Alum

Alum

Description

A general name used for double sulfate salts of aluminum. The most common alums are double salts with potassium [KAl(SO₄)₂·12 H₂O], sodium [NaAl(SO₄)₂·12 H₂O] or ammonium [NH₄Al(SO₄)₂·12 H₂O]. All of these have some properties in common, such as they are water-soluble and have a sweet taste. Additionally they react as an acid by turning blue litmus paper to red. Uses for the specific types of alum salts include:

• Potash alum, refers to Aluminum potassium sulfate. It is a colorless, crystalline material used in the tawing of skins and as a mordant, or fixative, for many natural dyes. Potash alum is also used as the base for most Lake pigments.
• For papermakers, the term alum, refers to Aluminum sulfate or the mixture of aluminum sulfates obtained by treating pulverized Bauxite with Sulfuric acid.
• In cooking, the term alum refers to Aluminum ammonium sulfate. It is used in baking powder where it acts as a whitener and drying agent in wheat flour. It also acts as a flocculant to clarify cloudy liquids.
• Other materials referred to as alum are Ferric ammonium sulfate, chromium sodium sulfate, and chromium ammonium sulfate.

FTIR (MFA)

Synonyms and Related Terms

aluminum potassium sulfate; aluminum ammonium sulfate; aluminum sulfate; alum cake; alunite

Risks

• Alums will corrode metal

Resources and Citations

• Irene Bruckle, "The Role of Alum in Historical Papermaking", Abbey Newsletter, Volume 17(4), September 1993. Link
• R. J. Gettens, G.L. Stout, Painting Materials, A Short Encyclopaedia, Dover Publications, New York, 1966 Comment: aluminum hydrate: uses the term 'alum'
• G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 32
• The Dictionary of Paper, American Paper Institute, New York, Fourth Edition, 1980
• E.J.LaBarre, Dictionary and Encyclopedia of Paper and Paper-making, Swets & Zeitlinger, Amsterdam, 1969
• Matt Roberts, Don Etherington, Bookbinding and the Conservation of Books: a Dictionary of Descriptive Terminology, U.S. Government Printing Office, Washington DC, 1982
• Susan E. Schur, Conservation Terminology: A review of Past & Current Nomenclature of Materials, Technology and Conservation, Spring (p.34-39); Summer (p.35-38); Fall (p.25-36), 1985
• Wikipedia: Alum Accessed March 2025.
• Bill Bryson, ‘At Home: A Short History of Private Life’ Doubleday Publishers, London 2010.