodium acetate trihydrate, respectively. The analysis of the corrosion was carried out by x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive x-ray microanalysis, and ion chromatography analysis. Conservation materials, as well as storage and environmental conditions, were found to play a role in the development of these corrosion products. Most of the objects have been chemically cleaned to various degrees. The role of cleaning and stabilizing agents containing sodium in creating the corrosion is considered. The contributing roles of acetic acid emissions from wooden storage materials and relative humidity are examined. Preliminary analysis of conservation coatings by FTIR was carried out to determine the effects of such coatings on the condition of the copper alloy collection. Solubilities of coatings used as lacquers and consolidants on the artifacts are reported. The coatings identified by FTIR were hydrocarbon waxes and cellulose nitrate. Observations regarding the conditions necessary for the development of corrosion are made. Recommendations for conservation and storage to prevent new corrosion or to alleviate further corrosion are given.