Gypsum

Description

Gypsum is a soft, crystalline sulfate mineral characterized by the chemical composition calcium sulfate dihydrate (CaSO4·2H2O). Formed as an evaporite or through the hydration of anhydrite, it exhibits a hardness of 2 on the Mohs scale and appears in various forms, including selenite. In ceramics, gypsum is primarily known as the raw material for plaster; however, its application in glazes is strictly avoided due to its chemical behavior. During firing, the mineral releases sulfur trioxide (SO3), which poses health risks and compromises the physical integrity and aesthetic quality of the glaze surface. Gypsum also undergoes two distinct dehydration phases that release water vapor at temperatures below 500°C. In clay bodies, gypsum frequently acts as a problematic, partially soluble impurity. It is responsible for efflorescence, where soluble salts migrate to the surface during the drying phase, resulting in post-firing discoloration. Ceramicists often mitigate this effect by introducing barium carbonate to precipitate the gypsum. Furthermore, in slip casting, gypsum can interact with soda ash or sodium silicate through calcium-for-sodium ion exchange, creating products that may obstruct the porous structure of plaster molds, though this is manageable through the application of polyacrylate deflocculants.