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Gypsum

Gypsum is hydrated calcium sulphate, CaSO4 2H2O. It is the crystalline mineral from which plaster is made. Although high in CaO, it is not practical as a source for 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).

Gypsum has two decomposition stages that generate water vapor. These start below the boiling point of water and end well before 500C. 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 on 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. In the slip casting process this is a concern because the reaction product can eventually clog the pore structure of the plaster (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
Minerals Selenite
Hazards The Use of Barium in Clay Bodies
Considerations regarding the use of barium carbonate in pottery and structural clay bodies for precipitation of soluble salts.
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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