Timothy Joko-Veltman on tue 30 jan 07
I have inherited from my mentor a cobalt glaze which has a problem I
haven't found mentioned much in the books I have nor on the internet:
it spits. By which I mean, after opening the kiln, little spots of it
are frequently found on the elements, kiln furniture, and even other
pots which are some distance away - which is, of course, why I never
used it - but it is a popular glaze with students, so I'd like to fix
it, if I can.
The materials are all Brazilian, so giving the recipe won't help much.
Here is the Unity formula, however:
0.23 CaO
0.22 MgO
0.22 KNaO
0.33 ZnO
0.32 Al2O3
2.67 SiO2
0.02 CoO
This was fired to 1265C, and ran a bit, too. Cobalt was initially
supplied with Cobalt Oxide, but we've also tried Cobalt Carbonate,
without much luck (though that gave us a much more even colour).
I know the silica and alumina are a bit low, and I'm planning on
increasing them both as well, while I'm fiddling with the spitting
issue.
Regards,
Tim
Rikki Gill on tue 30 jan 07
Hi Tim,
Have you tried to mix the cobalt with hot water before adding to the rest of
the ingredients? It works for me.
Also, do you run the entire formular, wet, through a sieve? You should, if
you don't.
Hope this helps,
Rikki Gill
rikigil@cwnet.com
www.rikkigillceramics.com
www.berkeleypotters.com
----- Original Message -----
From: "Timothy Joko-Veltman"
To:
Sent: Tuesday, January 30, 2007 6:02 AM
Subject: Spitting Cobalt Glaze
>I have inherited from my mentor a cobalt glaze which has a problem I
> haven't found mentioned much in the books
Timothy Joko-Veltman on tue 30 jan 07
On 1/30/07, Rikki Gill wrote:
> Hi Tim,
>
> Have you tried to mix the cobalt with hot water before adding to the rest of
> the ingredients? It works for me.
This I have not tried, but I'll definitely do so in the near future.
> Also, do you run the entire formular, wet, through a sieve? You should, if
> you don't.
We sieve all our wet glazes - through a rather fine mesh, actually
(forget which, at the moment).
Cheers,
Tim
Chris Trabka on tue 30 jan 07
Tim,
John Britt mentions spitting glazes in his book "The complete guide to high-
fired glazes" on pages 152 & 153.
John is discussing single firing a pot. However some of the things he talks
about firing may apply to your situation.
Chris
Timothy Joko-Veltman on wed 31 jan 07
On 1/30/07, Chris Trabka wrote:
> Tim,
>
> John Britt mentions spitting glazes in his book "The complete guide to high-
> fired glazes" on pages 152 & 153.
>
> John is discussing single firing a pot. However some of the things he talks
> about firing may apply to your situation.
>
> Chris
Chris,
Thanks for the tip. We don't do single-firings, but I looking through
it, the only thing that might make a big difference is the firing
speed. I'll keep that in mind for the next firing.
Regards,
Tim
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