sodium sulfate, chemical compound, Na2SO4. It is a white, orthorhombic crystalline compound at ordinary temperatures; above 100°C it assumes a monoclinic structure, and above about 250°C it assumes a hexagonal structure. Sodium sulfate is soluble in cold water and very soluble in hot water. It forms two hydrates; the decahydrate is Glauber's salt. Anhydrous sodium sulfate is found in nature as the mineral thenardite. The major commercial source of sodium sulfate is salt cake, a byproduct of the production of hydrochloric acid from sodium chloride (common salt) by treatment with sulfuric acid. It is obtained (with other chemicals) by evaporation of natural brines. It is also obtained as a byproduct of viscose rayon manufacture and in several other, less important ways. The principal use of sodium sulfate is in processing wood pulp for making kraft paper. It is also used in glass manufacture, textile dyeing, and synthetic detergents.