Marble-imitating renders in the Monumental Cemetery in Bologna (Italy): formulation and compatibility of repair mortars

In this paper, an interesting study case of marble-imitating renders, used to decorate the walls of a XIX century family vault in the Monumental Cemetery in Bologna (Italy), is presented. Due to degradation processes owing to rising moisture in the last decades, large portions of the renders, detached from the underlying masonry, were lost. Hence, possible filling of the missing parts with new renders had to be considered. The feasibility of reproducing compatible repair renders, in terms of both aesthetic outcome and microstructural properties, was here evaluated. Samples of original renders were withdrawn and characterized by XRD, TGA, SEM/EDX and MIP. The renders were identified as examples of “scagliola carpigiana”, [gypsum-based works typical of Carpi, Italy], a technique for imitating precious materials, such as coloured marbles and semiprecious stones, using gypsum. Basing on characterization results and information collected in historical recipes, repair mortars were prepared and their compatibility evaluated by visual comparison with original renders and by microstructure characterization using the same techniques as above. The repair renders proved to be fairly compatible with the original ones, in terms of aesthetic outcome, composition and porosity, so that a fair physical and mechanical compatibility could be expected as well. The conservation and restoration of such marble-imitating renders with compatible and suitable repair renders would reach the double goal of preserving the original image of the tomb and preserving the “know-how” of a sophisticated technology, today too often forgotten.