A non-invasive archaeometric protocol for characterizing possible Corallium rubrum-related carbonate materials in decorations on Iron Age bronze fibulae of the Golasecca culture

In the Iron Age, the Golasecca culture in Northwestern Italy connected Italic populations with transalpine and Celtic ones. From the 7th century BCE, in some jewellery artefacts (bronze fibulae and pendants) a whitish material constituting either part of the objects or nestled decorations is observed. Archaeologists identify it as coral (Corallium rubrum), its red hue whitened by the passing of time. However, few archaeometric studies have been carried out so far, also because of their invasiveness. 46 artefacts with whitish decorations from the archaeological sites of Como, Castelletto Ticino and Golasecca (stored in the Museo Civico Archeologico ‘P. Giovio’, in Como) are investigated here with a novel, non-destructive, multi-analytical protocol (i.e., μ-Raman spectroscopy, μ-X-ray diffraction, optical microscopy and SEM-EDS in low-vacuum), which completely safeguards their integrity. This approach, applied on the artefacts as such, allows identifying their constituent materials and degradation byproducts, extrapolating information about their nature, history and manufacturing process. Ca-carbonates – in the form of calcite – are identified in the decorations of most studied artefacts. This evidence alone does not account for their biologic origin (i.e., from C. rubrum), but other aspects can help in solving such an issue. In several artefacts, high-Mg calcite (HMC, a marker for biogenic carbonates) + polyenes (the typical pigments of red coral), sole HMC or even a mixture of HMC + LMC are identified – often coupled to typical coral micro-morphologies. A biogenic origin for these carbonates (i.e. red coral, though other similar species cannot be completely ruled out) can certainly be ascertained – lack of polyenes being justified by possible denaturation over time. In other artefacts, however, low-Mg-calcite (LMC) or even pure-calcite (i.e., with no detectable Mg) are found with no polyenes – hinting a possible inorganic origin. Most of these, however, show typical coral micro-morphologies. In these decorations, Mg-depletion might thus depend on the meteoric diagenesis of originally biogenic carbonates, responsible for transformation of HMC to LMC (or even pure-calcite) and decay of polyenes. Other materials are also seldom detected – i.e., gypsum, probably resulting from sulfation of an original coralline matrix interred in acidic soils, and Ca-phosphates, due to sporadic use of bones. All these evidences point to the fact that C. rubrum was feasibly used in at least ≈ 70 % of the studied decorated artefacts. For what concerns the composition of the alloy, bronze or lead-bronze can be found in different artefacts – or even different portions of the same item – possibly depending on sharp technological issues, dictated by fluidity or workability during manufacture.