Microstructural characterization of early western Greek incuse coins

In this research, we studied the compositional, crystallographic and microstructural properties of a series of incuse silver didrachmae stemming from the Achaean colonies of Metapontum and Caulonia. In this paper, we address the following points: (i) the metal sources, (ii) the fabrication process and (iii) degradation phenomena, such as incrustation and embrittlement. In this investigation, we employed energy-dispersive X-ray fluorescence spectroscopy, X-ray diffractometry, and scanning electron and optical microscopies. The patina is mainly composed of chlorargirite. The coins consist of a silver-rich alloy containing similar to 1% of Au and Cu. Metallographic and local compositional analyses revealed a complex scenario of inclusions. In one instance, unalloyed copper grains, two-phase copper/bismuth globuli and high-bismuth filaments were observed. In other cases, globular Cu2S (chalcocite) inclusions were noticed. The presence of SiO2 and iron oxide inclusions is ubiquitous in these samples. Distorted twin lines and strain lines can be detected, denoting work-hardening of recrystallized flans. Grain polygonalization can occasionally be noticed, hinting at secondary recrystallization processes. The irregularly shaped iron oxide particles often act as crack initiation sites. Fracture facets are generally intergranular. On some areas, intergranular decohesion is also observed. Open cracks sometimes contain AgCl. The strain lines that can be noticed on the fracture surfaces indicate work-hardening and residual microstructural deformation. Information regarding inclusions and the presence of significant amounts of gold can be tentatively used to address provenancing and fabrication issues.