Richard mahaffey on sat 7 feb 98
Hello,
The definition of a Celadon glaze that I learned was of a limestone glaze
with .5 to 4% iron fired in a reduction atmosphere. Am I wrong? How can
one have a cleadon without the iron? Are we talking about a limestone
glaze?
IMHO and from my humble experience: If you want your glaze to stop
crazing make sure your clay is mature at the temperature you are firing
to, otherwise it may take a looooong time to make the crazing go away.
Just another old guy checking to see if his knowledge is out dated.
Rick Mahaffey
Tacoma Washington, USA
rmahaffe@linknet.kitsap.lib.wa.us
Craig Martell on sun 8 feb 98
At 02:51 PM 2/7/98 EST, Rick Mahaffey wrote:
>----------------------------Original message----------------------------
>The definition of a Celadon glaze that I learned was of a limestone glaze
>with .5 to 4% iron fired in a reduction atmosphere. Am I wrong? How can
>one have a cleadon without the iron? Are we talking about a limestone
>glaze?
Hi Rick:
If you are referring to the clear celedon I posted, let me clarify the
"clear" thing. I should have called it a transparent celedon to avoid
confusion I guess. You are correct of course, a celedon glaze requires
iron. I add a native Oregon clay that is from parent rocks of Basalt and
Andesite. Basalt is very iron rich and this clay addition gives the glaze
enough iron to make the celedon color. I have seen transparent glazes fired
in reduction turn a very pale celedon from trace amounts of iron in the
feldspar and kaolin, without added iron oxide.
>IMHO and from my humble experience: If you want your glaze to stop
>crazing make sure your clay is mature at the temperature you are firing
>to, otherwise it may take a looooong time to make the crazing go away.
Yes, the clay needs to be mature to ensure that there isn't a volume change
due to the absorbtion of moisture which will cause a delayed crazing effect.
BUT, the glaze has to be adjusted in expansion rate, so that it is under
slight compression after firing to maturity, or it is going to craze no
matter how mature the claybody is. Fitting a fusion bonded glass to a
claybody can be real tricky, and I'm sometimes amazed that we can do it, but
it is done all the time. One needs to know what to do to the claybody, as
well as the glaze to promote good fit. You can't fit a well formulated
glaze to an unbalanced, poorly formulated claybody and vice versa. It's not
as simple as just firing the ware to maturity, there is some molecular
engineering that needs to be done from time to time.
regards, Craig Martell-Oregon
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