Ca-oxalate and alteration films on the stones of the medieval Romanesque Sardinian monuments (Italy)

Le résumé fourni par la source

The chromatic aspect that the rocks are taking at the interface with the atmosphere is usually different from what of unal-tered (fresh) rock substrate. This difference may be due to weathering and bio-deterioration processes, which involve physi-cal and / or chemical modifications of the rock, or deposition of organic and inorganic substances on the surface (e.g. at-mospheric particles). \nThe research intends to analyse the surfaces of geomaterials used in the Sardinian medieval churches (XI-XIV sec.) with high historical-cultural relevance, using different analytical methodologies: SEM-EDS, LIBS, XRD. Regardless of the lithol-ogy of the substrate (basalts, pyroclastic rocks, limestone, marbles), the analytical results lead to the identification of three different cases: i) simple chromatic alteration of stone, ii) chemical alteration of stone with formation of coatings that com-positionally are also linked to the stone substrate, iii) chemical processes with the formation of compounds which have no connection with the substrate and the presence of Ca-oxalate films (i.e. weddelite, wevellite). The LIBS analysis is very use-ful because not being destructive can analyse the stone artifacts of monuments without changing its structure. Furthermore, this analytical method allows to define not only the compositional characteristics of the solid phases (amorphous or crystal-line) present in the stone surface, but also the micro-stratigraphic aspects of different film levels. \nThe presence of the Ca-oxalate on the stone of ancient monuments, as noted, is a testimony of application of organic sub-stances on the surfaces themselves, with aesthetic, to standardize or improve the tone colour of the stone, or conservative, to limit the negative effects of weathering. \nIn the specific case, the discovery of these films in the Romanesque churches in Sardinia, constitutes a first step towards a comprehensive study of these films, also to the understanding of their historical and material significance in terms of use and function.

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