ABSTRACT
A very common cause of building deterioration associated to atmospheric pollution is the formation of black crusts that are primarily comprised of gypsum (Schiavon 1991, Felix 1992, Price 1996, Moropoulou 1998). Air pollution has since mid-19th century been suspected of accelerating the degradation of many types of construction materials (Winkler 1994). Moreover, air pollution is an important factor in the degradation and, in some cases, the loss of large parts of our cultural heritage (López-Arce et al. 2003, Török 2003). This problem is especially marked in industrialized or urban areas, where sulphur-containing combustibles are used, which causes an increase in the SO2 concentration in the atmosphere. SO2 is a stable gas at room T and in dry, clean air, but in the presence of humidity and other pollutants it transforms into sulphuric acid. This acid readily reacts with most of the minerals present in building materials (e.g. carbonates) and with dust deposits, resulting in gypsum crystallization and black crusts formation. Novakov et al. (1974) established a relationship between sulphating processes from atmospheric SO2 and the presence of carbonaceous particulate matter. This process accelerates in the presence of some metals (Fe, V, Cr, Pb, etc.), which catalyse the oxidation and hydrolysis of SO2 to form sulphuric acid, which is a
significant component of and contributor to acid rain and stone sulphation (Urone et al. 1968, RodriguezNavarro & Sebastian 1996).
