Diagnostic analysis of stone materials from underwater excavations: the case study of the Roman archaeological site of Baia (Naples, Italy).

This work belongs to the framework of the na- tional research project “COMAS” (Planned COnservation, “in situ”, of underwater archaeological artifacts), funded by the Italian Ministry of Education, Universities and Research (MIUR), concerning the submarine archaeological area of Baia (Naples, Italy). The site includes remains of the ancient cities of Baiae and Portus Iulius, which, since the 4th cen- tury AD, started to be submerged because of the bradyseism phenomenon. The work aims to the characterization of four different types of stone materials collected from the site, specifically marbles, limestones, ignimbrites, and bricks, in order to in- vestigate their state of conservation. In particular, specimens were sampled from some masonry structures and pavement slabs (opus sectile) in a specific area of the submerged site, called “Villa a Protiro”. In order to characterize archaeological samples from a mineralogical-petrographic point of view, polarized opti- cal microscopy and X-ray diffraction analyses were car- ried out, while to assess their conservation state, the surface colonization by biodeteriogen agents and their interaction with the substrate were studied through observations un- der a stereomicroscope, scanning electron microscopy and Fourier transform infrared spectroscopy. Marble and limestone samples revealed an intense bio- erosion phenomena, attributable to epilithic and endolithic forms, particularly boring sponges. On the contrary, ign- imbrites suffer a lower degree of biological colonization re- lated to the activity of other species, such as serpulids and bryozoans. In bricks, biocolonisation is correlated to the type of temper used in the artifact, the quartz pastes having a greater susceptibility to biological attack than the volcanic ones.