Aluminum potassium sulfate

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Efflorescence on ceramic plate


A white odorless powder with transparent crystals. Aluminum potassium sulfate occurs naturally in the minerals Alunite and leucite. It has been used since ancient times as a Mordant in dyeing textiles and for tawing skins. Aluminum potassium sulfate, or potash alum, is also used as a filler in Paper, Cement, and paints. It is used to harden Gelatin, Plaster, and cement. Potash alum has also been used as a substrate in the preparation of lake pigments.

Chemical structure

Aluminum potassium sulfate.jpg

Synonyms and Related Terms

potassium aluminum sulfate; aluminum potassium sulphate (Br.); potash alum; alum NF; potassium alum; kalinite; alum flour; alum meal; cube alum; alumstone; common alum


  • Noncombustible.
  • Harmful by ingestion and inhalation.
  • Contact causes irritation.
  • Fisher Scientific: SDS

Physical and Chemical Properties

Soluble in water. Insoluble in ethanol.

Composition Al2(SO4)3-K2SO4-24H2O
CAS 7784-24-9
Mohs Hardness 3.5 - 4.0
Melting Point 92 C
Density 1.75 g/ml
Molecular Weight mol. wt. = 474.38

Resources and Citations

  • Irene Bruckle, "The Role of Alum in Historical Papermaking", Abbey Newsletter, Volume 17(4), September 1993. Link
  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 32, 33
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • Hermann Kuhn, Conservation and Restoration of Works of Art and Antiquities, Butterworths, London, 1986
  • Rosalie Rosso King, Textile Identification, Conservation, and Preservation, Noyes Publications, Park Ridge, NJ, 1985
  • Palmy Weigle, Ancient Dyes for Modern Weavers, Watson-Guptill Publications, New York, 1974
  • 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
  • Michael McCann, Artist Beware, Watson-Guptill Publications, New York City, 1979
  • Book and Paper Group, Paper Conservation Catalog, AIC, 1984, 1989
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983
  • A Glossary of Paper Conservation Terms, Margaret Ellis (ed.), Conservation Center of the Institute of Fine Arts, New York City, 1998