Multi-analytical characterization of archaeological fragments of pietra ollare from the Medieval site ‘Piana di S. Martino’ (Piacenza, Italy): A mobile and laboratory approach

The archaeological excavations in the site of ‘Piana di San Martino’ (Pianello Val Tidone, Piacenza, Italy) discovered several fragments of pietra ollare (soapstone) artefacts, dating back to the Medieval period. A selection of fragments was characterized with micro-Raman spectroscopy in order to determine their composition, in terms of main and accessory species. The morphology of the crystals and the mineral phases were also investigated with polarized-light microscopy observations, X-ray powder diffraction and Scanning Electron Microscopy with Energy-Dispersive System analyses, which corroborate the Raman results. A detailed characterization was obtained on 12 fragments ascribable to pots and a millstone, made of talc-schists or chlorite-schists, including garnet-bearing chlorite-schists. Interest findings about the activities related to the use of these artefacts and the alteration processes occurred during their burial period were highlighted by the identification of specific products. Additionally, mobile Raman measurements were performed on the fragments, and a quick and effective evaluation of their composition was successfully achieved. These results highlight the use of mobile Raman instruments in the field of archaeology, specifically referred to pietra ollare findings, for which a preliminary identification may be obtained directly in situ.