Mineralogical study of salt crusts formed on historic building stones

A historic building stone, Baumberger sandstone, was exposed to gaseous SO2 under dry and wet surface conditions in an atmospheric test chamber. The extent of salt formation resulting from the exposure to pollutant gases as well as the variations in mineralogical composition were studied based on petrological thin sections. Results reveal that the salts are formed only over the surface of the sample. The thickness of the formed salt crusts is determined. Calcite dissolution as well as porosity decreases are due to gypsum formation.