Alum is any of various double sulphates of a trivalent metal such as chromium, aluminum, or iron and a univalent metal such as potassium or sodium, especially aluminum potassium sulphates. It is translucent in appearance and is available both as loose crystals and small solid blocks. They are soluble in water and have a primary nature to crystallize. They have an astringent, sweetish and acid taste. When heated, the crystals liquefy and on continued heating, they lose all the crystalline form and turn to a amorphous powder. Alums are useful for a range of industrial processes.