Investigation about ancient mortars and plasters in the Mondovì cathedral (Cuneo, Italy)

The S. Donato Cathedral of Mondovi (Cuneo Province, North-West Italy) is the last project of the famous architect Francesco Gallo (1672-1750). The edification started in 1744, but the dedication was achieved only in 1763. The building is an impressive brick-made church with a nave and two aisles, in baroque style with some neoclassical additions, showing a considerable length of 56 meters and a width of 24 meters; noteworthy are the 32 monolitic columns made by Frabosa gray marble, the wooden chorus and Episcopal cathedra, and the polychrome lateral altars by some of the most famous architect and designer of that period. After the edification, the internal decoration of the church walls was very simple, probably with a white finish of smooth plaster made of lime and magnesian lime mortar. There are some analytical evidences of a very simple decoration like a uniform bluish or grayish tinge. Only around the 1850 a plan of new decorations was made, committing some of the most important sacred art painters of that period (Andrea Vinaj, Luigi Hartman and Paolo Emilio Morgari between others) to paint sacred subjects on the vault and apse, and provide for decorations of lateral walls. Started in early 2012, some important restoration works are in progress at the Cathedral, and one of the main goal of the Restoration Team was (and still is) to obtain scientific and historical information about the previous works of painters and decorators, through analytical investigations by means of SEM-EDS microscopy and other techniques. The analyses carried out on some samples clearly showed the historical stratification of the different layers. It starts with some mortars and plasters of the middle of ‘700, having different compositions and very neat stratification. It ends with a more recent mortar plaster, coarse-grained, used as background for the new decoration of the middle ‘800 artworks. Backscattered Electron Imaging, EDS analysis and maps performed on sections showed skillfulness of the ancient decorators (1740-1750) in the applications of very fine plasters in multiple layers. Analyses showed up to seven layers of plaster of different composition, some more calcic, some more magnesiac, with very few or no inert phases at all. Contrasting, the more recent mortar used as background and preparative surface for the new decorations (~1850) has a monotone composition, with rough and irregular monolayer presenting coarse grained inert phases. This could suggest some rush work, regarding a surface considered less important, used only as a base for the most impressive sacred decoration. In retrospective, the use of a coarse grained mortar had unforeseen degradation effects with loss of tint and decoration in correspondence of inert phase grains, due to lack of adhesion between mortar and painting.