Ammonium iron(III) sulfate, NH4Fe(SO4)2·12 H2O, also known as ferric ammonium sulfate (FAS) or iron alum, is a double salt in the class of alums, which consists of compounds with the general formula AB(SO4)2 · 12 H2O. It has the appearance of weakly violet, octahedrical crystals. There has been some discussion regarding the origin of the crystals' colour, with some ascribing it to impurities in the compound, and others claiming it to be a property of the crystal itself
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.
Alum solution has the property of dissolving steels while not affecting aluminium or base metals, and can be used to recover workpieces made in these metals with broken toolbits lodged inside them. As considerable expense and/or effort may have gone into machining a specialist part this can be a worthwhile excercise.
- Alum in block form (usually potassium alum) can be used as a blood coagulant.
- Styptic pencils containing aluminium sulfate or potassium aluminium sulfate are used as astringents to prevent bleeding from small shaving cuts.
- Alum may be used in depilatory waxes used for the removal of body hair or applied to freshly waxed skin as a soothing agent.
- In the 1950s, men sporting crewcut or flattop hairstyles sometimes applied alum to their hair as an alternative topomade. When the hair dried, it would stay up all day.
- Alum's antiperspirant and antibacterial properties contribute to its traditional use as an underarm deodorant. It has been used for this purpose in Europe, Mexico, Thailand (where it is called sarn-som), throughout Asia and in the Philippines (where it is calledtawas). Today, potassium or ammonium alum is sold commercially for this purpose as a "deodorant crystal", often in a protective plastic case.
- Alum powder, found in the spice section of many grocery stores, may be used in pickling recipes as a preservative to maintain fruit and vegetable crispness.
- Alum is used as the acidic component of some commercial baking powders.
- Alum was used by bakers in England during the 1800s to make bread whiter. The Sale of Food and Drugs Act 1875 prevented this and other adulterations.
- In Nigeria, it is used in the removal of snail slime before cooking
- Solutions containing alum may be used to treat cloth, wood, and paper materials to increase their resistance to fire.
- 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 ofsedimentation.
- Alum is an ingredient in some recipes for homemade modeling compounds intended for use by children. (These are often called "play clay" or "play dough" for their similarity to "Play-Doh", a trademarked product marketed by American toy manufacturer Hasbro).
- Alum is used in the tanning of animal hides to remove moisture, prevent rotting, and produce a type of leather.
- Alum has been used as an adjuvant to increase the efficacy of vaccines since the 1920s. See adjuvant for details on the mechanism.
- Alum can be used as a coagulant to help stop internal bleeding of organs. Its mechanism of action is believed to be due the same preservative properties for which it is used in food storage, by causing shrinkage of the pores and decreasing humoral secretions.
- Alum is used to fix pigments on a surface, for example in paper marbling.