The anhydrous form occurs naturally as a rare mineral millosevichite, found for example in volcanic environments and on burning coal-mining waste dumps.

However, the name "alum" is more commonly and properly used for any double sulfate salt with the generic formula XAl(SO4)2·12H2O, where X is a monovalent cation such as potassium or ammonium.

The mass is now systematically extracted with water, and a solution of aluminium sulfate of specific gravity 1.16 is prepared.

[6] In the preparation of aluminium sulfate from clays or from bauxite, the material is gently calcined, then mixed with sulfuric acid and water and heated gradually to boiling; if concentrated acid is used no external heat is generally required as the formation of aluminium sulfate is exothermic.

[6] Aluminium sulfate is sometimes used in the human food industry as a firming agent, where it takes on E number E520, and in animal feed as a bactericide.

"[citation needed] Aluminium sulfate is used in water purification and for chemical phosphorus removal from wastewater.

It causes suspended impurities to coagulate into larger particles and then settle to the bottom of the container (or be filtered out) more easily.

Research suggests that in Australia, aluminium sulfate used in this way in drinking water treatment is the primary source of hydrogen sulfide gas in sanitary sewer systems.

The carbon dioxide generated also served to propel the foam out of the container, be it a portable fire extinguisher or fixed installation using hoselines.