Abstract
Black varnish commonly develops on rain-washed fine-grained monument sandstone. Stone modifications are, to the naked eye, limited to 10-μm thick black film and underlying modified zone about 1-mm thick. Transfer properties (absorption and drying kinetics and permeability) are, however, modified several centimetres under the surface. The present study investigates the modifications of black-varnish covered siliceous sandstones taken from Alsatian monuments (East of France) and of fresh sandstone undergoing wetting–drying cycles in the laboratory. Double-coloured thin-sections revealed gradual changes in the porous network, up to 3 cm under the black varnish. SEM observations showed that the film was mainly composed of iron and phosphorus while the modified zone was rich in calcium and sulphur. Fifty capillary absorption–drying cycles were carried out on fresh sandstone. Absorption kinetics was measured at each cycle. A continuous decrease of sandstone absorption kinetics over the fifty cycles was interpreted as a reorganisation of the porous network, reducing the connectivity of the porous network although total porosity remained unchanged. Wetting–drying cycles carried out under an environmental microscope (ESEM) showed a displacement of the finest particles (clay clusters), filling the macroporosity and decreasing the connectivity.
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Acknowledgments
The authors would like to thank the Japan Society for the Promotion of Sciences (JSPS) for providing financial support for this research. We would like also to thank C.T. Oguchi and T. Hatta (Japan International Research Center for Agricultural Sciences of Tsukuba) for helping the SEM and chemical analysis.
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Thomachot-Schneider, C., Gommeaux, M. & Fronteau, G. Modifications of the porous network of sandstone accompanying the formation of black varnish. Environ Geol 56, 571–582 (2008). https://doi.org/10.1007/s00254-008-1443-5
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DOI: https://doi.org/10.1007/s00254-008-1443-5