Potassium alum

Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound defined as the double sulfate of potassium and aluminium, with chemical formula KAl(SO4)2. It is commonly encountered as the dodecahydrate, KAl(SO4)2·12H2O. It crystallizes in an octahedral structure in neutral solution and cubic structure in an alkali solution with space group Pa3 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.

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

Antiquity

Egypt Potassium alum was also 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.

Mesopotamia According to the expert on Middle Eastern history of chemistry Martin Levey, potassium alum is one of the few compounds known to the ancients that can be found relatively pure in nature, as well as one of only a few chemicals used in Mesopotamian chemical technology that can be identified with certainty. Both native and imported potassium alum was used. Together with other agents, potassium alum was used in glass-making, tanning, and in the dyeing of cloth, wood, and possibly hair. A tanning process using potassium alum is described in tablets from the first millennium BCE. When Levey wrote his article in 1958, no description of the dyeing process had been found, so it is not known how potassium alum was used in it. In Mesopotamian medicine potassium alum was used extensively, for example against itch, jaundice, some eye condition, and unidentified ailments. According to Levey, potassium alum was used in "classical times" as a flux when soldering copper, in the fireproofing of wood, and in the separation of silver and gold, but that there is no evidence that these uses existed in Mesopotamia.

Greece 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.

Rome Potassium alum was described under the name alumen or salsugoterrae by Pliny, and it is clearly the same as the stypteria (στυπτηρία) described by Dioscorides. 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".

India and China Potassium alum is mentioned in Ayurvedic texts namely Charak Samhita, Sushurta Samhita, and Ashtanga Hridaya with the name such as sphaṭika kṣāra, phitkari or saurashtri. It is used in traditional Chinese medicine with the name mingfan .

Middle Ages 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 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 Bruges, and many 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.

Modern era

England 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 ceased following the excommunication of Henry VIII. With state financing, attempts were made throughout the 16th century, but without success until the early 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 are 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, crystallisation, 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.

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