The Gassino stone in the architecture of Torino (Italy): different features for different uses

The Gassino Stone, the most widely used carbonate sedimentary rock in Torino until the XIX century, is also known as Calcare di Gassino or Gassino Marble (Borghi et al., 2014). It was quarried from the Torino Hill domain near Torino and its formation can be referred to the Eocene. It is a biocalcirudite containing mainly red algae, macroforaminifera, bivalves, echinoids, and locally abundant intraclasts of biocalcarenites. Although it has not been exploited for a long time, its occurrence in the city is still considerable. Its uses began in the end of the 1600 till mid 1800 and was installed not only in the churches but also in the historical buildings in plinth, columns, fireplaces and furnishings (Campanino et al., 1991). It is possible to find applications of this rock in many famous historical buildings of Torino and surroundings such as in the colonnade of the courtyard of the Rectorate of the Torino University, in the portal of Palazzo Carignano, on the façades of the Santa Cristina Church and of the Torino Town Hall of, and in the Basilica of Superga external columns. In the interior it has been usually installed with polished finishes (for example in the San Lorenzo church in the center of Torino). This kind of surface finishes gives a brownish aspect to the stone, in addition to a sense of compactness. When instead the Gassino stone is installed outdoor, with increasing time of exposure, its colour turns white and the compactness decreases emphasizing the presence of nodules consisting of calcareous red algae and intraclasts surrounded by thin clay seams. The portal of  Palazzo Carignano  represents the best example of the differential decay of this kind of stone outdoors, while the holy eater font in San Lorenzo church is the perfect example of optimal maintenance indoors.