Post-depositional modification of atmospheric dust on a granite building in central Rio de Janeiro: implications for surface induration and subsequent stone decay
Published:January 01, 2007
B. J. Smith, J. J. McAlister, J. A. Baptista Neto, M. A. M. Silva, 2007. "Post-depositional modification of atmospheric dust on a granite building in central Rio de Janeiro: implications for surface induration and subsequent stone decay", Building Stone Decay: From Diagnosis to Conservation, R. Přikryl, B. J. Smith
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Extensive contour scaling of a 200 year old granite church is associated with the breaching of an apparently iron-rich crust and the widespread deposition of atmospheric dust within the canyon-like streetscape of Rio de Janeiro. Contemporary dust, accumulated dust from within a depression on the building surface, the surface crust and the underlying granite are examined by a combination of total element analysis and sequential extraction, X-ray diffraction and energy dispersive X-ray fluorescence. Results indicate an increase in total organic carbon and marked decrease in pH within the accumulated dust, and a rapid mobilization of anions and cations from the water-soluble and carbonate phases. It is considered that the latter is linked to salt accumulation within and eventual salt weathering of the granite. Post-depositional alteration of the dust is also linked with the de-silicification of clay minerals (illite to kaolinite) and the loss of silica from the amorphous Fe/Mn phase of the accumulated dust under the initially saline and progressively more acidic conditions experienced at the stone–atmosphere interface. This mobilization of silica is associated with the formation of what is, in effect, a thin silica-rich surface crust or glaze. Within the glaze, accessory amounts of extractable iron are concentrated within the amorphous and crystalline Fe/Mn phases at levels that are significantly elevated with respect to the underlying granite, but much lower than in the equivalent phases of the accumulated dust from which it is principally assumed to derive. The protection afforded to the stonework by the crust is not, however, permanent and within the last 15 years it has been possible to observe a rapid increase in the surface delamination of the church close to street level.
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Building Stone Decay: From Diagnosis to Conservation
Stone buildings and monuments from the cultural centres of many of the world's urban areas. Frequently these areas are prone to high levels of atmospheric pollution that promote a variety of aggressive stone decay processes. Because of this, stone decay is now widely recognized as a severe threat to much of our cultural heritage. If this threat is to be successfully addressed it is essential that the symptoms of decay are clearly identified, that appropriate stone properties are accurately characterized and that decay processes are precisely identified. It is undoubtedly the case that successful conservation has to be underpinned by a comprehensive understanding of the causes of decay and the factors that control them. The accomplishment of these demanding goals requires an interdisciplinary approach based on co-operation between geologists, environmental scientists, chemists, material scientists, civil engineers, restorers and architects. In pursuit of this collaboration, this volume aims to strengthen the knowledge base dealing with the causes, consequences, prevention and solution of stone decay problems.