Sodium sulfate is the sodium salt of sulfuric acid. Anhydrous, it is a white crystalline solid of formula Na2SO4 known as the mineral thenardite; the decahydrate Na2SO4.10H2O has been known as Glaubers salt or, historically, sal mirabilis since the 17th century. Another solid is the heptahydrate, which transforms to mirabilite when cooled. With an annual production of 6 million tonnes, it is a major commodity chemical and one of the most damaging salts in structure conservation: when it grows in the pores of stones it can achieve high levels of pressure, causing structures to crack.