Travis Maxwell, PE on mon 25 jun 01
=20
I thought I would share some findings on unwanted glaze blisters in cone =
6 oxidation electric firing that I have found recently. I have had a =
terrible time with glaze blisters on my pots. It started out as a minor =
problem with just a few blisters and became a major problem when I =
changed to a matt glaze. Every attempt to remedy the problem with =
firing changes failed. Pots that were refired came out with even more =
blisters. I tried slowing my firing way down, both cone 04 bisque and =
cone 6 glaze, and soaking at peak temperature but did not have any =
improvement. I even tried the crazy idea of putting in double shelving =
to see if I could hold heat in the kiln longer. The blistering I was =
having was very heavy with a typical coffee cup size piece having 30 or =
more smoothed wall blisters. The glaze I am using is a cone 6 opulence =
matt white with one of their "reduction look" blue glazes on top of it =
in places. This is a very nice combination. The blue glaze seemed =
viscous enough to heal from the clay gassing through the glaze but not =
the matt. Unwilling to change my glaze, I decided to try a change in my =
clay. The clay I was having all the blisters with has some grog in it =
and probably some iron. So, I made some pots from another finer clay, =
Standard 181. This is a very white smooth fine clay. I had stopped =
using it since I did not like its look in unglazed areas and it is hard =
to throw large thin pieces with it. I fired some identical pots made =
from my old clay and the 181 clay and found my solution. I had pots in =
the last firing that covered all my bases. I had pots that had been =
glazed thick and thin, pots that had been glazed a long time ago and =
very recent(two days before firing), pots that had very matt and very =
glossy glaze, pots that had been washed before glazing, and pots that =
had been bisque fired from cone 06 to 6. The pots made from the 181 =
clay in all cases in the firing had no glaze blisters at all and the =
ones made from the other clay had many blisters. Some of the pots with =
the problem clay and a viscous glaze had just a few blisters but the =
ones with the matt glaze had many blisters. I fired the load in a =
"typical" manner with a soak at peak temperature. I did not try to do =
anything tricky with the firing. I guess I have learned that the clay I =
had been using is not a terrible clay for causing blisters. A good soak =
at peak temperature takes care of most of the blisters with all my =
glazes that are not matt. It is just a terrible clay when trying to =
keep glaze blisters out of my matt white. This is definately a case =
where I put two non ideal components together and created a big problem. =
I am sure there are some other things I could try to fix this problem =
instead of changing clays but sometime you just have to throw your hands =
up and make a major change. I'll miss the old clay since it was a =
breeze to throw but absoutely love the look of this matt white glaze =
without blisters on the 181. =20
Travis Maxwell
Snail Scott on mon 25 jun 01
At 05:26 PM 6/25/01 -0400, you wrote:
>
>I thought I would share some findings on unwanted glaze blisters in cone 6
oxidation electric firing...a matt glaze.
>Travis Maxwell
Yes, this duplicates my experience very closely.
High-viscosity matt glazes trap blisters more
than gloss glazes, and it appears that high-frit
glazes do this more than most. It's also worst
on rough surfaces. I use very groggy clays, and
I get more blisters in 'scraped' areas where the
grog is exposed than I do in areas that are
smoothed with a rib, on the same clay.
-Snail
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