Difference between revisions of "Gypsum crust"
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== Description == | == Description == | ||
| − | A surface degradation layer on [http://cameo.mfa.org/materials/fullrecord.asp?name=marble marble] and [http://cameo.mfa.org/materials/fullrecord.asp?name=limestone limestone] produced by the chemical reaction of [http://cameo.mfa.org/materials/fullrecord.asp?name=sulfur | + | A surface degradation layer on [http://cameo.mfa.org/materials/fullrecord.asp?name=marble marble] and [http://cameo.mfa.org/materials/fullrecord.asp?name=limestone limestone] produced by the chemical reaction of [http://cameo.mfa.org/materials/fullrecord.asp?name=sulfur%20dioxide 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. [http://cameo.mfa.org/materials/fullrecord.asp?name=gypsum gypsum], is formed. Often other airborne pollutants such as [http://cameo.mfa.org/materials/fullrecord.asp?name=dust dust], [http://cameo.mfa.org/materials/fullrecord.asp?name=soot 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 == | == Synonyms and Related Terms == | ||
| − | black crust; gypcrust; crosta | + | black crust; gypcrust; crosta gipsífera (Port.); |
== Authority == | == Authority == | ||
Revision as of 06:52, 24 July 2013
Description
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.);
Authority
- Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996