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Mineral: 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).

Out Bound Links

  • (Minerals) Limestone, Calcium Carbonate

    Also called GCC (Ground Calcium Carbonate), limestone is a very common sedimentary rock. Calcite and Aragonite minerals are the pure crystalline forms of CaCO3 (limestone contains them), but limestone...

  • (URLs) Gypsum at Wikipedia


  • (Hazards) The Use of Barium in Clay Bodies

    Hazards of barium carbonate, considerations regarding its use in clay bodies for precipitation of soluble salts

  • (Glossary) Efflorescence

    A term describing the whitish or brownish dry or glassy scum (depending on iron content and firing temperature) left on the surface of a fired clay body (most often red earthenware or raw stoneware and fireclays). Many clays contain soluble sulphates that migrate to the surface with the water and ar...

  • (URLs) Gypsum mineral data


  • (URLs) Calcium sulfate at Wikipedia


  • (Materials) Barium Carbonate - BaCO3

    Barium Carb, Witherite

In Bound Links

  • (Minerals - Related) Selenite

    A transparent crystal form of the mineral gypsum. This can occur in clays and is the cause of efflorescence.

  • (Temperatures) Calcium Sulphate decomposition (80C-250C)

    Water vapor is expelled. Many clays contain small amounts of calcium sulphate.

By Tony Hansen

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