Calcium Oxide - CaO

Alkaline Earth Flux Unit 6 Topic 4
Oxide Qualities

Powerful flux at high temperatures - moderate expansion - bleaches red iron oxide


  • A very commonly used flux at temperatures over C.4 - 1100 deg.C
  • OK to use in small amounts for temperatures below C.4
  • Has moderate coefficient of expansion and high surface tension.
  • Adds whiteness, hardness and durability.
  • Excessive amounts cause crystal growth through devitrification (lime matts).


  • Most commonly whiting CaCO3 (limestone, calcite).
  • Fluorspar CaF2 - in limited quantities at lower temperatures (releases poisonous fluorine).
  • Dolomite CaMg(CO3)2 - also brings MgO (usually primarily a source of MgO).
  • Wollastonite CaSiO2 - also brings SiO2. Gives a good glaze at higher temperatures. Can be used to avoid problems associated with the release of CO2 from whiting.
  • Colemanite, Gerstley Borate, Calcium Borate Fritt - these are usually used as a source of B2O3
  • Bone ash Ca3(PO4)2 - usually used as a source of P2O5.

Colour Responses

  • Tends to give alkaline effects with small amounts of colourants (copper blues, cobalt mauves).
  • Can bleach red iron oxide causing yellowish colourations.
  • An excellent flux for reduced iron glazes - not suitable for oxidised tenmokus.
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