Investigations on alloy-burial environment interaction of archaeological bronze coins

This study is focused on ten Roman leaded coins from the archaeological site of the Magna Mater Temple (Rome, Italy). These coins, buried for centuries in a peculiar environment of the archaeological area (i.e., the latrinae), were investigated to explore the chemical composition and microstructure of the inner alloy. The opportunity to examine the core of the alloy permitted to disclose information about the original composition as well as to infer the interaction and the modification of the metal core during burial condition over centuries. Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and Electron Microprobe Analysis (EMPA) techniques have been employed to study each sample from rim to inner un-corroded core. The results showed that the coins are a Cu-Sn based alloy enriched in Pb. The content of Cu in the Cu-rich area (i.e., the α-phase) is ca 99 wt% and that of the Pb islands (i.e. the β-phase) reaches 97 wt%. The X-ray maps of the interface between the subexternal layer/inner-core revealed the occurrence of high chloride content, in which the distribution of Cl and Pb indicated the presence of lead chloride minerals. SEM-imaging highlighted also a deep corrosion of the patina with cracks and pits. Underneath the corrosion layer, an area of the almost unaltered β-phase composing the bulk material of the coin has been detected.