Secondary mineral phases and ancient treatments on limestone from Monte de Prama nuragic statues (Sardinia, Italy)

The study intends to analyse with micro-destructive methods some original fragments of stone materials used in sculptures dated to Nuraghic Age (XI-IX cent. BC) from Sinis area (Cabras, central-western Sardinia). The important nuraghic sculptures, found in the archaeological site of Monte de Prama, consist of porous limestone surely locally quarried. Several historical, archaeological and geological studies have been already done on these sculptures, but up to date information and data on chemical and physical weathering processes occurring in the millennia, are missing. The main purpose of this work is analysing by LIBS and SEM, well tested techniques in the Cultural Heritage field, the Limestone finds from nuraghic statues, first of all to know the degree of the chemical and physical alteration. Furthermore, the LIBS methodology, with the aid of XRD analysis of the particles adhering to the surface, allows us to analyse the composition of the films / coatings / incrustations present on the original surface of the statues (remained buried for centuries), and the definition of the secondary mineral phases not belonging to the limestone substrate. A careful compositional analysis of the surface also allows us to identify any original ancient treatments, aimed to colour the statues and / or conservation purposes to preserve and protect them from the weathering. The chemical and physical alteration processes have been addressed through a comparative study of mineralogical, petrographic and physical characteristics among artifacts and geological samples from the Sinis area. Through PLT tests, Hepycnometry, Hg-porosimetry and BET analysis (by Brunauer-Emmet-Teller method) on field geological samples some physical and mechanical properties of the stone (density, porosity, specific surface area, resistance to compression, tensile, elastic modulus, etc.) were also determined to understand the resistance to tensional stresses in the original position of the sculptures. The analysis of the limestone alteration mechanisms also allows us to understand the ideal micro-environmental conditions in which to keep these important sculptures inside the museum spaces, to avoid further deterioration of the stone.