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

Raku Base Glazes

Since Gerstley Borate has a high LOI and it seems logical that it would tend to create bubbles and imperfections in low fire glazes. However in practice it is amazing how transparent and crystal clear some of the glazes can be. It is common to see Gerstley Borate in low fire glazes that also contain significant amounts of boron frit. Boraq 1 is highly likely to work well in these recipes. Since its borate content is slightly higher it may melt a little better. In addition it will lower the thermal expansion of glazes slightly making crazing less likely or less severe.

Circumstances where the GB amount is not too high and there is plenty of boron frit in the recipe provide a perfect opportunity to use ceramic calculations to remove the GB completely and use frit to supply all of the needed boron. However you will only be able to do this if you employ frits that are low in alumina so that at least 10-15% kaolin can be included to suspend the recipe (kaolin supplies alumina).

INSIGHT 5x Comparison for 2826B

This recipe has a 60% frit mixture, 20% GB, along with a little wollastonite and 15% kaolin to suspend the slurry. It has some suspended bubbles in the glass and crazing is not too severe. Adding 1-5% silica would likely eliminate the crazing.

06-05 Raku Base with GB
06-05 Raku Base with Boraq 1
Analysis Mole%
CaO 0.72 14.4 18.0
MgO 0.05 0.7 1.2
K2O 0.01 0.5 0.3
Na2O 0.22 4.8 5.4
Al2O3 0.33 11.9 8.1
B2O3 0.61 15.2 15.2
SiO2 2.07 44.5 51.6
  LOI 7.9  

This closeup shows crazing lines. Glaze is transparent and shows body color through.
Analysis Mole%
CaO 0.73 13.6 16.9
MgO 0.03 0.4 0.8
K2O 0.02 0.5 0.4
Na2O 0.22 4.5 5.1
Al2O3 0.36 12.2 8.3
B2O3 0.70 16.3 16.3
SiO2 2.25 44.9 52.1
  LOI 7.2  

Glaze is transparent and shows body color through.

Nepheline Syenite:GB Raku Base

While there are many variations on this base, the 50:50 GB:Nepheline mix is typical. We have seen versions all the way to 80% GB and 20% Nepheline. This range of blends melts very well at cone 06 and most are remarkably stable and bubble free. The higher the nepheline syenite the greater the likelihood of crazing (a desired effect in many raku applications). It should be remembered however that a very fine craze pattern will severely weaken the already weak raku ware. For mixes toward the 80:20 end we suggest adding some silica, perhaps 5-10% to reduce the crazing. This base is a good demonstration of the effect of the lower LOI and slightly better melting of Boraq 1. The Boraq version is clearer with fewer suspended bubbles and surface dimples and pinholes. The craze pattern is also slightly less severe.

GB:Nepheline Syenite Raku Base
Boraq 1:Nepheline Syenite Raku Base
  Amount %  
Gerstley Borate 50.00  
  Amount %  
Boraq 1 50.00  
Analysis Mole%
CaO 0.51 11.2 15.2
MgO 0.11 1.8 3.3
K2O 0.07 2.5 2.1
Na2O 0.31 7.6 9.3
Al2O3 0.31 12.3 9.2
B2O3 0.53 14.5 15.9
SiO2 1.50 35.2 44.8
  LOI 14.8  

Analysis Mole%
CaO 0.49 9.1 12.3
MgO 0.08 1.1 2.1
K2O 0.09 2.7 2.2
Na2O 0.34 7.0 8.6
Al2O3 0.38 13.1 9.7
B2O3 0.74 17.2 18.8
SiO2 1.82 36.5 46.1
  LOI 13.0  

A similar version of the glaze employs 80% GB and 20% Nepheline. It melts and flows much better and that above 50:50 mix and fires to a crystal clear glass. We have compared Boraq and GB and found that the Boraq 1 version crazes a little less, goes on thicker and melts to a very similar finish.

Tony Hansen

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