ClayCoyote on mon 22 dec 97
Here's another possibility if the work has been refired. We were using a
fireclay based white stoneware that frequently (50%) blew up with the same
very small shard and throughout the kiln characteristics. (Needless to say we
quit refiring after only a couple of experiences.
Then, when we fired up a new gas kiln, we had saved bisqueware seconds to use
as unglazed kiln filler for the first few firings. On the third time through,
starting at about 400 deg. F. it sounded like a war going on in the kiln. (We
kept firing because we were really just trying to hit temp at that point
(short stack problem earlier).) Upon opening, every single one of the
multiple fired pots was "dust". My supposition is that each time we had fired
these pots, more and more crystobalite had been formed, until there was so
much that everything was reduced to bits. I'm making the assumption from the
fact that crystobalite undergoes expansion at about 400 deg. Also, in the
earlier glazed blows, the blow had been on the way up, not cooling, as
evidenced by the way the glaze had flowed over the cracked edges. Actually
Vince, this could be a secondary explanation for the pitcher blow up. Suggest
looking at Hamer re: Crystobalite. Also, Jim Robinson's article about silicas
in Studio Potter 4 or 5 years ago.
Anyway, the (commercial) clay formula now has so kyanite in it which seems to
have alleviated the problem. I know Jonathan K vasltly prefers Pyrophyllite. I
look forward to the day when we can get our own formula tested and make our
own clay.
Tom Wirt
Clay Coyote Pottery
Hutchinson, MN
claypot@hutchtel.net
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