(B2O3 , Molecular weight 69.6, Expansion: 0.031, Fusion: 577° C)
Boric oxide is the ‘Wild Card’ of glaze chemistry, acting as a flux, stabilizer, and glass former depending on firing temperature and other oxides in the glaze. It softens and deforms over a range of temperatures, from 300-700 C, and does not recrystallize on cooling, giving a smooth, glossy glass.
Boric oxide strengthens the clay/glaze interface and has a low thermal expansion, discouraging crazing. However, this effect only holds true for amounts up to 10-14%; after this point, boric oxide starts to decrease the elasticity of the cooled glaze, increasing crazing.
B2O3 is considered a neutral oxide for Seger calculations, but is an active flux at all temperatures and is a glass former at low temperatures. In fact, boric oxide can work as the primary flux in very low temperature glazes, replacing lead. It can replace some alumina at all temperature ranges, but its effectiveness decreases with higher temperatures, as it starts to contribute to fluidity. In addition, it retards crystals like alumina, but at lower temperatures.
Boron Color Response:
Boron is similar in color response to the alkalis, helping to produce bright glazes. It pushes copper to turquoise, produces more luscious copper reds, gives manganese purples, makes milky iron blues, and strengthens cobalt blue. Boron boils in the glaze at high temperatures, producing mottled surfaces.
Boric Acid (B2O3•3H2O, Molecular weight 124)
Boric Acid is expensive and absorbs atmospheric water, so it needs to be stored in a lidded container. It’s solubility makes it difficult to use in glazes, but it is widely used in frit manufacture.
Borax (Na2O•2B2O3•10K2O, molecular weight 382)
Like boric acid, borax absorbs atmospheric water, and should to be stored in a lidded container. It does help engobes stick to raw pots, though it is problematic in most glazes.
Gerstley Borate (highly variable composition, 2CaO•3B2O3•5H2O, molecular weight 206)
Gerstley Borate is no longer being mined, though several suppliers have introduced substitutes, and a supply seems to be still on the market.
A source of calcium and boron, Gerstley Borate often contained as much as 10% sodium. It is a slightly soluble material and can flocculate glazes, sometimes severely. This property was sometimes used to help suspend low-clay copper reds.
Frits (formula varies, see Frit pages)
Commercial frits are often the best source of boron for glazes.
This article taken wholly from
Grimmer, Stephen. “A Short Course on Glaze Chemistry for Ceramic Artists.” Winnipeg: 2009.
Currie, Ian. Revealing Glazes Using the Grid Method. Maryvale, Queensland, Australia: Bootstrap Press, 2000.
Currie, Ian. Stoneware Glazes : A Systematic Approach. Maryvale, Queensland, Australia: Bootstrap Press, 1986.
Hamer, Frank and Hamer, Janet. The Potter's Dictionary of Materials and Techniques. Philadelphia: University of Pennsylvania Press, 19??.
Hansen, Tony. The Magic of Fire Reference. Medicine Hat, Alberta, Canada: Digital Fire Corporation, 1998.
Rhodes, Daniel. Clay and Glazes for the Potter. New York: Chiltons, 1973.