Susan Davy on tue 25 nov 97
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.
Sheryl VanVleck on wed 26 nov 97
ValCushings AA Cobalt Blue cone 10 reduction glaze
<< Cornwall stone-------46
Whiting--------------34
EPK------------------20
cobalt Carb.--------- 2
>>
Craig Martell on wed 26 nov 97
At 08:08 AM 11/25/97 EST, Susan wrote:
>----------------------------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.
>Here is the receipe:
>
>Cornwall stone-------46
>Whiting--------------34
>EPK------------------20
>cobalt Carb.--------- 2
Hi Susan:
My first inclination about the bubbling pinholes would be to look at the
Cornwall Stone. It usually contains some fluorine which can and will gas at
high temps. It has been said too that Cornwall Stone is a very inconsistent
material and will vary from batch to batch. Have you cracked a new bag of
it recently? This glaze is also too low in silica to be considered durable
and good for functional pots. It is 1.76 moles of SiO2 to 1 mole flux. The
silica for a cone 10 glaze should be at least 3.0.
The fact that you are only having problems in the lower part of the kiln
makes me think that there is a possibility of overfiring. Is the lower part
of the kiln firing a bit hotter than the top? The alumina in the glaze is
at the low limit of acceptable which could put the glaze into a narrow temp
range for maturation.
Just some thoughts, Craig Martell-Oregon
Ray Carlton on wed 26 nov 97
hi susan it should not be hard to solve your problem..the glaze is very
simple i would suggest that pinholing is caused by material within the
glaze volatising and blowing off the pot leaving small holes.. an incresae
in either your main flux [whiting] to soften the glaze a bit or incresae
your soak time because these holes will heal with more heat. also if the
glaze is a bit thin there is not enough material to melt over the pinhole.
With that amount of cobalt in te glaze your tolerance for changes to the
glaze composition should be wide. Is the are on the piece with pinholes by
chance facing directly at or above a burner??
cheers
At 08:08 25/11/97 EST, you wrote:
>----------------------------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.
>
>
raycarlt@valylink.net.au
Ray Carlton
McMahons Creek Victoria Australia 3799
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