Technological evolution in ancient Etruria: The production of a tri-metallic complex artefact and corrosion model

The production technology and the effects of long-term corrosion of an Etruscan trimetallic sauroter were explored. This artefact, unearthed in the archaeological site of Pyrgi (Italy), dates to the 5th-4th century BCE. The structure-configuration consists of Fe, Pb, and bronze shells, involving galvanic interfaces among different metals and producing complex corrosion dynamics, further intensified by microstructural manufacturing defects and severe coastal burial conditions. Optical microscopy, SEM-EDS with multilayer elemental mapping, and μ-Raman spectroscopy were employed to characterise the chemical composition, microstructural features, and corrosion products. The results indicated that variations in environmental conditions coupled with galvanic reactions and crevice effects led to partial dissolution of the metallic shells and the formation of corrosion products. Atacamite, cuprite, and tenorite occur in the outer shell bronze surface, while at the interface between the bronze and the Pb shells, atacamite, cuprite, and cerussite/hydrocerussite are present. Laurionite is predominant on the interface between the Fe core and the Pb intermediate shell (IS/IC); finally, the Fe core (IC) is completely corroded, composed of magnetite and goethite. The microstructure and chemical analyses indicated a controlled assembly sequence, which involved bronze casting, insertion of a work-hardened iron core, and final pouring of molten lead to secure the components and improve weapon balance. Over two millennia of burial, the initial galvanic hierarchy (Fe ' Pb ' Cu) evolved into multiple localised corrosion domains. Lead temporarily reduced the galvanic driving force between iron and bronze; however, the poor electrical conductivity and thickness of the Pb shell promoted chloride accumulation and crevice-driven reactions at OS/IS and IS/IC interfaces. The growth of Pb and Cu corrosion products in both the interfaces generated internal mechanical stresses, leading to embrittlement, loss of adhesion, and detachment of bronze fragments. These results highlighted technologically advanced Etruscan metallurgy with significant implications for the interpretation and conservation of composite metal artefacts within archaeological contexts.