Chemical-physical decay of volcanic stone from Santa Maria Church in Tergu (Anglona, Italy)

The Church of Santa Maria di Tergu is located by the Island’s Northern coast, the Gulf of Asinara, a short drive from Castelsardo, in the historical region of Anglona. At the Age of the Giudici the Church was the place of a Montecassino priory, mentioned in 1122 under the title of “Sancta Maria de Therco”. Situated over an area where recent archaeological excavations have revealed traces of a thriving late Byzantine domus, the Church features a single nave and a transept, a gabled front and a characteristic deep red bulk counterbalanced by the white arches and the white panels with black inlays in the façade wall surface. In post-Medieval times it was built a cover with a barrel vault (probably in the second half of the nineteenth century, as in many other similar cases dating the same moment) which caused severe static problems to the monument, since nobody had apparently considered the static requirements of the building and the load on the side walls, not large enough to support such a heavy load. The stone vault was later eliminated in favour of a new cover in wooden trusses. The Church was build using in duotone two main different stones: reddish volcanic stones and whitish limestones. The macroscopic and petrographic analysis showed that the volcanic rock are Oligo-Miocenic pyroclastites (i.e., rhyolites and rhyodacites). The volcanic stone show a petrographic affinity with samples coming from a local historic quarry, already mentioned in the restoration work documents of the 1940s. The limestone materials taken from the monument show a variable chemical-physical decay, due to dissolution and decohesion processes of the carbonate matrix. Also pyroclastic rocks show decay problems, consisting of chemical-mineralogical transformations on the outer stone surface, with devitrification of glassy groundmass and formation of secondary phases.