Cone 6
These pages were a study of how we substituted Boraq 1, Boraq 2 and Boraq 3 into common recipes. Boraq was developed by Plainsman Clays as a substitute for Gerstley Borate (under code number L3127E). The development effort took place during the early 2000s, the initial period when the demise of Gerstley Borate appeared imminent. Other companies, including Laguna Clays, introduced similar products at the time. Later Laguna Clays began processing a last stockpile of the material they found at the mine and interest in substitutes waned. In 2023 the cycle appears set to repeat so these pages are pertinent again.
Gerstley Borate was used to create fluid melt glazes at cone 4-6 to host the development of transparent colors and crystalization. Yellow colors can be achieved adding just rutile (up to 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. Of course leaching and hardness issues could arise with this type of glaze.
This is a variation on the 50:20:30 recipe (with 50 GB along with 15-18 kaolin and 32-35 silica) with 3-4% iron and rutile for the color and variegation.
Since this glaze is so fluid, darkly colored versions of it work well as overglazes on lighter colored more melt-stable mattes.
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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 is "tweaked" with the addition of a little lithium to variegate it 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 chart shown here compares the mole% formula of the Gerstley Borate version of the glaze with the Boraq versions.
An interesting comment: "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 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. |
We have removed a couple of oxides showing trace amounts to reduce clutter. |
 With GB
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 With Boraq1
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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.
| 2826A1 Butterscotch with GB |
2826A2 Butterscotch with Boraq 3 |
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| By Tony Hansen Follow me on          |  |
