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oxidation cleanup after reduction firing

updated tue 31 dec 96


Vince Pitelka on wed 18 dec 96

Marshall and Raphael -
In my experience, the decision whether or not to do an "oxidation cleanup"
at the end of a reduction high-fire should be based on the types of glazes
in the firing, and the thickness of glaze application. Reduction induces
volatization within the glaze, and therefore causes the glaze to bubble or
foam. In thick glazes, a short oxidation soak should stop the volatization
while maintaining the mobility of the glaze-melt, allowing the glaze
surfaces to "heal."

In a reduction firing, if the kiln is shut off without an oxidation cleanup,
even if the damper is closed tight, the interior will initially drop in
temperature quite fast, and glaze viscosity will increase rapidly. This can
lock bubbles in place, and it can also affect glaze color. Many of our
favorite reduction glazes are those which break from one color to another in
thick and thin areas. In most cases color breaks are due to reoxidation of
the glaze where it is thin, while reduced effects are locked in the thick
areas. Again, the degree to which the thin areas reoxidize depends on
mobility of the glaze melt, and if the kiln is cooled too quickly, the
degree of glaze reoxidation will be limited. This is often the case with
temmokus which come out all black with very little brown. A good
fifteen-minute oxidation soak will drastically increase the shift from black
to brown in the thinner areas.

In production in California I applied my glazes very thinly, and I only did
a light glaze reduction. I shut down the kiln right after the
glaze-reduction, and left the damper wide open. I unloaded the kiln ten
hours later, and none of my stoneware glazes crazed.
- Vince

Vince Pitelka - vpitelka@Dekalb.Net
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts, Smithville TN 37166