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Laser cleaning of black weathered Obernkirchen sandstone

T.G. Nijland, T.J. Wijffels, C.W. Dubelaar, J.A. Larbi
TNO Built Environment & Geoscience, Delft / Utrecht, the Netherlands

Formation of thin, black, well adhering weathering layers on pure sandstones used as building stone is a well known phenomenon, described in several countries and on several types of sandstones. In the Netherlands, sandstones from the Lower Saxony basin in Germany, notably the Bentheim and Obernkirchen sandstones, have been used for many prominent monuments from the 11th century onwards, and especially in the 14th till 17th century. Both sandstones show the formation of thin black weathering layers. In the present case, after characterization of the black weathering layer on a late 18th century fašade cladded with Obernkirchen sandstone, a pilot test was performed to investigate the possibility of cleaning, and, especially, of any harmful effects to the original stone resulting from cleaning. Commercial firms have been invited to clean test panels on the fašade. Two firms using a Nd-YAG laser have been invited, a third one using EDTA pastes. The latter proved not successful. In both cases, laser visually succeeded to remove the black layer. Possible deleterious effects of stone cleaning are twofold, viz. direct damage, such as removal of grains or patina from the original stone and damage to working of the stone, or indirect damage, that may arise from a different hygric behaviour of the stone (and fašade as a whole) after cleaning. The latter have not been evaluated. Direct damage was evaluated by combination of polarization-and-fluorescence (PFM) and scanning electron microscopy (SEM). Cleaning using a Nd-YAG laser under commercially realistic conditions did not result in any direct damage to the Obernkirchen sandstone.

Key words: Laser cleaning, Obernkirchen sandstone