alum

Alum Bay on the Isle of Wight, a site of great natural beauty, famed for its coloured sands. Alum, a double sulphate of aluminium and potassium used in papermaking, tanning, and medicine, was first mined in the area during the reign of Elizabeth I.

Any double sulphate of a monovalent metal or radical (such as sodium, potassium, or ammonium) and a trivalent metal (such as aluminium, chromium, or iron). The commonest alum is the double sulphate of potassium and aluminium, K₂Al₂(SO₄)₄.24H₂O, a white crystalline powder that is readily soluble in water. It is used in curing animal skins. Other alums are used in papermaking and to fix dyes in the textile industry.

The raw material of manufacture of common alums is alum rock, composed chiefly of alunite or alum stone. This is mixed with fuel in a furnace and after roasting exposed to the air. The mass is then leached with hot water, the clear liquid drawn off and allowed to crystallize. Alum is also made from alum shale, which is either allowed to decompose by exposure, or roasted. During the process, free sulphuric acid is formed, which acts upon the clay, producing aluminium sulphate, which is then dissolved out. Potassium sulphate or ammonium sulphate is added to the solution, when potash alum or ammonia alum is produced.

Most alums are acidic, but, if an alkali is slowly added to the solution, a precipitate is formed which is redissolved on stirring. If this is done until no more precipitate can be dissolved, the product is a neutral alum, which is much used in dyeing, as it helps the dye to adhere to the fabric.