You know how they say things happen for a reason. Well this recent firing made me read up on a lot of stuff I'd already learned but had maybe not remembered lately. It also made me do a bit more digging to find some things I did not know.
For some time now I've been interested in doing crystalline glazing in my studio. In fact it was Wally Schwab who first introduced me to crystalline glazes when I studied under him at PCC Rock Creek. I'd also done quite a lot of research into microcrystalline glazes at MHCC with Don Sprague. Since then some advances have been made by other potters to fire these glazes at cone 6. When I looked at these before I'd found a cone 1 crystalline glaze as well, but had lost the recipe. Well I found it yesterday, so I'll be tinkering with that one soon thanks to Charlie Cummings post on Clay Art Thread. I also did some research into the firing schedules and found out that the crystallization is happening upon cooling, but a very slow cooling where the kiln is held at the 1830 - 1900 degree mark for four hours. What's interesting is this is when the iron in cone 5 iron reds is held to facilitate crystallization as well.
As you may know there is a huge difference between Orton cones. The large ones drop much faster than the small ones, in fact a large cone 6 drops at 2228 where a small doesn't drop until 2291. Sixty three degrees doesn't seem like a lot when you're talking about thousands of degrees, but it's enough that it could make a glaze go from stable to runny. It's not just the temperature but also the time it takes to get to the higher temperature.
Charlie Cummings
Macrocrystalline Cone 1
Frit 3110 - 43
Zinc Oxide - 23
Silica - 16
Lithium - 9
Laguna Borate - 9
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