Nikom Chimnok on thu 20 jan 00
Dave,
In my experience, how fast you can bisque fire depends on two
things--your clay, and the thickness of the pieces. At one extreme, I've
taken little porcelain ornaments to ^10 in 2 hours with no damage. At the
other extreme, I've lost half a kilnload to explosion and cracking when
firing our local tight-bodied red clay as mural blocks 8X8X3 inches thick
with 24 hours of candling then a temperature rise from 100 C on of less than
50 degrees per hour.
Using the same basic clay, I have then taken a hint from the tile
producers who fire in 30 minutes, and added talc, wollastonite and grog,
then fired slipcast pieces in 6 hours with 100% success. Grog is explosion
insurance, and talc and wollastonite improve thermal shock resistance. Large
particle size along with a lack of organics and sulfur means you can fire
fast. To oversimplify, if it's easy to throw it's hard to fire, and if it's
hard to throw it's easy to fire. The right mix can be pretty easy to throw
and pretty easy to fire. After 3 or 4 years of development, I now mix a clay
that the potters don't complain about, and for pieces let us say not over a
foot high we once fire them to ^6 in 14 hours without losing 1 in a thousand.
On the other hand, mural blocks made of the same clay (mural blocks
should not be made out of the same clay; they should be made of mural clay
but I don't run things around here) sometimes still explode or crack on an
18 hour schedule. But mostly not. Not enough that I can get anyone
interested in mixing up some guaranteed mural clay.
I think Jonathan Kaplan explains all this in an article on Digital
Fire's website, the title having something to do with "Stop the Whining..."
(sorry, too lazy to look it up). Along with a lot of experience and
experimentation, I learned the most about making clay from Jonathan, first
in an article in Ceramics Monthly years ago, and more lately from the
article referred to above. Highly recommended. So far as how fast you can
fire your particular mix: Try it and find out. If you go too fast you gain
both knowledge and grog.
To Progress,
Nikom
******************************************************
At 14:06 19/1/00 EST, you wrote:
>----------------------------Original message----------------------------
>Kiln gurus,
> Ginny's successful bisque firing--great work, by the way!--raises a
>question in my mind. I, too, have encountered rapid temperature rise
>immediately at the start of a gas firing. My burners have a lot of
>turndown, though and I can manage just fine. I am wondering, though, how
>slow you really do need to go between candling to really bone dry (less than
>200 degrees F) and say 400 to 500 degrees. At what point do you start
>kicking off chemically bound water that could ruin the ware? Is there a
>gap between drying the greenware and driving off bound water where the ware,
>being somewhat open and porous, can handle rapid heating early in the
>firing?
> Right now I am following a very careful 150 degree F/hour temperature
>increase from candling to 1,600 F for greenware firing. I am just
>wondering if I can safely speed up this process in some areas. It seems
>like it would save time and fuel!
> Thanks for considering this question.
> Dave Finkelnburg pondering why there's no snow, it's 45 degrees F,
>and raining in January Idaho
> dfinkeln@cyberhighway.net
>
>
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