Alum, also known as alum, raw alum and alum. Colorless cubic octahedral crystal or monoclinic cubic crystal. Tasteless, slightly sweet or slightly astringent. The density is1.757g/cm3. The melting point is 92.5℃. At 64.5℃, nine crystal waters disappeared, and at 200℃, all crystal waters disappeared, turning into white sponge-like anhydrous gypsum and white powdered alum. The melting point is 92.5℃. Soluble in water (12.2%, 25℃), the pH value of 1% aqueous solution is 4.2, and it is hydrolyzed into colloidal precipitation of aluminum hydroxide in water, which is almost insoluble in ethanol. Very soluble in hot water, soluble in cold water and dilute acid, insoluble in ethanol and acetone. It is made of natural alunite through crushing, roasting, dehydration, leaching, precipitation and crystallization. It can also be prepared by decomposing bauxite powder with sulfuric acid to generate aluminum sulfate solution, reacting with potassium sulfate, filtering, crystallizing, centrifugal dehydration and drying. The paper industry is used as a settling agent for rosin size, a settling agent for purifying turbid water, a hardening agent for photographic paper, a foaming agent for foam rubber, a conductive agent for electro-zinc plating, a mordant for printing and dyeing, and an anti-discharge corrosion agent. Can be used for astringent cosmetics. Medicine is used for astringency, emesis and hemostasis. Used as a loosening agent in food.