Gypsum is a natural form of calcium sulfate, CaSO4, with varying degrees of water of crystallization (usually 2 H20). It is relatively insoluble in water and is the main constituent of permanent hardness in water. When in the crystalline or dry powder form, it is also known as “Alabaster” or “Plaster of Paris.”

In brewing it is perhaps best known as the main mineral in the well water of Burton Upon Trent, England, and is widely attributed to be the essential component of the water for pale ale and India pale ale brewing; as such it is the main ingredient in the Burtonizing salts for brewing water treatment. See burton-on-trent. It is added by dissolving the gypsum in the mashing and sparging water or directly as a powder into the grist or mash vessels at mashing.