Craig Martell on fri 1 dec 00
Hi:
Lois wanted to know what to do about cracking platters. Here's how I
solved it.
I make porcelain platters up to 24 inches in diameter. A lot of them used
to crack from 15" dia on up to the bigger ones. I stopped this from
happening by doing a very simple, painless thing. I raised by bisque
temperature from cone 06 to cone 03. Cone 04 would probably be ok for most
bodies but higher kaolin Clays may need some more temp.
The deal is that you will never get even heating and cooling. You can come
close but that's about it. So you have to give the bisque fired piece some
strength by firing to a point where the feldspar at least starts to
sinter. Cone 06 just isn't hot enough to give strength to most porcelains
or porcelaineous bodies. A lot of forms really don't need this higher
bisque but big bowls and platters sure do. On the way up to the target
temp the clay is more open because the hydroxyls haven't been altered by
heat and the pieces will get thru the quartz inversion alright but on the
way down there's a much more intimate orientation of the clay lattice and
strength is required to get thru quartz in one piece.
I fire in an electric kiln to cone 03 in about 8 hours. I don't do
anything special to the platters. No grog, sand, shims, any of that
stuff. I just put the platters on a full shelf, by themselves and fire
them. I haven't lost one since I've fired to 03. Sometimes, when I'm in a
hurry to make a deadline, like now, I will put platters that aren't fully
dry in the kiln and turn the bottom ring on low over night and fire the
usual schedule the next morning. No cracks with this rush job either. So,
I think the higher bisque is the key.
later and good luck, Craig Martell in Oregon
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