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This is G2826A3, a transparent amber glaze at cone 6 on white (Plainsman M370), black (Plainsman 3B + 6% Mason 6666 black stain) and red (Plainsman M390) stoneware bodies. When the glaze is thinly applied, it is transparent. But at a tipping-point-thickness, it generates boron-blue that transforms it into a milky white. Glazes that are very glassy but on the edge of structural instability do this. So they are not good for functional ware.
This is an adjustment to the 50:30:20 Gerstley Borate base recipe (historically used for reactive glazes, often on functional surfaces! This cuts B2O3 and adds significant SiO2. But it still has double the boron of a typical functional glaze. While the chemistry of the original was within the territory of boron blue development (relatively low Al2O3), this one is better because of the increased SiO2 (the high MgO:CaO ratio is likely also helping). Boron blues like the lower Fe2O3 content or Gillespie Borate. One more factor: I am using 325 mesh silica here, it dissolves in the melt better.

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The melt fluidity tester was fired at cone 6. The glaze on the left is G2826A2, the 50:30:20 historically popular Gerstley Borate base recipe used for reactive and transparent glazes. The B2O3 in that glaze was 3x the normal amount in cone 6 glazes, making it so melt fluid it can eat through a firebrick!
The glaze on the right is G2926A3, an adjusted version that cuts the B2O3 level and adds SiO2 (from silica and nepheline). The result is more sane, although still very melt-fluid glaze. This is also a lesson in the chemistry that produces boron-blue: High B2O3 is not the key; my adjustment lowers it significantly. CaO/MgO is also lower, so that is not the key. The SiO2 appears to be the enabler here; it is much higher (from 2.6 to 3.48, a hugh increase). And, I am using 325 mesh silica, so it dissolves in the melt better, delivering even more SiO2. Boron blue thus seems to thrive on enough SiO2 coupled with high B2O3 and low Al2O3 with some MgO/CaO.
| Glossary |
Boron Blue
Boron blue is a glaze fault involving the crystallization of calcium borate. It can be solved using glaze chemistry. |
| Glossary |
Glaze thickness
Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed. |
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