THE INFLUENCE OF CONSERVATION TREATMENTS AND ENVIRONMENTAL STORAGE FACTORS ON CORROSION OF COPPER ALLOYS IN THE ANCIENT ATHENIAN AGORA
ALICE BOCCIA PATERAKIS
During a survey of the copper alloy objects excavated from the North Slope of the Acropolis in the 1930s, a blue corrosion was noted on many of them (Hall 1995). The blue material was often found on top of a dark brown corrosion layer and, in many cases, was intimately associated with a white crystalline corrosion. The blue corrosion is mossy in appearance. Since the survey, a more extensive investigation has been carried out on the cataloged copper alloy collections housed in the Agora, which consist of approximately 72,000 objects. The North Slope objects, which constitute 3.5% of that number, show blue corrosion on 20% of the coins and 40% of other objects. Significantly less than 20% of copper objects with this corrosion are observed in the other Agora collections.
Blue corrosion on copper alloy objects from the North Slope was tentatively identified as a compound based on copper acetate by x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis (Paterakis 1998). The XRD patterns of five samples from five copper alloy objects were published for comparative purposes. In 1999 further analysis of the blue, the white, and the dark brown corrosion was carried out by XRD, FTIR, ion chromatography (IC), and scanning electron microscopy with energy dispersive x-ray micro-analysis (SEM-EDS). The blue corrosion was identified as a sodium copper carbonate acetate [NaCu(CO3)(CH3COO)], the white as sodium acetate trihydrate (CH3COONa.3H2O), and the dark brown as a copper oxide (cuprite, Cu2O) and tin oxide (cassiterite, SnO2) mixture. Blue corrosion on one object was identified as copper (II) hydroxide and shall be referred to as turquoise blue throughout this paper to distinguish it from the blue acetate compound (Paterakis 1999).
Two sodium copper carbonate acetate compounds and sodium acetate trihydrate have been identified in the British Museum on Egyptian copper alloys stored in wooden cupboards since the 1930s (Thickett 1998; Thickett et al. 1998; Thickett and Odlyha 2000). A comparison of the corrosion present on these copper alloys with those in the Agora, which also have been kept in wooden storage cases since the 1930s, is consistent with similar published observations suggesting that the buildup of acetic (ethanoic) acid is a major contributor to the formation of the acetate corrosion products (Tennent and Baird 1992; Tennent et al. 1993). The Agora storage cases are constructed of softwood and thin plywood. In the Agora collections the blue corrosion has been found solely on chemically treated objects, whereas in the British Museum it was found also on objects that retained their original corrosion products.
The purpose of this article is to present in some detail the results from the recent campaign of corrosion analysis on the Agora collection and to explore the role that conservation methods and materials, as well as environmental storage conditions, may have played in the formation of this corrosion. To this end, the collection was surveyed for corrosion and conservation coatings, a few coatings were selected for analysis by FTIR, and the preliminary results of FTIR analysis are presented.