Laser cleaning of black weathered Obernkirchen sandstone
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Abstract:
Formation of thin, black, well adhering weathering layers on pure sandstones used as building stone is well known phenomenon, described from 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 11 century onwards, and especially in the 14 til 17 century. Both sandstones show the formation of thin black weathering layers. Whereas layers on the Bentheim sandstone are generally thicker (up to a few quartz grains), layers on the Obernkirchen sandstone are very thin. In both cases, the layers are made up by opaque matter, such as soot and metallic particles, Fe(hydr)oxides and gypsum, whereas algae and/or fungi are present in many cases.Keywords:
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