This article investigates the impact of sulfur dioxide attack, deposition of dark particles in urban environments and laser cleaning with Nd:YAG 1064 nm on color change in a range of ornamental limestones. We have used the CIELAB and CIELCH systems to compare the relative importance of the variation of each coordinate for the color change. Sulfur dioxide and dark particle deposition seems to increase the chroma, most particularly in the yellow component. Particle deposition also leads to an obvious darkening of stone surfaces. Laser irradiation at 1064 nm affects the red component of limestone, particularly if they already possess a reddish color. In general, the more intense the original color of the stone the greater are the chromatic changes, but the direction change of the color-parameter affected by a particular process remains the same. It has always been apparent in an atmosphere heavily polluted with soot that the main changes to light-colored stones are the exponential decrease in the parameter L* (darkening-blackening). This has important aesthetic and social implications. However, in the near future it may be that in cleaner atmospheres, perhaps more dominated by organic pollutants, a yellowing process may be of greater concern.

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Las sales solubles en el deterioro de rocas monumentales.Revisión bibliográfica

A methodology to collect and analyse atmospheric particulate matter has been developed at the Cathedral of Burgos (Spain). Particles were collected in a portable particle sampler on carbon layers and stone surfaces. The analyses were undertaken under SEM-EDX by means X-ray mapping and Featurescan (a program for the automated characterisation of particles). To determine their possible sources, particles collected in the sampler and on carbon layers were classi"ed according to their composition, mainly by cluster analysis. Then, they were compared with those deposited on stone surfaces. This classi"cation is useful when a plan of preventive conservation for monuments is to be undertaken. In general, no important di!erences are observed between the chemical composition of particles directly collected from the atmosphere and those deposited on di!erent substrates. Fine particles present the highest sulphur contents (almost 100%), while calcium is the major element in the medium and coarse particles. Other abundant elements are silicon, chlorine and phosphorus. The number of iron-rich particles is small. The study of the material deposited on carbon layers and stone substrates has con"rmed the presence of gypsum in all cases.

The application limits of the laser-cleaning technique for different types of building stones have been investigated by measuring colour variations. The selected stones differ in their chemical and mineralogical composition, colour, texture and crystallinity degree. The experimentation was carried out with a Q-switched Nd:YAG laser. The colour variations on stones associated with different operative fluences were measured using a colorimeter. Further, surface morphological changes were examined under SEM. From the calculation of colour differences, a damage threshold fluence was established for each stone type. The response of the stones to laser radiation at a particular fluence was found to be mainly conditioned by their chemical and mineralogical composition and, to a less extent, by their textural characteristics.