Alums are useful for a range of industrial processes. They are soluble in water; have an astringent, acid, and sweetish taste; react acid to litmus; and crystallize in regular octahedra. When heated they liquefy; and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains. Industrial uses
Potassium alum is the common alum of commerce, although soda alum, ferric alum, and ammonium alum are manufactured.
Alum has been used at least since Roman times for purification of drinking water and industrial process water. Between 30 and 40 ppm of alum for household wastewater, often more for industrial wastewater, is added to the water so that the negatively charged colloidal particles clump together into "flocs", which then float to the top of the liquid, settle to the bottom of the liquid, or can be more easily filtered from the liquid, prior to further filtration and disinfection of the water.
- Solutions containing alum may be used to treat cloth, wood, and paper materials to increase their resistance to fire.
- Alum is also a component of foamite.
- Alum is also used in fire extinguishers to smother chemical and oil fires.
- Alum is used to clarify water by neutralizing the electrical double layer surrounding very fine suspended particles, allowing them to flocculate (stick together). After flocculation, the particles will be large enough to settle and can be removed.
- Alum may be used to increase the viscosity of a ceramic glaze suspension; this makes the glaze more readily adherent and slows its rate of sedimentation.
- Alum is an ingredient in some recipes for homemade modeling compounds intended for use by children.