Boric Oxide - Boron - B2O3

Unit 6 Topic 13
Oxide Qualities

Low temperature melt - lowest expansion - creates own glass network.


  • A glass-former and flux at low temperatures.
  • One of the most useful oxides - a substitute for lead at low temperatures.
  • Has a very low coefficient of expansion - can replace silica to lower maturing temperature of a glaze (replace alumina in matt glazes).
  • Amounts over 15% in a glaze may increase crazing rather than inhibit it.
  • Does not crystallise in the melt and tends to inhibit crystallisation of other oxides in the glaze.
  • Boric oxide glazes were developed as an alternative to lead glazes.
  • Decreases the solubility of lead in a glaze.

Insoluble Sources

  • Commercial fritts are the most common source.
  • Gerstley Borate - mined material which tends to vary in oxide balances.
  • Colemanite 2CaO.3B2O3.5H2O - also varies and can give spitting and crawling problems due to high molecular water content.

Soluble Sources

  • Borax - Na2B4O7.10H2O
  • Boric acid - B(OH)3

Colour Responses

  • Has a strong solvent action on colourants helping to disperse colour through a glaze.
  • Excess harms underglaze reds and greens.
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