Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures.
Part 7: Atmospheric Soiling and Black Crusts
Atmospheric soiling is a discolored build-up of airborne pollutants. It can build up on all materials, but this discussion focuses on its effects on masonry and concrete. Black crusts are accumulations of gypsum deposited at the surface of masonry materials and discolored by atmospheric soiling.
Atmospheric soiling results from airborne pollutants – usually soot, soil particles, and fly ash – accumulating on the surface of a building material. Porous and rough-textured materials, such as brick and stone, are more susceptible to atmospheric soiling than metals and painted surfaces. Some types of atmospheric pollution contribute to the deterioration of masonry materials via acid rain. Black crusts are accumulations of insoluble gypsum formed by the reaction of particulate pollutants within the masonry material, and darkened by atmospheric soiling. Black crusts typically form in sheltered areas, where moist air delivers a high concentration of dissolved pollutants.
Atmospheric soiling may be cleaned in order to prevent further deterioration, or for cosmetic purposes.
Next in this series: Biological Growth