Stone-imitating plasters in the Renaissance Ducal palace in Mantua (Italy): characteristics and decay with relation to microstructure

The plasters of the XVI cent. Cavallerizza Courtyard in the Ducal Palace in Mantua (Italy), designed to imitate natural stones (rustication work, carved marble, etc.), were realized, for both aesthetic and manufacturing needs, with highly variable formulations, resulting in highly variable microstructure of the final elements. As in the last decades the plasters have been exposed to high concentrations of atmospheric pollutants, they currently exhibit severe decay, differing on the basis of microstructural characteristics. The aim of this paper is to analyze the complex interaction between environmental aggressiveness, plasters microstructure and degradation phenomena, in view of their restoration. Plaster samples were withdrawn from the Loggia side of the courtyard, at short distance one another, so that exposure conditions could be considered as constant. The samples were characterized in terms of mineralogical composition, overall open porosity and pore size distribution, and, for each sample, the degree of the chemical attack was assessed by comparing the salts content in the external layer and in the inner part. In light of microstructure characterization and chemical attack evaluation, hints for restoration of the plasters were proposed.