Characterization of two Roman coins from an archaeological site in Portugal

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 aluminium 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, Fe₂O₃ 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.