Quantification of surface changes and volume losses of selected rock types due to different cleaning processes

The restorative cleaning of natural stones has a special significance for the preservation of important cultural assets or the slowing of their deterioration. Organisms such as fungi, lichens or mosses, but also emission dirt such as soot soften and otherwise damage both the surface and the internal structure of the building stone. In order to quantify the effects and in particular the abrasiveness of selected cleaning methods, cleaning experiments were carried out on six different naturally and artificially weathered rocks using cold water under high pressure, hot water under high pressure as well as hot-water steam. The types of rocks studied include marble, limestone, granite, sandstone and tuff. Surface changes in roughness and topography were quantified using two surface-sensitive methods: confocal microscopy as well as 3D shadow triangulation. The two high-pressure cleaning methods were found to have a significantly stronger abrasive effect than steam cleaning when the distances were too small. The cleaning performance, which was compared using biologically weathered samples, was lowest for steam cleaning. However, the high temperatures of the steam also permanently eliminate much of the biological matter on and under the surface, as observed in the field test. The results presented should make it possible for the conservator to assess, which cleaning procedures to use for the different rock varieties depending on the degree of weathering.