Bruce Davis on sun 1 feb 04
All this talk about warts and bloats must be "catching". This week one of
my students was using a new Spectrum glaze--Bright Red--on a Laguna 402
stoneware mug which was bisqued to 04. I fast fired it to Cone 5, held at the top
for 30 minutes, slow cooled at 150 per hour. It came out with big warts all
over the inside. I refired it to see if anything could be done. This time I
slow fired it with the same hold and cooling....same result. I also included
several other pieces that I made to see if the problem was repeatable. It
was.
I glazed a stoneware bowl with Red, white and Blue sections. The red
portion came out with small warts on the surface but the blue and white appear to
be normal.
I also glazed a B-mix vase with the red. It came out normal.
I love this glaze because it truely is a great red color--the brightest
red I have seen, but so far the wart problem has me concerned. We have never
had any warts before for any reason. Any ideas as to why this glaze might be
doing this with stoneware clay???
Best regards,
Bruce Davis, Mud Run Pottery
Snail Scott on sun 1 feb 04
At 07:36 AM 2/1/04 EST, you wrote:
> All this talk about warts and bloats must be "catching".
I see this effect most often on glazes with a lot of
frit, with matte high-viscosity glazes, and more often
on rough clay. I have heard that frits melt and 'skin
over' earlier in the firing process than some other
fluxes, and this seems to correlate with effects I've
observed.
If the gasses are still trying to escape from the clay,
and they're sealed in by a glaze that melted early
in the firing, they may be trapped. A very runny glaze
(or low surface tension, or low viscosity, or whatever)
will allow the bubbles to boil out, but many glazes
won't. I also see this effect most on rough clay.
On the same piece, I sometimes get blisters on clay
areas that I scraped after smoothing, while the areas
that were rib-smoothed were fine. Underfired areas of
otherwise 'good' glazes may be vulnerable, too, since
the bubbles that might be released at full melt remain
trapped.
This fits your symptoms, as well. No blisters on your
(smooth) B-mix, but blisters on the rougher clay. Also,
the presence of blisters only on the interior of some
forms is consistent with the notion that the inner
surface hadn't quite matured. It's an effect many of
us have seen with raku, but it still holds true at
higher temperatures. Try firing a little longer. The
jar may say "^5", but it may need to be a 'hard' ^5
or even a bit more for best results. YMMV.
-Snail
Ron Roy on mon 2 feb 04
Hey Bruce - this sounds like it could be a bisqing problem - maybe that
glaze is sealing over early and trapping combustables. You stacking ware
in the bisque - like bowls rim to rim or inside each other?
Once the gas in trapped under the glaze you can't get it out so refiring
won't help.
RR
> All this talk about warts and bloats must be "catching". This week one of
>my students was using a new Spectrum glaze--Bright Red--on a Laguna 402
>stoneware mug which was bisqued to 04. I fast fired it to Cone 5, held
>at the top
>for 30 minutes, slow cooled at 150 per hour. It came out with big warts all
>over the inside. I refired it to see if anything could be done. This time I
>slow fired it with the same hold and cooling....same result. I also included
>several other pieces that I made to see if the problem was repeatable. It
>was.
> I glazed a stoneware bowl with Red, white and Blue sections. The red
>portion came out with small warts on the surface but the blue and white
>appear to
>be normal.
>I also glazed a B-mix vase with the red. It came out normal.
> I love this glaze because it truely is a great red color--the brightest
>red I have seen, but so far the wart problem has me concerned. We have never
>had any warts before for any reason. Any ideas as to why this glaze might be
>doing this with stoneware clay???
>
>Best regards,
>
>Bruce Davis, Mud Run Pottery
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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