Study of origin and composition of coatings in a monument built with granitic rocks, by SEM, XRD, XRF and DTA-TGA

The Casas de Paredes (A Coruña, Spain) is an example of 18th Century neoclassical architecture built with granitic rocks. There are neoformation coatings on the main façade of one of the buildings, producing a visible soiling. Sixteen samples of coatings were collected and studied by SEM (surface and cross‐sections), including analyses by EDS and cross‐sections by x‐ray maps, in order to know their textural and chemical characteristics and to elucidate their origin. Studies by XRD, XRF and DTA/TGA were carried out in most of the samples. These analytical methods have allowed us to recognise up to six types of coatings. Three of them are gypsum‐rich coatings, derived from ancient protection plasters and also, by migration of solutions from lime mortar joints. Air pollution contributes to the blackening of these gypsum coatings and to the formation of black crusts by deposition of fuel combustion particles (Pb‐ and Fe‐rich aggregates, fly‐ash) and other particles (airborne soil particles) on the surface. One of the coatings is a dust crust (with significant amounts of iron, lead and other metals) originated by deposition of atmospheric pollution on ashlar surfaces (without gypsum aggregates). Also, Si‐rich detritic coatings originated by surface alteration of the granitic rock and particle deposition and accretion have been found. The last group is made by coatings originated by deposition (and alteration) of bird excrement. These results could help to define an appropriate cleaning program for the façade, and highlight the relevance of coatings as evidence of pollution agents affecting heritage buildings. Copyright © 2008 John Wiley & Sons, Ltd.