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
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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 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.
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| G2826A | GB Mole% | Boraq1
Mole% | 80% as much Boraq1 | Boraq2
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.
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With GB
With Boraq3
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With Boraq1
With 80% as
much 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.
HTML Recipe report from INSIGHT 5.3
2826A1
Butterscotch with GB |
2826A2
Butterscotch with Boraq 3 |
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