Ammonium alum is a NH<sub>4</sub>Al(SO<sub>4</sub>)<sub>2</sub> 12H<sub>2</sub>O, a white crystalline double sulphate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in deodorants (though potassium alum is more commonly used), in tanning, dyeing and in fireproofing textiles.