An integrated Raman and petrographic characterization of Italian mediaeval artifacts in pietra ollare (soapstone).

Ten fragments of pots made by pietra ollare (basic and ultrabasic metamorphic rocks belonging to greenschist facies) found in the archaeological site of Crocetta di Sant'Agata Bolognese (Bologna, Italy) have been studied to characterize them and to define their provenance. The fragments, dated between the 9th and 10th century AD, show traces of blackening from fire, indicating the use of these findings as pots by fire. The extensive use of a non-destructive technique, Raman spectroscopy, in both laboratory and portable forms, for the provenance analysis of pietra ollare artifacts has been evaluated. Micro-Raman spectroscopy was used for a detailed study of the main components, to identify the secondary minerals and to study the distribution of the mineral phases in the samples. The samples show schistose texture, are fine grained and consist mainly of talc and chlorite, and subordinate carbonates, oxides and serpentine. Olivine is rare. Based on the mineralogical composition, the material can be defined as chlorite and carbonate talc-schist. Our samples of pietra ollare may be assigned to chlorite and magnesite-bearing talc-schists. The alpine areas of origin of this lithotype are Valtellina, Valchiavenna and Val Bregaglia. In particular, we can hypothesize that the origin of these findings is Valchiavenna, which had great commercial importance in the Middle Ages. Further measurements, obtained with a portable Raman spectrometer, directly on a quarry near Chiavenna, support our hypothesis.