Silikatischer Sandstein unter städtischen Umweltbedingungen: Verwitterung und Reinigung der Kirche Betlem (Barcelona, Catalonien) / A siliceous sandstone in an urban environment: the decay and cleaning of the church of Betlem (Barcelona, Catalonia)

The aims of this paper are to study and characterize the siliceous sandstone used in the construction of an important Barroque church placed in the down town of Barcelona. The degree of resistance to the urban polluted environment is evaluated and the decay forms and weathering mechanisms studied by means of mineralogical analytical techniques. A complete restoration campain was carried out including the cleaning of the whole facade and sculptural elements. We studied the rock previous to, during and after the cleaning processes. The main results achieved are as follows: i) two rock types have been identified as building stones which may be differenciated by the mineralogy of the cement leading to a different behaviour and durability; ii) several decay forms have been identified, namely the darkening of the surfaces by dirt accumulation, the weathering of the feldspars forming the rock and the transformation of formed pyrite nodules; iii) the mechanisms leading to the decay forms identified are studied and a model for the decay (aesthetical and physical) of this kind of stone is proposed for urban environment. On the basis of these results a cleaning process with water and neutral detergent using non metallic brushes as proposed was applied. The results of this cleaning on each lithotype are evaluated by means of microscopical analyses of fragments of stone after cleaning. The different behaviour of both types of stone are analyzed and documented on a microscopical scale.