Characterisation of five coins from the archaeological heritage of Portugal

This paper reports a study carried out on three Roman and two Portuguese coins found in the archaeological site of São Pedro, in Fronteira (Alentejo, Portugal). The three Roman coins have been identified as a Follis, an AE 2 and an AE 3 (bronze alloys), while the Portuguese coins have been identified as a Ceitil (copper) and the "6 vinténs" (Ag-Cu alloy). Scanning electron microscopy (SEM) coupled to energy dispersive spectroscopy (EDS) has allowed the semi-quantitative determination of the elemental composition of both the corrosion products and the alloy used in the manufacture of the coins. The crystalline corrosion products constituents of the patinas were identified by X-ray powder diffraction spectroscopy (XRD). The more deteriorated coin, the AE 3, was submitted to electrochemical studies in Na ₂SO ₄ aqueous solution and the corresponding data analysed. After 3 weeks, the E _OCP was still quite stable, ranging between -0.050 and -0.070 V vs. SCE; the corrosion resistance, R _p, was of the order of 5 to 3×10 ³ Ω. Chemical treatment of the sample by 1-h immersion in 0.1 M NaOH produced a more active surface, with R _p showing a decrease of a factor of about ten. On the other hand, it was concluded from voltammetric data that polarisations of E _a≥0.050 V vs. SCE led to copper oxidation, with no reduction of the other corrosion products.