A general name used for aluminium sulphate salts. Most commonly the term alum, or potash alum, refers to aluminium potassium sulphate. It is a colourless, crystalline material and is used in the pictorial technique as a mordent or an anti-decayer in natural and animal glues. In restoration, it is also utilized as a conservative, for example, in the lining mixture. Other materials referred to as alum are aluminium sulphate or the mixture of aluminium sulphates obtained by treating pulverised bauxite with sulphuric acid, ferric ammonium sulphate, chromium sodium sulphate and chromium ammonium sulphate. Alum is listed among the inorganic additives used for stucco marble preparation according to literature. See also: alum-gypsum. Synonyms: aluminium potassium sulphate; aluminium ammonium sulphate; aluminium sulphate; alum cake; alunite. Wittenburg (1999); Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]