Study of long-term corrosion layers grown on high-tin leaded bronzes by means of the combined use of GDOES and SEM+EDS

Glow discharge optical emission spectrometry (GDOES) and scanning electron microscopy + energy-dispersive spectrometry (SEM + EDS) are used for studying the microchemistry of the long-term corrosion products, i.e. the patina, on archaeological high-tin leaded bronzes used by Romans, most likely as mirrors. The results show that via GDOES it is possible to obtain reliable and detailed elemental composition-depth profiles for the corrosion products of the patina and for the bulk chemical composition of the bronzes that are in good agreement with the microchemical features obtained via SEM + EDS. The results provide good insight into the corrosion layers, showing copper and lead selective corrosion and depletion phenomena leading to the formation of a complex microchemical structure and, further, showing clearly the interaction between the inorganic and organic constituents of the soil, such as iron, silicon and phosphorus, in contact with the corrosion products grown on the bronzes. Finally, the results show that GDOES, with its ability of routine and rapid analysis of layers of thickness up to 150 μm, in combination with SEM + EDS, has significant potential in studies of the corrosion products for the restoration and conservation of ancient high-tin leaded bronzes.