Someone is having a problem with a cone 6 glaze going glossy and crystallizing, this article rationalizes the problem in terms of chemistry
Following is a letter we got from someone who was wondering whether INSIGHT software could help fix a glaze problem:
The main thing I am trying to fix currently is this: I have a Cone 6 base that I like very much, but it sometimes goes a little glossy and crystallizes on me. Here is the recipe:
Potash feldspar 39.0 Zinc Oxide 25.0 Whiting 16.1 Flint 14.2 Kaolin (EPK) 6.7
A friend suggested I try Neph Sy in place of the feldspar and/or replacing the Zinc Oxide with tin oxide. I can't tell where to look in the program or website to tell if either of these or both would help or if I need to do something else. Can you direct me to the right place? Thanks for you help.
If you put this recipe into INSIGHT and then open a typical cone 6 target formula beside it you will see that the zinc is more than double the recommended maximum. If you research ZnO using the Info button in the oxides dialog of INSIGHT you will see that it causes crystallization and is of course a strong flux.
CaO 0.31* MgO 0.00* K2O 0.13* ZnO 0.56* Target 0-0.2 TiO2 0.00 Al2O3 0.17 Target 0.25-0.5 SiO2 1.27 Target 2.5-4.0
You will also see that Al2O3 and SiO3 are dramatically lower than the minimum recommended. Alumina gives the glaze body and silica is the glass. If these are low the glaze will run and be soft and easily scratched and disollved by acids.
Crystalline glazes are low in alumina and high in zinc. However they need silica to form silicate crystals and the lack of silica in your glaze explains why you did not get crystals all the time. You probably got them when you cooled slower and gave them a chance to grow or when the glaze was able to obtain SiO2 from the body or an underglaze or a stain. Your glaze is probably going glossy when the circumstances are right for zinc to also impose its personality as a flux.
Your problem is a classic example of when you need to learn the function of the ten or so oxides from which the fired glass is built and to see glazes as oxides rather than materials. INSIGHT makes it easy to convert a recipe to a formula of oxides and back again.
Your friends idea of replace zinc with tin is curious. Zinc is a flux, tin is a refractory opacifier. Also Nepheline and feldspar are quite similar materials. Nepheline has more alumina so that might have impeded the crystals a bit but the glaze would still have been dramatically off balance and subject to scratching, leaching and running.
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By Tony Hansen