TRADITIONAL METHODS FOR SURFACE TREATMENT OF VILLAMAYOR SANDSTONE USED IN HISTORICAL BUILDINGS IN SALAMANCA (SPAIN)

A study has been carried out on the surface layers formed (both naturally and artificially) on several historical buildings in Salamanca (Spain) built with ViUamayor sandstone. Ageing of this sandstone spontaneously develops a patina on its external surface, thus providing a golden-like appearance, and improving its resistance to damage and deterioration. On studying the nature of the patina formed on several historical buildings, a different sort of patina has been found, that contains foreign materials (mainly gypsum and iron-rich clays) that do not exist in the bulk of the stone (this is free of calcium carbonate), and cannot be formed under the environmental conditions at which the buildings are submitted. This type of artifical patina has been found in nearly all buildings studied that were built or restored after the Baroque period (s. XVII-XVIII), suggesting that patinating was probably a usual technique in order to fit the external colour of stones from different quarries or to match the colour of new stones to that of aged ones during restoration works. The nature of these different patinas has been studied by X-ray diffraction, thermal and chemical analyses and by infrared spectroscopy.