Craig Martell on sat 11 jul 98
Hi:
Some of the most frequently discussed glaze problems on Clayart involve
Gerstley Borate. So here are some of my thoughts about the stuff for any of
you that are interested.
The two main problems with GB seem to be: 1) It's highly variable in
analysis and 2) It will release soluble alkalis into the glaze batch and
cause flocculation. Bummer! So what can be done about this??
Well, you can use another material such as a calcium-borate frit or another
raw material such as Cadycal 100. A note about Cadycal. While cadycal and
GB have about the same fusion rate, GB contains more boron and sodium and
will give more of an alkaline color response. Actually, Cadycal has
practically no sodium at all. I tested some glazes with direct subs of
Cadycal for GB and the color response was affected. With GB, cobalt and
chrome mixes produced a nice teal blue but with Cadycal, the color was
simply green. Cadycal is reported to hold a true analysis and is being used
by Ferro (so I'm told) for boron frit manufacture. It's more expensive than
GB, but a bit less than frits.
When using GB for glazes, you could mix only enough glaze for your current
batch of pots. It takes a bit of time for the solubles to flocculate the
glaze so it's not an immediate problem. There will probably be some glaze
left from the glazing session and you can add it to the next fresh mix.
One could "wash" the solubles out of the GB as is often done with ash
glazes. Weigh out the proper amount of GB for your glaze and hydrate it with
plenty of water, stir it really well, and let it sit for a day or two, or
even a week if you have the time. Decant the water and then mix in the rest
of the glaze materials. If a quick wash like this doesn't do the job, wash
the stuff twice, or three times, the next time you mix glaze. Of course the
drawback here is that you have to do some advance planning and can't mix at
the last minute. Washing out soluble alkalis will lower the fluxing action
of the GB a bit so you might have to compensate by increasing the percentage
in the glaze. This will have to be determined by trial and error. I've had
lots of flocculation problems with my ash glazes over the years and am now
washing the ash, which totally eliminates this problem. I can store the
glazes, fully hydrated, indefinitely with no flocculation occuring. I think
this process could be used with GB as well.
just some thougnts on a beaten to death topic, Craig Martell-Oregon
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