Raman spectroscopy as a tool for provenancing black limestones ( bigi morati ) used in antiquity

Among precious stones used in antiquity, bigi morati stand out for their importance. They consist in black limestones mainly employed in sculptures and architectural heritages and quarried from several sites through the Mediterranean area. Their provenance is difficult to assess due to their similar appearance to the naked-eye observation and the possible confusion with black marbles (i.e., bigi antichi). Classical methods used for studying ancient marbles, such as minero-petrographic and stable isotopes analyses (δ13C and δ 16O), resulted to be powerful in provenancing these black lithotypes. However, these methods are microdestructive and cannot be always applied in the case of precious artefacts for which sampling is not allowed. Bearing carbonaceous matter conferring them the typical black-dark grey colour, Raman spectroscopy might support studies on these limestones, bringing important information about the structural state of organic/carbonaceous matter and their geological evolution. In this study, the potential of Raman spectroscopy in determining geological and geographic provenance of bigi morati was tested on quarry samples from six among the most famous ancient exploitation sources. The obtained results demonstrated that the use of Raman parameters typical of low-ordered carbon matter is an effective tool in discriminating provenance. Moreover, the laboratory-based approach lays the groundwork for future development and validation of the method for in situ measurements on artefacts by using portable equipment.