The weathering of sandstone is connected with the activity of water penetrating the fabric through its pores. The microclimate is of particular importance for weathering phenomena. To understand the reactions on the surface of a stone, the variations of temperature and humidity have to be determined. From this data it can be concluded that cycles of drying and wetting are responsible for the weakening of stone. Artificial stone behaves like natural stone, but there are problems in the contact zone, since there the humidity penetrates more rapidly than through the normal fabric.