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Home | Overview | Description | Substitutes | Boraq | Glaze Recipes | Learning | Contact

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

Amber, Honey, Butterscotch, Warm Brown Fluids

Gerstley Borate is used to create very fluid base glazes at cone 4-6 to host the development of crystals (from coloring oxides and rutile) and variegation (because of the phase and opacity discontinuities that occur with additions like lithium, tin, and titanium). Yellow colors can be achieved using additions of rutile (eg. 5%) and these can be darkened with iron oxide (fluid transparent boron glazes can support significant amounts of iron and yet still be quite light in color). Very small amounts of lithium can have a remarkable effect on rutile iron versions, transforming them into a much more interesting variegated surface. Needless to say, these glazes tend to run off pots and pool in the bottoms of bowls. Also be aware that there are solubility and hardness issues with this type of glaze (you can do simple tests to check leaching and hardness).

We have separated this recipe from the page of transparent base recipes for its simplicity and popularity. This particular iron based variation is known by many names, but all are essentially the same base recipe, 50 GB along with 15-18 kaolin and 32-35 silica. All use 3-4% iron and rutile for the color and variegation. This simple base transparent recipe appears to have been developed by many people independently (a testament to the fact that it is likely an optimal mix).

Since this glaze is so fluid darkly colored versions of it work well as overglazes on lighter colored glazes, especially mattes. The contrast of a variegated highly colored glossy glaze that 'feathers into' a pastel matte is a very effective decorative technique.

Conclusions from Feedback 

The upper two cone 5 porcelain tiles show the maximum difference we have been able to achieve using Boraq 1 for GB (most are a much better match). This variation on the 50:35:15 base is 'tweaked' with the addition of a little lithium to variegate it even more than the rutile:iron mix will do alone. While the Boraq 1 glaze is more interesting, its darker color is curious. The GB version goes on much thinner and takes longer to dry (the shade of  transparent colors varies with thickness).

The chart shown here compares the mole% formula of the Gerstley Borate version of the glaze with the Boraq versions. 

Here is one interesting comment we got: "I would think the Mg and Ca would have a bleaching and "greening" effect on the iron in this glaze. Dropping the Al2O3 and B2O3 content of Boraq glaze may help to bring about a more generalized opacity helping to lighten the colour." This supports the idea of dropping the amount of Boraq 1. However notice the 80% Boraq 1 sample. It is much darker. Also notice that the Boraq 2 version supplies plenty of CaO and MgO and not as much boron. It is almost identical.

 G2826AGB Mole%Boraq1 Mole%80% as much Boraq1Boraq2 Mole%
CaO 13.99 11.27 8.83 15.01
Li2O 0.97 0.97 0.95 0.96
MgO 3.11 1.93 1.52 1.71
K2O 0.14 0.26 0.22 0.23
Na2O 2.60 1.97 1.55 1.75
TiO2 3.57 3.56 3.49 3.55
Al2O3 4.97 5.47 6.07 5.35
B2O3 14.81 17.52 13.70 15.53
SiO2 53.95 55.09 61.76 53.98
Fe2O3 1.84 1.83 1.80 1.82

We have removed a couple of oxides showing trace amounts to reduce clutter.
Also note that the last digit of precision is not significant.
Click here to learn about Mole% calculations.

With GB

With Boraq3

With Boraq1

With 80% as
much Boraq1

This glaze is really quite puzzling. Although we have a good match with Boraq 2 and Boraq 3 it would be good to understand what is happening with the others. All of these are white porcelain tiles. Do you have any idea why the 80% Boraq 1 is so dark?

Here is an interesting comment from Rick Shanks: Even though the analysis indicates little difference in CaO:MgO ratios, I think if you do a line blend that increases MgO and decreases CaO you will find browner iron colors as MgO increases in relation to CaO, regardless of total MgO+CaO. Eliminating CaO as much as raw material choises will allow for the high MgO end member and eliminating MgO as much as raw material choices will allow for the high CaO end member and also trying to match flow of both end members and reference GB glaze would constitute a fair and revealing evaluation using the line blend method. Of course, the higher the SiO2:Al2O3 ratio the more phase separation or milky white opalescence. This will opacify/whiten the color but all things being equal higher MgO:CaO ratio will give browner iron color. If the SiO2:Al2O3 ratio is high enough (even without changing total refractory / SIO2+Al2O3), the glazes will be nearly white or solid opaque phase. Also the phase separation of the "80% as much Boraq1" isn't flowing or floating downward as much as the other samples, indicating less flow or fluidity. Fluidity is one of the most important variables to consider or reference standards to measure against when trying to duplicate effects with the "marginal" glazes that potter's envy.

HTML Recipe report from INSIGHT 5.3

Butterscotch with GB
Butterscotch with Boraq 3
Analysis Mole%
CaO 0.67 10.1 14.0
Li2O 0.05 0.4 1.0
MgO 0.15 1.6 3.1
K2O 0.01 0.2 0.1
Na2O 0.12 2.1 2.6
TiO2 0.17 3.7 3.6
Al2O3 0.24 6.5 5.0
B2O3 0.71 13.3 14.8
SiO2 2.59 41.8 54.0
Fe2O3 0.09 3.8 1.8
  LOI 16.4  

Analysis Mole%
CaO 0.68 8.0 11.3
Li2O 0.06 0.4 1.0
MgO 0.12 1.0 1.9
K2O 0.02 0.3 0.3
Na2O 0.12 1.6 2.0
SrO 0.01 0.1 0.1
TiO2 0.22 3.6 3.6
Al2O3 0.33 7.1 5.5
B2O3 1.06 15.5 17.5
SiO2 3.34 42.1 55.1
Fe2O3 0.11 3.7 1.8
  LOI 16.4  

Tony Hansen

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