ELSEVIER The Science of the Total Environment 167 (1995) 87-101 Nature and decay effects of urban soiling on granitic building stones N. Schiavonma, G. Chiavarib, G. Schiavonb, D. Fabbri” aDepartment of Earth Sciences, Cambridge Universi& Cambridge, UK bDipartimento di Chimica ‘Ciamician’, Bologna University, Bologna, Italy ‘Laboratorio di Chimica Ambientale, Ravenna, Bologna University, Bologna, Italy Abstract A detailed microscopical and chemicalexamination by SEM + EDAX, XRD and Py/GC-MS of black patinas coating the surfaces of urban granitic monuments in Aberdeen and Dublin has revealeda variety of decayfeatures together with a complex chemical composition. Beside sulphate-rich thick patinas composed of a framework of gypsumcrystals in which both particulate and gaseous inorganic/organic pollutants are depositedand which are similar to gypsum crusts developing under polluted atmosphericconditions 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 layersshow, relative to the substrate, higher % of P, Ca, S, and Cl. The occurrence of a patina’stype against the other is probably controlled by the availability of local SO, sources. Physicaland chemical decayeffects on the granitic substrateare more severe when gypsum is present but dissolution and micro fracturing episodes are alsoobserved in iron-rich patinas. The chemicalinorganic/organic composition of thesesurficial layers suggests that pollution induced soiling processes are the main factors responsible for their build-up on building facades. On the other hand, microscopic and preliminary chemical evidencedoes not excludebiological activity asan important agent in the development of iron-rich thin black layers. Keywords: Granitic buildings; Urban soiling; Black patinas; Gypsumpatinas; Iron-rich patinas; Granite buildings; Gypsum/silicate replacement; Black patinas;Analytical pyrolysis 1. Introduction The surface soiling of stone monuments and works of art exposed to polluted urban atmo- spheres has become the object of intense re- search amongst conservation scientists in recent *Corresponding author. years. The problem that these surficial patinas cause to the preservation of our artistic cultural heritage is not only an aesthetic one; weathering and decay processes acting on the stone substrate underneath are a common feature. This is true not only in ‘soft’ highly porous stones such as limestones and calcite-cemented sandstones but also in ‘hard’ low porous stones such as granite [l-5]. Granite is an igneous rock with a multimin- 0048-9697/95/$09.50 0 1995 Elsevier Science BV. All rights reserved. SSDI 0048-9697(95) 04572-5