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Orange uranium glaze,<be>MFA# 2012.1112
Byzantine plaque


A naturally radioactive metallic element. Uranium was discovered in 1781 by Sir William Herschel and isolated by Klaproth in 1789. It has an abundance of 2.1 ppm in the earth's crust and primarily occurs in pitchblende, coffinite, and carnotite ores. Uranium ores are mined in France, Zaire, Canada, South Africa, Australia, Russia, and the U.S. (Colorado, New Mexico, Utah). Uranium is a silvery white metal that is ductile and malleable. It polishes to a high gloss but tarnishes quickly in air. Uranium undergoes a series of nuclear fission reactions to eventually produce Lead. Because these reactions are used in nuclear weapons and nuclear powder plants, radioactive uranium has not been commercially available since 1944. Uranium salts and depleted uranium oxides are used as mordants for dyes, toners in photography, and pigments in Glass, paints, and ceramics. Uranium oxides produce red and orange colors at about 940 C while slightly higher temperatures give a soft yellow color (Kawai's yellow). It can turn gray or black at temperatures above 1260 C.

Uranium ore

Synonyms and Related Terms

U; uranium salts; Kawai's yellow


  • Flammable. Burns in air at 150-175 C. Powder ignites spontaneously in air.
  • Highly toxic.
  • Radioactive.
  • Contact causes burns.
  • Suspected carcinogen.

Physical and Chemical Properties

Reactive with all metals, water, acids and peroxides. Inert to alkalis.

Composition U (atomic no. 92)
CAS 7440-61-1
Melting Point 1132-1133 C
Density 19.05 g/ml
Molecular Weight atomic wt = 238.029
Boiling Point 3818 C

Resources and Citations

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 834
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • Robert Fournier, Illustrated Dictionary of Practical Pottery, Chilton Book Company, Radnor, PA, 1992
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 9990
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998