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So-called opal or opalescent glazes are commonly ones having a combination of boron blue coupled with plenty of titanium or rutile to enhance the effect. However such glazes bring along significant "baggage". For example, 6-7% titanium, which, on cooling in the kiln, would matte or crystallize many glazes, can be tolerated because of the high melt fluidity. Unfortunately runny glazes bring issues for production. And, glazes of this type commonly have a high percentage of feldspar (to source the needed KNaO), unfortunately that means they will craze. Plus, the high melt fluidity needed for the effect requires a low percentage of Al2O3. That has other implications. First, hardness of the fired glaze is affected. Second, since Al2O3 is being contributed by the feldspar, there is no room left in the recipe for kaolin or ball clay (which normally sources the Al2O3). In the past, Gerstley Borate, a very active melter that is also plastic was an ideal solution. Unfortunately, Gillespie borate, the recommended substitute, does not have the same suspending properties. One solution is to source the KNaO from Ferro Frit 3110, using glaze chemistry. It has almost no Al2O3 so significant kaolin can be added to source it. However, that still does not fix the issue of crazing. An alternate solution is to find another melt fluid lower expansion base glaze and add the titanium to that.
Glossary |
Boron Blue
Boron blue is a glaze fault involving the crystallization of calcium borate. It can be solved using glaze chemistry. |
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URLs |
https://glazy.org/recipes/19676
Example of opalescent glaze on Glazy |
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