Ama Menec on thu 6 oct 05
An ex-student of mine did a Raku firing recently at her home and fired a
sculpture of mine for me. It had a deep purple underglaze, (fired on
already) and then a clear raku glaze on top. She brought it back to me in an
'interesting' state....most of the purple underglaze had vanished, except, I
suspect, where the sculpture might have been cooler. The underglazeless
areas had very fine pinholes, lots of 'em, so I suspect it had gotten too
hot and the underglaze has boiled underneath the glaze and fired out.
I've never seen the likes of this before. Has anyone else experienced
underglaze firing out in a raku firing? She says it was only 1000C, but
wasn't using cones to check the heatwork. I'd be very interested if anyone
else has seen this happen. Thanks,
Ama Menec, Totnes, Devon, UK.
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Vince Pitelka on thu 6 oct 05
> The underglazeless
> areas had very fine pinholes, lots of 'em, so I suspect it had gotten too
> hot and the underglaze has boiled underneath the glaze and fired out.
Rather than burning out, it is more likely that the underglaze was simply
absorbed into the glaze. This just came up in another thread about terra
sig. At the clay-glaze interface there is a lot going on as the glaze
reaches maturity. There is an old aphorism "The glaze feeds on the body,"
and this may be another good example. Being quite fluid and chemically
reactive, the glaze acts as a solvent on the surface of the body,
incorporating materials into the glaze. If the glaze becomes quite liquid,
it can dissolve quite a bit of material from the surface of the pot, and
thus can absorb a thin layer of slip. The solution in the future is
probably just to apply several more coats of the underglaze before glazing.
I know that there are many underglaze colors that do not survive to
high-fire, but I would think that most of them would survive raku
Good luck -
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111