The article focuses on the building stone of Alsace (France). Topics examined are: sandstone patina; physical and chemical decay; fine and thick crusts; desquamation; the formation of cavities; disaggregation of solid stone into sand grains; differences between decay types in the city and in the country; gypsum-rich layers; clay-rich layers with calcite; iron-rich film on stone surfaces. A discussion of the possible roles of the various layers in decay processes is included.