Michael McDowell on thu 7 dec 00
I was going to just post this note directly to Vince, but considered that many
on the list might appreciate the story. My pottery production has become very
sporadic as I'm experimenting with small scale vegetable farming on the side.
The last time I fired my kiln was to clear out the shop in preparation for
Vince's Summer Workshops here, back in mid July. BTW we are planning to do
that again this summer. That couple of firings
produced some beautiful results, but easily half of the pieces went straight
to the trash as I was plagued by shivering of the glaze on a great number of
the pieces.
It caused me some embarrassment, to be unloading large amounts of seriously
defective wares just as my place was being inundated with potters of various
levels of accomplishment. At the time we speculated on the cause. Of course
shivering in glaze is caused by a mismatch between the thermal expansions of
clay and glaze such that the glaze is considerably less expansive than the
clay. Vince speculated that I had reduced the pots so much that my heavy iron
bearing clay had changed it's coefficient of thermal expansion. Knowing that I
had used clay from two different batches, my guess was that the second batch
of clay was just more expansive than the first (to which I had fitted my
glazes). The resolution of this question waited on further firings.
So, now after several months, I'm firing again. I tried giving the kiln a
little less reduction in case Vince was right, but not a lot less, in case I
was right and my glazes suffered from a lack of it. I had made a number of
small cups out of the suspect batch of clay, but avoided it for the rest of my
production this time. Then I mixed up an "expansion series" of glazes, as our
friend and glaze guru Ron Roy had coached me to do some time ago. Just to get
a general idea of what range of expansion would "fit", I tested with glazes
having expansions from 400 (which previously fit) on up to 550. I also did the
same tests with a couple of other clay bodies that I'm considering for use.
The other two bodies returned the kind of results I've gotten used to by now.
Crazing with 550 glaze, and less crazing with reduced glaze expansion.
Interestingly, both of those bodies seem to fit well with the lowest expansion
glaze.
The suspect clay body does seem to be the cause of the shivering I
experienced. The results were rather dramatic. Applying the test glaze to the
inside of the cup only, as per Ron's directions, the clay was so much more
expansive than any of the glazes tested that all four of the test cups were
shattered by the stress! But stress has been relieved for me! Now I can go
back to firing my kiln as I think I should. Maybe I'll mix up some more tests
to find what will work with the 750 lbs. remaining of the questionable clay.
Michael McDowell
Whatcom County, WA USA
mmpots@memes.com
http://www2.memes.com/mmpots
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