Potassium alum, potash alum, or potassium aluminum sulfate is a chemical compound: the double sulfate of potassium and aluminum, with chemical formula KAl(SO4)2. It is commonly encountered as the dodecahydrate, KAl(SO4)2·12H2O. It crystallizes in cubic structure with space group P a -3 and lattice parameter of 12.18 Å. The compound is the most important member of the generic class of compounds called alums and is often called simply alum.

Potassium alum is commonly used in water purification, leather tanning, dyeing, fireproof textiles, and baking powder as E number E522. It also has cosmetic uses as a deodorant, as an aftershave treatment and as a styptic for minor bleeding from shaving.


In antiquity

Potassium alum was known to the Ancient Egyptians, who obtained it from evaporites in the Western desert and reportedly used it as early as 1500 BCE to reduce the visible cloudiness (turbidity) in the water.

Potassium alum was described under the name alumen or salsugoterrae by Pliny, and it is clearly the same as the stupteria described by Dioscorides.[10] However, the name alum and other names applied to this substance — like misy, sory, chalcanthum, and atramentum sutorium — were often applied to other products with vaguely similar properties or uses, such as iron sulfate or “green vitriol”.

The production of potassium alum from alunite is archaeologically attested on the island Lesbos. This site was abandoned in the 7th century but dates back at least to the 2nd century CE.

Potassium alum is mentioned in the Ayurveda with the name phitkari or saurashtri. It is used in traditional Chinese medicine with the name mingfan

In the middle and modern ages

Potassium alum was imported into England mainly from the Middle East, and, from the late 15th century onwards, the Papal States for hundreds of years. Its use there was as a dye-fixer (mordant) for wool (which was one of England’s primary industries, the value of which increased significantly if dyed).

These sources were unreliable, however, and there was a push to develop a source in England especially as imports from the Papal States were ceased following the ex-communication of Henry VIII.

Historically, potassium alum was used extensively in the wool industry from Classical antiquity, during the Middle Ages, and well into the 19th century as a mordant or dye fixative in the process of turning wool into dyed bolts of cloth.

In the 13th and 14th centuries, alum (from alunite) was a major import from Phocaea (Gulf of Smyrna in Byzantium) by Genoans and Venetians (and was a cause of the war between Genoa and Venice) and later by Florence. After the fall of Constantinople, alunite (the source of alum) was discovered at Tolfa in the Papal States (1461). The textile dyeing industry in Bruge, and many other locations in Italy, and later in England, required alum to stabilize the dyes onto the fabric (make the dyes “fast”) and also to brighten the colors.

With state financing, attempts were made throughout the 16th century, but without success until early on in the 17th century. An industry was founded in Yorkshire to process the shale, which contained the key ingredient, aluminium sulfate, and made an important contribution to the Industrial Revolution. One of the oldest historic sites for the production of alum from shale and human urine is the Peak alum works in Ravenscar, North Yorkshire. By the 18th century, the landscape of northeast Yorkshire had been devastated by this process, which involved constructing 100-foot (30 m) stacks of burning shale and fuelling them with firewood continuously for months. The rest of the production process consisted of quarrying, extraction, steeping of shale ash with seaweed in urine, boiling, evaporating, crystallization, milling and loading into sacks for export. Quarrying ate into the cliffs of the area; the forests were felled for charcoal and the land polluted by sulfuric acid and ash.


Medicine and cosmetics

An alum block sold as an astringent in pharmacies in India (where it is widely known as Fitkiri (Bengali), Fitkari (Hindi) or Phitkari (Urdu)

Potassium alum is used in medicine mainly as an astringent (or styptic) and antiseptic.

Styptic pencils are rods composed of potassium alum or aluminum sulfate, used topically to reduce bleeding in minor cuts (especially from shaving) and abrasions, nosebleeds, and hemorrhoids, and to relieve pain from stings and bites. Potassium alum blocks were rubbed over the wet skin after shaving.

Potassium alum was also used topically to remove pimples and acne, and to cauterize aphthous ulcers in the mouth and canker sores, as it has a significant drying effect to the area and reduces the irritation felt at the site. It has been used to stop bleeding in cases of hemorrhagic cystitis.[35] and is used in some countries as a cure for hyperhidrosis.

It is used in dentistry (especially in gingival retraction cords) because of its astringent and hemostatic properties.

Potassium and ammonium alum are the active ingredients in some antiperspirants and deodorants, acting by inhibiting the growth of the bacteria responsible for body odor. 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 called tawas). Today, potassium or ammonium alum is sold commercially for this purpose as a “deodorant crystal”. Beginning in 2005 the US Food and Drug Administration no longer recognized it as a wetness reducer, however, it is still available and used in several other countries, primarily in Asia.

Potassium alum was the major adjuvant used to increase the efficacy of vaccines and has been used since the 1920s. But it has been almost completely replaced by aluminium hydroxide and aluminium phosphate in commercial vaccines.

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 to pomade, to keep the hair standing up.


Potassium alum may be an acidic ingredient of baking powder to provide a second leavening phase at high temperatures (although sodium alum is more commonly used for that purpose.

Alum was used by bakers in England during the 1800s to make bread whiter. This was theorized by some, including John Snow, to cause rickets. The Sale of Food and Drugs Act 1875 prevented this and other adulterations.

Potassium alum, under the name “alum powder”, is found in the spice section of many grocery stores in the US. Its chief culinary use is in pickling recipes, to preserve and add crispness to fruit and vegetables.

Flame retardant

Potassium alum is used as a fire retardant to render cloth, wood, and paper materials less flammable.


Potassium alum is used in leather tanning, to remove moisture from the hide and prevent rotting. Unlike tannic acid, alum doesn’t bind to the hide and can be washed out of it.