Microstructural and electrochemical characterization of Roman mirrors

Ancient mirrors, made of copper alloys with a high tin amount, up to 24%, are metallic artefacts of noticeable archaeological interest for different reasons: i) for the study of the alloy characteristics, as brittleness, several mirrors crack during the excavation itself; ii) for the study of the complex production techniques; iii) for the study of the deposition techniques of Sn, Au and Ag carried out for increasing the brightness degree; iiii) for the study of the corrosion mechanism and consequently of the selection of the proper restoration and protection methodology. Microstructural and microchemical characterization have been performed by means of OM (optical microscopy), SEM+EDS (scanning electron microscopy + energy dispersive spectrometry) and GDOES (glow discharge optical emission spectroscopy), together with electrochemical tests (impedance measurements, polarization curves), on several fragments of roman mirrors coming from Phoenician-Punic Sardinian excavation sites enhancing, at the end of the study, the common features of these objects, as the alloy composition and the typology of corrosion products. The shiny sides of the mirrors, which have a patina 20-100 mm thick, are generally attached by the aggressive agents (soil, atmosphere) to a lower extent with respect to the dull one, generally covered by wide isles of corrosion products. Both the patina and the corrosion products show a noticeable enrichment in Sn. Phosphorus, carbon and silicon have also been detected, thus indicating a deep interaction between the buried artefacts and the soil.