RPeckham@COOKSONELECTRONICS.COM on tue 27 aug 02
Sounds like a drying problem.
Check out variables in your drying process, from when you made that form
successfully to what is happening now.
My last studio was forced hot air.
I was doing fine in the summer, then in the fall I started doing larger
shapes. All of them cracked.
First thing I discovered is that I was using very hard clay to compensate
for a lack of ability, and the throwing process either caused, or didn't
help cracks to form together. Later during drying or firing the cracks
showed up.
Then I got some softer clay, and concentrated on throwing techniques to
discourage cracks, compressing the bottom, and rim etc. I still got
cracks, whenever I attached something, and on particularly wide fattish
pots, like platters.
Then in the spring I seemed to have found the right attachment technique,
clay softness, and throwing technique. No cracks all summer. The next
fall, everything started cracking again.
Forced hot air.
I used plastic bags, and got a rain forest effect that soake the to, or
middle of a pot, and it cracked. I ripped a hole in the bag to let the
moisture out, and it dryed too fast. I went from too fast to not at all
drying.
I was making bathroom sinks, and made three for a customer, hoping one
wouldn't crack. They all did.
I made another one, and in desparation tried a wax emulsion that is for
waxing feet etc. I painted a thin coat all over the whole sink. No
cracks! Fired beautifully.
Dryness, air flow, keeping windows open in the summer, etc. all variables
that can give you hell.
Snail Scott on tue 27 aug 02
>I used plastic bags, and got a rain forest effect that soake the to, or
>middle of a pot, and it cracked. I ripped a hole in the bag to let the
>moisture out, and it dryed too fast. I went from too fast to not at all
>drying.
Whenever I use plastic bags, I put cloth inside, over
the clay piece. It absorbs the condensation that forms
at the top of the bag, and helps maintain an even
humidity in the bag. If I need to soften a piece that's
over-dry, I dampen the rag before wrapping the piece.
(Sprayed water on clay just runs off and causes puddles
at the bottom; this keeps the water in ongoing contact
with the clay.) When I am storing work in progress, I
keep the rags moistened. When I am done, I quit wetting
the rags, and let the piece sit in the bag (under the
rags) 'til it's dry. That's pretty fast around here.
Thick cotton knits work best; wovens and synthetics
don't hold the moisture as well. If they start to grow
mildew, I just wash them with a bit of bleach. I also
use damp rags inside my clay-storage bins to even out
and control the moisture there.
-Snail
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