A multi-analytical approach for the characterization of ancient Roman coins in orichalcum

A selected number of Roman orichalcum coins, from private collections, have been studied. Numismatic analysis indicates that (Crawford, 1974; Sutherland, 1984) they are asses, sestertius and dupondium, minted by Julius Caesar, Augustus and Claudius. The aim of this study was to disclose the chemical composition of the orichalcum alloy, the nature of the patina and corrosion products; orichalcum is an ancient copper based alloy with a variable percentage of zinc (Craddock, 1978). With this aim a multi-analytical approach was involved, i.e. X-ray fluorescence (XRF), scanning electron microscope (SEM-EDS), electron micro probe analyser (EMPA). In particular, XRF analysis was performed on different spots of coins’ surfaces to have information about the chemical composition of the external layers. SEM-EDS analysis allowed to investigate alloy’s microtexture (e.g. segregation micro-domains) and to obtain morphological information. In addition, X-ray maps, acquired across section of coins, give information about elemental distribution inside the alloy; whereas EMPA analysis permits a quantitative chemical composition of major, minor and trace elements composing this ancient alloy. All these techniques confirmed that copper and zinc are the main components of the coins. Other metals, i.e. Fe, Pb, Sn, Co, Ni, As, Sn, Sb and Bi, were also found in the alloy. These elements can provide information about sourcing raw materials sites. Si, S, Cl, K and Ca were found on the surfaces, being contaminant from the soil. Quantitative analysis with microprobe permitted to evaluate the different percentage of Cu and Zn in all the samples. The use of invasive technique (EMPA) was necessary to quantify the abundances of each elements composing the original alloy, thus eliminating soil contaminants and alteration product of the layers developed through time on surfaces. These results, could contribute to fill the lack of knowledge about the orichalcum alloy