Microstructure, chemistry and corrosion products of copper and copper alloys of artefacts from Pyrgi (Italy) through a multi-analytical approach

In this study, several archaeological artifacts, i.e., arrowheads, nails, and aes rude, made of copper or copper alloys were examined (Esposito et al., 2019). They were found at the Etruscan site of Pyrgi (Santa Severa, Italy), dating back to the 6th to 4th century B.C. We explored the microstructure and composition of each sample, considering its relationship to the environmental storage, and the possible corrosion phenomena triggered on its surface. Such phenomena can produce irregular multilayer structures (patina) and possible selective enrichment or depletion processes of alloying elements. A multi-analytical approach, based on Scanning Electron Microscope, X-ray maps, and µ-Raman spectroscopy was used. The chemical composition of nails and aes rude is pure copper, while arrowheads are made of a ternary Cu-Sn-Pb alloy. In addition, it was possible to distinguish artifacts produced by simple casting from those that underwent additional processing. The µ-Raman spectra of the surface of the artifacts indicated the occurrence of cuprite, malachite, and copper trihydroxy-chlorides, indicating the occurrence of type I and type II patinas (Robbiola et al., 1998). The presence of chlorine even within the cores in some samples indicates a very corrosive environment. The chemical and microstructural information allowed hypotheses on the production and metallurgical processes of the artifacts and their application. The choice of using copper nails was quite unusual, but as reported in the literature (Dilo et al., 2010; Pomey et al., 2013), it could be due to their use in ship carpentry. As for the arrowheads, the data suggest that they are votive objects as their composition and structure would make them not particularly suitable as weapons. The aes rude, almost all composed of pure copper, represent one of the earliest forms of coinage. They are semi-finished products, rich in inclusions due to the process of smelting raw materials. These data provide valuable information on the processing techniques and materials used by the Etruscans, contributing to the understanding of their technological capabilities and conservation processes of artifacts found in a coastal archaeological context such as Pyrgi. Dilo T. et al. (2010) - Archaeometallurgical characterization of some ancient copper and bronze artifacts from Albania. AIP Conference Proceedings, 1203, 985-990, https://doi.org/10.1063/1.3322596. Esposito A. et al. (2019) - The production of metal artefacts in Southern Etruria (Central Italy): Case studies from copper to Iron Age. Sci. Technol. Archaeol. Res., 5.2, 64-84, https://doi.org/10.1080/20548923.2019.1660496. Pomey P. et al. (2013) - On the transition from Shell to Skeleton. Int. J. Naut. Archaeol., 42(2), 434-438. Robbiola L. et al. (1998) - Morphology and mechanisms of formation of natural patinas on archaeological Cu–Sn alloys. Corros. Sci., 40(12), 2083-2111, https://doi.org/10.1016/S0010-938X(98)00096-1.