A surface degradation layer on marble and limestone produced by the chemical reaction of sulfur dioxide pollutants, moisture, and the stone. Acidic pollutants react with the basic stone to form insoluble salts; in this example calcium sulfate, i.e. gypsum, is formed. Often other airborne pollutants such as dust, soot, and heavy metals are incorporated into the gypsum making the degraded surface appear dark or black. Gypsum is softer and more water soluble than limestone and marble and thus the crust will flake away or dissolve in areas that are washed by rain.
Synonyms and Related Terms
black crust; gypcrust; crosta gipsífera (Port.);
Sources Checked for Data in Record
- Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996