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Posted by Bob Masta on October 11, 2007, 8:10 am
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>
>
>> No, this aluminum should not cause any problem at all. Note that
>> aluminum oxide melts at a very high temperature (stiffens glazes),
>> whereas whatever is causing the bubbles is something that is actually
>> boiling, or more likely burning, at your firing temperatures. The
>> combustion gasses are released as bubbles that appear in the
>> slow-to-heal glaze. (There are other possibilities, like release of
>> chemically-bound gasses from some minerals, which don't actually
>> constitute "burning" as such, but the result is the same.)
>
>
>Hi Bob
>
>If aluminium stiffens glazes, couldn't it be that the release of it from the
>clay, trying to get out through the glaze - causes the glaze to stiffen and
>thus the bubbles not to melt back "in position"?
As Dewitt mentioned, aluminum oxide is very common in clays and
glazes, and the amount from your oxidized wheel is trivial by
comparison. However, even if you had a lot more (say, if you
deliberately worked powdered alumina into the clay or something),
you'd find that alumina by itself wouldn't melt at kiln temperatures
without a flux. It wouldn't "release" from the clay.
Alumina that was on the surface and mixed with glaze fluxes
would retard their melting in the mix zone. My guess is that
this surface alumina would not migrate far up into the glaze
itself... I don't think it would dissolve in it very easily and raise
its overall melting (freezing) point, unless you held the kiln
at a high temperature that would keep the glaze molten long
enough to allow the alumina to mix. my guess is that if you
held it that long, assuming the glaze didn't run off the pot,
that all the bubbles would have time to work out through
the molten glaze, anyway.
But to reiterate, I don't think alumina is your problem at all.
>I know that may be a stupid question, but I have found that I only learn if
>I ask questions about things I don't understand :-)
>
>Marianne
>
More than one instructor has said that "the only foolish question
is the one you don't ask"!
Best regards,
Bob Masta
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