Nature and decay effects of urban soiling on granitic building stones

A detailed microscopical and chemical examination by SEM + EDAX, XRD and PyGC-MS of black patinas coating the surfaces of urban granitic monuments in Aberdeen and Dublin has revealed a variety of decay features together with a complex chemical composition. Beside sulphate-rich thick patinas composed of a framework of gypsum crystals in which both particulate and gaseous inorganic/organic pollutants are deposited and which are similar to gypsum crusts developing under polluted atmospheric conditions on calcite-bearing stones, thinner, gypsum-free, black surface layers made up of particulate matter embedded in a fine-grained, iron-rich matrix are commonly found. These soiling layers show, relative to the substrate, higher % of P, Ca, S, and Cl. The occurrence of a patinaʹs type against the other is probably controlled by the availability of local SO2 sources. Physical and chemical decay effects on the granitic substrate are more severe when gypsum is present but dissolution and micro fracturing episodes are also observed in iron-rich patinas. The chemical inorganic/organic composition of these surficial layers suggests that pollution induced soiling processes are the main factors resoonsible for their build-up on building facades. On the other hand, microscopic and preliminary chemical evidence does not exclude biological activity as an important agent in the development of iron-rich thin black layers.