Characterization of two Roman coins from an archaeological site in Portugal

摘要

This paper reports a study carried out on two Roman coins found in the archaeological site of Sao 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 aluminium silicate, Pb_4Al_4Si_3O_(16), and the lead copper silicate, Pb_8Cu(Si_2O_7)_3, pyromorphite, Pb_5(PO_4)_3Cl, on the Antoninianus and only oxides, namely Cu_2O, and iron oxides, Fe_2O_3 and/Fe_3O_4, 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.