rballou@mnsinc.com on mon 1 dec 97
Hi Susan,
This is, perhaps, a bit late. Blame it on the turkey! Some of us on Clayart
have been doing the experiments in Ian Currie's Stoneware Glazes. And some
others of you have asked how it is going. I thought I'd apply the testing
to your glaze.
The analysis for your glaze is:
..12 KNaO Percentage analysis 4.69%
..88 Ca 24.34%
..002 MgO .05%
..35 Al2O3 18.00%
1.8 SiO2 53.00%
There's support for both of the previous comments about your glaze from the
test tiles for the experiments from Currie's book. One of the experiments
is for .9 CaO, which is very close to your glaze. This experiment varies
the alumina and silica over a series of 35 glazes. The calcium is at .9
mole in all of them. A couple of the glazes are pretty close to your glaze,
though neither is an exact match. Both show a great deal of variation in
different firings. Also, the glazes that are close to the analysis of your
glaze show a lot of vaporization of calcium on the surrounding clay.
Examination with a hand lens shows that both also have bubbles trapped
under the surface of the glaze and some of those bubbles have pinholes.
There's also a lot of crazing in all samples of these two glazes. There are
some glazes with .9 mole of calcium that look like they might be more
stable and show less vaporization. They have more alumina and silica than
your glaze (in the range of .4 to .55 moles of Al2O3 and 2.8 to 4 moles of
silica). Only more testing will show whether these glazes will look
anything like your glaze when cobalt is added (and on your clay body). In
any event, .9 mole of calcium is a lot for a glaze, over the limit for most
charts. This tile gives a practical demonstration of the meaning of that
limit. I don't see how adding even more calcium would have any benefit.
Increasing the clay in 5% increments and then increasing the silica in 10%
increments in each of those tests might.
If the Cornwall is the problem, you could reformulate this glaze to use a
pot spar. However, the glazes for these tiles used a pot spar, not Cornwall
stone. If the calcium vaporization is the problem, you could try
substituting wollastonite for some of the calcium. But judging from the
test tiles, I think this glaze is going to continue to have problems.
Perhaps a different base formula is your best bet, or at least some
substantial monkeying with this one.
Ruth Ballou
rballou@mnsinc.com
>----------------------------Original message----------------------------
>I am using Val Cushings AA cobalt blue..cone 10 glaze. The initial
>results were excellent...now, a month down the road and it is starting
>to pinhole bubble on only one side of some of my pots. I fire in a 36
>cu ft downdraft Bailey car kiln...in reduction from 1800F to cone 10
>just tipping. The bottom pots seem to be worse than those on the top.
>In fact, the top is usually pretty good.
>
>I gave up on my previous blue because it did the same thing only worse,
>and the answer I got from Clay Art was that it had zinc in it and that
>was the culprit....
>Here is the receipe:
>
>Cornwall stone-------46
>Whiting--------------34
>EPK------------------20
>cobalt Carb.--------- 2
>
>Thanks dearly in advance for any insight...Susan Davy in sunny ND, with
>a little snow on the ground.
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