Archaeometric investigation of bronze Iron Age fibulae with nestled coral insets from three archaeological sites of the Piedmont region (Northwestern Italy)

Bronze fibulae decorated with white-to-reddish circular insets were used during the VII century BC in Northwestern Italy – particularly in the Piedmont region. These insets, commonly described in the archaeological literature as ‘red coral’ (Corallium rubrum), have hardly been studied with a scientific approach. To fill this gap, an archaeometric study was performed on 33 ancient fibulae from three archaeological sites in the Piedmont region (Villa del Foro, Brignano Frascata and Castelletto Ticino). Tiny samples were extracted from two kinds of decorative patterns (i: stuck in circular punctures; ii: filling engraved lines on the fibulae surface) and analysed with a multi-analytical protocol (including FT-IR, μ-Raman, μ-XRD, optical and scanning electron microscopy with EDS) to characterize the constituent materials and infer their manufacture procedures. In most cases, a low to-high magnesian-calcite (LMC and HMC, respectively) is detected, at times coupled to organic pigments (polyenes). Contextual presence of polyenes and HMC clearly accounts for a biogenic origin of these decorations – specifically to Corallium rubrum. An analogous origin can be postulated when only HMC is present – lack of pigments being related, in these cases, to their possible denaturation under specific environmental conditions. Presence of sole LMC, on the other hand, is not sufficient per se to justify a biogenic provenance and may be justified by several factors – e.g., denaturation of corals over time and/or specific diagenetic conditions existing in the interment soil. Besides, the chance for these carbonates to originate from other marine organisms lacking pigments (e.g., white coral or others) must also be taken into account. These evidences are also supported by the microscopic observation of coral skeletons morphologies (e.g., fragments and sprigs) in some of the studied materials. Degradation byproducts of the bronze alloy and minerals of the interment soil are also found, some of which (e.g., talc) may have been purposely used as additional decorating agents. Feasibly, two distinct procedures were adopted for manufacturing the decorations: i) a pasty material, obtained by mixing ground coral powders and a (yet unknown) binder, was applied with a tool to facilitate spreading in the hosting cavities; ii) aptly shaped coral insets or tiny raw fragments were stuck in circular pores, with the aid of a gluing agent.