Identification of corrosion products on a medieval copper-silver coin

This work relates a study on a medieval coin found during archaeological works carried out at the site of Senhora do Castelo in Urros (Torre de Moncorvo, Portugal). The artefact has been identified as a coin of D. Afonso IX of Leon (1155-1188). The substratum showed composition typical of bullion, Cu-Ag (30-33%) with minor elements (1% Pb + 0.2% Sn + 0.4% Zn). The corrosion products developed on the coin during the archaeological burial was studied by means of optical microscopy (OM), scanning electron microscopy combined with energy dispersive spectrometry (SEM-EDS), energy dispersive X-ray fluorescence (EDXRF), and X-ray diffraction (XRD). The results show the presence of cuprite, atacamite, chalcocite, native silver, silver oxide and silver sulphide on the outer layers of the coin, which allows deducing the importance of the environment in the corrosion phenomena.