ABSTRACT
This chapter presents the role of black varnish in modifying the transfer properties of stones. Surfaces of thin sandstones used in buildings in Alsace are covered by a black varnish which modifies their surface and transfer properties. The weight increase per surface unit and the capillary height are plotted over the square root of time, according to the Washburn law. In some cases, a one-millimeter thick slab of sandstone in which black varnish adheres comes off in a parallel direction to the outside surface of the stone. To complete the petrophysical data, drying kinetics were also measured. The presence of black varnish at the surface of sandstones modified the surface properties of the rock. The surface was wet because water capillary transfer toward the surface was enough to equilibrate the loss by drying. Black varnishes develop on thin sandstones exposed in washed zones. Under these coatings, stones are coherent and don’t show visible weathering.
