Nondestructive investigations of tin-rich bronze mirrors discovered in Dacian sites from Sălaj County – Romania

Mirrors were a sign of power and wealth in antiquity, having an important role in wedding and funeral rituals. Bronze mirrors were commonly manufactured in the Roman and Hellenistic worlds, with a moderate tin content of up to 26 wt%. Roman and Greek mirrors entered Dacia through commercial relations or as spoils of war, and therefore, are often found in Dacian sites. The present article is focused on the investigation of unusual white bronze mirror fragments discovered in Dacian sites in Salaj County, Romania, which were subjected to non-destructive analysis. The general aspect indicates that they were produced by casting. Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS) elemental analysis reveals bronze with high tin amounts ranging from 39.1 to 44.6 wt% and small amounts of 1 – 4 % Pb regarding Cu. Elemental maps reveal tin-rich grains identified by X Ray Diffraction (XRD) as intermetallic compound Cu6Sn5 and copper-rich grains identified as Cu₄₁Sn₁₁. Casting such thin-walled specimens causes rapid cooling, which doesn’t allow for the proper crystallisation of Cu3Sn through the eutectoid reaction, thereby maintaining the Cu₄₁Sn₁₁ structure. Thus, these mirrors have brittle structures due to the development of intermetallic compounds, which enable proper abrasion polishing using ancient methods. Atomic Force Microscopy reveals that Cu₆Sn₅ grains are more affected by the polishing action, inducing a roughness of 55.7 to 60.2 nm, while mirrors having more Cu41Sn11 have smoother surfaces with a roughness of about 21.7–36.8 nm. Both cases ensure clear mirror lustre with a silvery aspect, making these mirrors appreciated for their quality. Unfortunately, such brittle intermetallic compounds make these mirrors break like glass, a fact sustained by their findings as fragments. These findings suggest a deliberate technological choice aimed at enhancing reflectivity through the use of tin-rich alloys, despite their mechanical fragility. Comparative data from Celtic contexts suggest the possibility of shared metallurgical knowledge and similar alloying practices across Late Iron Age Europe. The absence of Greco-Roman parallels and the archaeological contexts point to local production, contributing to a revised understanding of Dacian metallurgy as innovative and regionally integrated.