Study of dezincification in orichalcum Roman coins

A group of orichalcum Roman coins, from private collections, have been studied for this research. Numismatic examination indicated that they are asses, sestertium and dupondius, minted from Julius Caesar to Nero. Orichalcum is an ancient copper based alloy with a variable percentage of zinc. The aim of this study was to investigate the process of dezincification from the external layers to the core of the samples. Furthermore, the research was aimed to disclose the real chemical composition of the orichalcum alloy. In scientific literature is reported that the percentage of zinc in the alloy is related to the age in which the coin was minted. The samples were firstly analysed by means of X-ray fluorescence spectroscopy to obtain information about the elemental composition (qualitative method) of the external layers. This non-invasive analysis allowed to discriminate coins made with zinc alloy from the bronze ones. Because of the degradation of the external layers of orichalcum and the dezincification; process, other analysis were also carried out on cross section of some zinc based samples, such as scanning electron microscope (SEM-EDS) and electron micro probe analyser (EMPA). The first technique was useful to investigate the depth of corrosion and dezincification processes and to examine alloy’s microtexture (e.g. segregation micro-domains); the second one permitted a quantitative chemical analysis of major, minor and trace elements composing this ancient alloy, understanding the difference of chemical compositions between the unaltered core and the altered layers. All the techniques performed both on the cross sections and on the external layers of the coins revealed that other metals, i.e. Fe, Sn, Pb, As, were also present in the alloy. Finally, results were chronologically ordered and compared with literature’s data.