Black crust

Description

A dark, thin crust that forms on the surface of outdoor limestone and marble sculpture. Black crust, or gypsum crust, contains airborne particulates (carbon, dirt, salts) deposited on the stone surface. The particulates are held in place by a gypsum layer formed by the reaction of acidic sulfur-containing pollutants with the calcite.

Synonyms and Related Terms

gypsum crust; black spots; Schwarze Kruste (Deut.); croute noire (Fr.); rosta negra (Port.)