Purple Amethyst-colored Alum geodes about 60 mm in diameter.

lums are chemical compounds, double salts of sulfates. The group of compounds is named after the substance KAl (SO4) 2.12H2O, which was already known as alum in ancient times. Without further specification, alum is therefore understood to mean potassium aluminum alum. In the formula of alums it is customary to place the monovalent ion in front, followed by the trivalent ion. The systematic naming is unambiguous: the ions are arranged alphabetically, followed by sulphate and the indication for the number of water molecules. Trivially, the alums are distinguished from each other by having the name of the monovalent cation followed by that of the trivalent ion. "Alum" closes the name, the water molecules are not explicitly mentioned. The Romans already knew alum and used it as a mordant to better adhere the precious purple dye to the mantles of the most important senators. This alum was mainly extracted in certain mountainous regions of Asia Minor. In the 15th century, a certain Bartholomeus Perdix mentions alum extraction on the island of Ischia. Giovanni de Castro founded an alum purification plant in the Vatican in 1458, which gave the Pope a monopoly on the alum trade in Europe for several decades. From 1471 onwards, the Medici undertook the exploitation of alum deposits near Volterra in Tuscany, the most famous application in everyday life being on the sink of many a man. Alum is used as a styptic by men who have cut themselves while shaving. This is because it binds to proteins of the damaged skin and makes them contract (astringent effect), which stops the bleeding.

Other applications: ingredient in the preparation of paper, it lowers the acidity (pH), removal / dissolving of coatings, mordant in dyeing textiles, tanning leather, antiperspirant in deodorants, disinfecting wounds (was used as an after shave) , provides relief for sore throats or mouth ulcers, killing algae in ponds.