Characterization of two Roman coins from an archaeological site in Portugal

Abstract

This paper reports a study carried out on two Roman coins found in the archaeological site of São Pedro, in Fronteira (Alentejo, Portugal). One coin was in a quite well preserved state, allowing its reading and historical identification, while the other was quite deteriorated and its reading difficult. The coins have been identified as: an Antoninianus of Galilenus (267-268 A.D.) and an AE 3 (337-340 A.D.) in which is impossible to identify the emperor. Energy dispersive spectroscopy (EDS) has given the following average composition: 42%C + 30%O + 12%Cu + 1%Ag + 2%Sn + 4%Pb + 6%Si and 17%C + 31%O + 5%Cu + 7%Pb + 27%Fe + 5%Si, for the outer layers of the Antoninianus and of the AE 3, respectively. The exact substrate composition of the AE 3 could be determined: 84%Cu + 10%Pb + 2%Ag + 2%Sn + 2%Fe. The chemical composition found for the AE 3 is consistent with historical investigations, since silver-plated Cu-Pb-Sn alloys was the material used in the production of the AE 3. X-ray powder diffraction spectroscopy (XRD) has identified as possible crystalline compounds: a mixture of two silicates, namely the lead al uminium silicate, Pb4Al4Si3O16, and the lead copper silicate, Pb8Cu(Si2O7)3, pyromorphite, Pb5(PO4)3Cl, on the Antoninianus and only oxides, namely Cu2O, and iron oxides, Fe2O 3 and/Fe3O4, on the AE 3 outer layer. The chemical composition of the corrosion products is related with the soil composition, a sandy neutral soil, rich in P, Mn, Si, K, Fe and organic matter (9%). The hydrogeological context plays also its role on the formation of the corrosion products. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA,.