Roman orichalcum coins: a deep investigation from patina to core

A set of Roman orichalcum coins, issued from 88 B.C. to 96 A.D., has been investigated using a multi-analytical approach.The aim of the study is to explore the corrosion processes of this alloy, with a special attention to the dezincification and decuprification phenomena, from the external layers to the unaltered core of the samples. Indeed, XRF, VIMP methodology and ATR-FTIR were used to obtain information about the elemental composition (qualitative method) of the external layers and to characterize both the patina and the corrosion pattern of the surfaces, allowing also the analysis of samples that could not be sacrificed. SEM-EDS and FIB-FESEM-EDX analysis allowed to investigate the corrosion micro-texture (e.g. segregation micro-domains) in depth and the unaltered metal core of the coins (Doménech-Carbó et al., 2018). In addition, cross-section analyses at high magnification showed cold-working evidences, i.e. deformed grains and strain lines.Quantitative data of major, minor and trace elements were obtained by means of EMP analysis. Moreover, differences in chemical compositions between the unaltered core and the altered layers of the samples were highlighted. All these techniques revealed the presence of other metal, besides Cu and Zn as the typical elements of orichalcum. Indeed, Tin, Fe, Pb and As are present at different concentrations (from 0.01 to 2.30 % wt). Finally, the interpretation of voltammetric data permitted to group coins, that belong to different monetary emissions, and the discrimination of the issues emitted by different monetary authorities (Di Turo et al., 2017). Di Turo F., Montoya N., Piquero-Cilla J., De Vito C., Coletti F., Favero G. & Doménech-Carbó A. (2017) - Archaeometric analysis of Roman bronze coins from the Magna Mater temple using solid-state voltammetry and electrochemical impedance spectroscopy. Analytica Chimica Acta, 955, 36–47. Doménech-Carbó M.T., Di Turo F., Montoya N., Catalli F., Doménech-Carbó A. & De Vito C. (2018) - FIB-FESEM and EMPA results on Antoninianus silver coins for manufacturing and corrosion processes. Scientific Reports, 8, 10676