Alum

Alum, in its commonest sense, is potassium alum: a double sulphate of potassium and aluminium, usually met as a hydrate holding twelve molecules of water, which forms white, octahedral crystals that dissolve readily and taste astringent. It turns up in nature as crusts of the mineral alum-(K) on weathered rock, and was long made from alunite; today it is manufactured from potassium and aluminium sulphates. It is one of the few compounds the ancients could find fairly pure, and Mesopotamian craftsmen used it in glass-making, tanning, dyeing and medicine, while Pliny knew it as alumen. For centuries it fixed dyes in wool, and the trade in it was one cause of war between Genoa and Venice. It still clears cloudy water, tans leather, fireproofs cloth and leavens some baking, and a block of it is rubbed on the skin after shaving or sold as a deodorant crystal. Cunningham gives alum to Saturn and to Earth and names protection as its one power: it has been worn as an amulet against evil in Egypt and along the north coast of Africa, a piece is kept in the home to guard it, and small amounts are sewn into children's hats to protect them.