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Gypsum, Calcium sulphate

CaSO4 2H2O is the crystalline mineral from which plaster is made. It is not practical as a source of CaO in glazes because its decomposition produces SO3 which is dangerous to health and it is destructive to the integrity of the glaze layer (and potentially the fired glaze quality).

Calcium sulphate has three different decomposition stages that generate water vapor and/or sulfur gases, two of which are reversible. These start below the boiling point of water and end well before 500C. Many ceramic clays, for example, contain sulfur, but either the percentage of slufur in the material is low or the percentage of the material used in glazes is low. Thus, in theory, calcium sulfate should be not responsible for glaze imperfections like blisters and pinholes (unless the glaze has a low melt fluidity or is fired very fast).

Calcium sulphate is often found in raw clay deposits as a partially soluble impurity that causes efflorescence, a plague to many ceramic industries (especially brick). During drying the salts are left behind of the surface of the clay to discolor it after firing. Barium carbonate is often used to precipitate this material.

Calcium sulphate reacts with sodium silicate and soda ash in a calcium-for-sodium ion exchange, and this process will eventually clog the pore structure of a mold used for casting slips containing it (the use of polyacrylate deflocculants will prevent this).

Related Information

Links

URLs http://en.wikipedia.org/wiki/Gypsum
Gypsum at Wikipedia
URLs http://www.handbookofmineralogy.org/pdfs/Gypsum.pdf
Gypsum mineral data
URLs http://en.wikipedia.org/wiki/Calcium_sulfate
Calcium sulfate at Wikipedia
Minerals Limestone, Calcium Carbonate
Minerals Selenite
Hazards The Use of Barium in Clay Bodies
Glossary Efflorescence
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
Materials Barium Carbonate
Temperatures Calcium Sulphate decomposition (80C-250C)

By Tony Hansen


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