Cobaltous oxide

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A dark gray crystalline powder. Cobaltous oxide was obtained by roasting native cobalt ores (cobaltite, smaltite). Commonly called zaffre, it is used to manufacture smalt, a blue pigment. Smalt is the fused product of finely ground zaffre mixed with potassium silicate (glassmakers fine white sand). In Europe, zaffre was used as early as the 14th century as a pigment for paints, glazes and glass. Cobalt oxide is used to give a blue color to structural glass, bottles, and optical filter glasses. It is also used to minimize the yellow tint from iron impurities in window glass.

Synonyms and Related Terms

cobalt(II) oxide; cobalt oxide; cobalt monoxide; óxido de cobalto (Esp., Port.); oxyde de cobalt (Fr.); ossido cobaltoso (It.); zaffre; smalt


  • Skin contact may cause allergies, especially on elbows, neck and ankles.
  • Chronic inhalation may cause asthma.
  • Ingestion may cause vomiting, diarrhea and the sensation of hotness.
  • ThermoFisher: SDS

Physical and Chemical Properties

Soluble in acids or alkalis. Insoluble in water.

Composition CoO
CAS 1307-96-6
Melting Point ~1935 C
Density 5.7-6.7 g/ml
Molecular Weight mol. wt. = 74.93

Resources and Citations

  • R. J. Gettens, G.L. Stout, Painting Materials, A Short Encyclopaedia, Dover Publications, New York, 1966
  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p.207
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • 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
  • R.D. Harley, Artists' Pigments c. 1600-1835, Butterworth Scientific, London, 1982
  • Henry Hodges, Artifacts: An Introduction to Early Materials and Technology, Ronald P. Frye, Kingston, Canada, 1988
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 2507