A white crystalline compound, Na₂SO₄, usually known as the anhydrous compound (orthorhombic; r.d. 2.67; m.p. 888°C) or the decahydrate (monoclinic; r.d. 1.46; which loses water at 100°C). The decahydrate is known as Glauber’s salt. A metastable heptahydrate (Na₂SO₄.7H₂O) also exists. All forms are soluble in water, dissolving to give a neutral solution. The compound occurs naturally as

mirabilite (Na₂SO₄.10H₂O),

threnardite (Na₂SO₄), and

glauberite (Na₂SO₄.CaSO₄).

Sodium sulphate may be produced industrially by the reaction of magnesium sulphate with sodium chloride in solution followed by crystallization, or by the reaction of concentrated sulphuric acid with solid sodium chloride. The latter method was used in the Leblanc process for the production of alkali and has given the name salt cake to impure industrial sodium sulphate. Sodium sulphate is used in the manufacture of glass and soft glazes and in dyeing to promote an even finish. It also finds medicinal application as a purgative and in commercial aperient salts.