Non-invasive archaeometric study of red coral on Iron Age jewellery: presentation of the analytical protocol and case studies

Bronze ornaments characterised by whitish insets are widespread in Northern Italy and Switzerland in the early Iron Age; this material is generally identified as Mediterranean red coral (Corallium rubrum) bleached over time (De Marinis R.C., 2000; Peroni et al., 1975; Perrin F., 2000). Despite the abundance of this type of material in archaeological contexts, its identification as C. rubrum is generally not supported by scientific analysis (Berruto et al., 2021; Mangani, C., 2016; Fürst S. et al., 2016; Schvoerer M. et al., 2000). The necessity to fill this scientific gap, combined with the need to preserve and protect these artefacts, inspired the development of a specific totally non-destructive analytical protocol, which allows the precise identification of the possible presence of this raw material (Berruto et al., 2021). This protocol has been improved in recent years, as part of Tech4Culture PhD project (University of Turin). It proved extremely useful in the comprehensive characterisation of Mediterranean red coral, as well as in identifying the use of other raw materials and traces of possible organic binders. The procedure involves the combined use of micro-Raman spectroscopy (µ-Raman; to identify the molecular composition and in particular the possible presence of trace amounts of polyenes, the organic pigments characteristic of Mediterranean red coral), micro-X-Ray Diffraction (µ-XRD; to identify crystalline phases) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectrometry (SEM-EDS; for the morphological observation and chemical analysis of artefacts). This approach is directly applicable on the archaeological finds in suitably equipped laboratories, or, if it is not possible to move the objects, the application of the complete protocol is also possible on micro-samples (≈ 300 μg) taken from the decorations. The developed protocol has been applied in the archaeometric study of the decorations of a typical class of bronze fibulae referable to the so-called Golasecca Culture: leech shaped fibulae with white or pinkish circular insets (north-western Italy, 6th-5th cent. BC).