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INSIGHT Software, the best way to deal with this: Watch the Gerstley Borate video here.

Low Fire Base Glazes

Since Gerstley Borate has a high LOI it tends to create bubbles and imperfections in low fire glazes. While this is not considered a problem in raku, generally people want a clear bright transparent base for electric fired earthenware. Thus it is generally better to use frit instead of GB in these glazes.

One case where Gerstley Borate is necessary is where frits do not contain enough boron for the glazes needs. Gerstley Borate is much higher in boron than almost any frit available and thus it will make more fluid and earlier melting glazes. We have found that Boraq 1 works well as a substitute for GB is these situations.

Frit GB Low Fire Base

For example, this glaze has a mix of GB and a boron frit along with nepheline syenite, kaolin, silica and a little lithium carbonate. The Boraq 1 version has a smoother surface with better color. The GB version has a bit more of a satin surface, if this is important you could use Boraq 2. In this sample we have added 8% iron to get an unexpectedly bright maroon color.

GB Low Fire Base
Boraq1 Low Fire Base
Analysis Mole%
CaO 0.51 9.3 12.3
Li2O 0.15 1.5 3.8
MgO 0.06 0.9 1.6
K2O 0.04 1.2 0.9
Na2O 0.24 4.8 5.8
Al2O3 0.32 10.9 7.9
B2O3 0.47 10.8 11.5
SiO2 2.16 42.5 52.7
Fe2O3 0.14 7.4 3.4
  LOI 10.7  

Analysis Mole%
CaO 0.49 8.3 11.0
Li2O 0.17 1.5 3.8
MgO 0.04 0.5 1.0
K2O 0.04 1.3 1.0
Na2O 0.25 4.6 5.5
Al2O3 0.37 11.2 8.2
B2O3 0.58 12.1 12.9
SiO2 2.40 43.1 53.2
Fe2O3 0.15 7.4 3.4
  LOI 9.9  


For more information see the page on Raku glazes.

Tony Hansen

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