Pietraforte, the Florentine building material from the Middle Ages to contemporary architecture

Pietraforte sandstone is the building material of the Medieval Florence (Italy). This was due to the proximity of its quarries to the city, its availability, ease extraction from the quarry face and to the particular characteristics of durability of the stone, suitable for the buildings and monuments cladding. Florence can be defined as a city in Pietraforte, and this makes it necessary its detailed characterization. The paper reports the geological setting of the Pietraforte sandstone, the mineralogical-petrographic characterization, the physical parameters and also a mineralogical method (based on clay minerals association) for the identification of the source quarries of the most important historical buildings. The knowledge of this lithotype allowed to better understand its decay phenomena. ant houses realized with Pietra Serena sandstone (Fig. 3b). In particular, Pietraforte was the primary building material (from Middle Ages) of the city, while Pietra Serena was used extensively with the advent of Renaissance, when large blocks of this latter sandstone were required to carve columns, capitals and other ornamental elements (Rodolico, 1964; Fratini and Rescic, 2013; Fratini et al., 2015). The extensive utilization of Pietraforte was mainly due to: 1) the proximity of its quarries to the city (the hills near to the left bank of the River Arno e.g. Boboli) (Fig. 4); 2) the ease of its extraction from the quarry face favoured by the thickness of the arenaceous beds (separated by shaly intercalations) that varies from a few decimetres to 1.5 m; 3) the presence of internal secondary discontinuities like fractures filled with calcite veins that allow an ease shaping of the material (Fig. 5); 4) its characteristics of durability. The aim of this paper is to summarize the lithological-sedimentary, compositional and physical features of Pietraforte, particularly of the Florence area, define the source quarries for many important buildings of the XIV-XVI centuries in the city, and also to describe the problems of its conservation.