vpitelka on tue 19 feb 08
Tom Sawyer wrote:
"The general question that I would like suggestions on is a problem that I
am
having firing large slabs. I wish to make a set of slabs about 20 x 25
inches in size. I've been using paper clay and get them dried without much
warping but when I fire them [lying flat], I get an inordinate amount of
cracking; when fired upright they slump. Does anyone have any suggestions."
Tom -
Most likely they are cracking because of differential shrinkage as they heat
up or cool down. That is always a problem when firing large flat things
right on the kiln shelf. I would suggest firing them on wadding - either a
network of small balls of wadding, or else parallel coils. Either will work
well and will allow the heat and atmosphere to pass beneath the piece and
eliminate the problem of the thick kiln shelf keeping the center of the
piece cold in heating and hot in cooling. Unless you are firing in salt or
soda, just mix any clay 50/50 with flint as a cheap, effective wadding. If
you use coils, don't criss-cross them, because that will create closed air
spaces and defeat the purpose.
The reason for using the 50-50 clay-flint mix is so that you can heat the
kiln as fast as you want to in the glaze firing. The 50% flint opens up the
body so much that steam and volatiles can escape without ever building up
pressure.
Some people advocate firing large clay slabs on a bed of sand or grog, and
while that does insulate the slab from the kiln shelf, it does not allow
heat and atmosphere to pass beneath the slab, and doesn't work as well as
wadding.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
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