The weathering of natural and artificial stones in the Baroque church of S.Pietro in Valle (Fano, Italy)

The church of San Pietro in Valle in Fano is with good reason considered one of the outstanding examples of baroque art in Central Italy. Currently the church is in a serious degradation status. For this reason, and in order to devise a correct conservative project, a scientific study has been carried out, aimed at characterizing mortar components used for the application of marble decorations and the different degradation products occurring in the church (i.e. pulverizations, encrustations, salt efflorescences, and condensation water). In the following, results obtained analysing the different sample typologies by optical microscopy, x-ray diffraction, electron scanning microscopy, and head-space solid phase microextraction-gas chromatography-mass spectrometry will be reported. The results showed that there are several causes responsible of the weathering, the most important being the use of mortar with gypsum aggregate concurrently with the occurrence of water capillary rise phenomena. This situation, together with ground settling episodes mainly in high altar area, is responsible of mortar pulverization and marble decorations detachment. Another important degradation factor is the condensation water present on the floor during the cold season that, interacting with salts present on the surface and within the pores, give rises to well known phenomena, i.e., hydration, dehydration, solution and crystallization able to exert considerable pressures and therefore material pulverization.