Michael Wendt on wed 29 oct 03
>but I'd say 5 out of 10 plates break during the bisque firing. Does anyone
have any pointers for me? I fire using a gas updraft kiln fueled by
propane.
Thanking you in advance........Gigia<
Gigia,
Fire the plates upside down, centered in the shelf, ringed by other items to
shield them from the edge of the kiln. Fire slowly, especially from 850 F to
1250 F. If you go too fast, part of the plate can go through quartz
inversion before the rest and cause breakages. Don't stack plates. If you
need to fire many plates at once, buy a box of insulating fire brick , a
cheap hand saw and cut the bricks up into 2 1/2" x 2" x 4 1/2 " blocks that
can be assembled into protective saggars that ring the shelves with plates
on them. This gives you more capacity and the saggars can be used to protect
any large flat items from too rapid heating and cooling.
I also found it helps to alternate taller layers with the shorter plate
layers to improve heat uniformity.
I rarely see more than 1 in 100 plates , or even 20" platters crack when I
use the saggars to ring them. I have even refired cone 10 plates and
platters using this method.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
wendtpottery.com
Craig Martell on thu 30 oct 03
Hi:
I've had this problem but I don't any longer. You don't say what kind of
claybody you are using or give your bisque temperature so I'm venturing a
general suggestion. Bisque to at least cone 04. Plates and many other
types of flatware need some strength to survive the return thru the quartz
inversion on the way down. The clay will survive this volume change on the
way up because it's a different structure and hasn't gone thru the phase
where the kaolin hyroxyls are lost and the clay lattice becomes
tighter. On the way down some fired strength is necessary for survival of
the piece. I make porcelain and I bisque fire to 1988F which is cone 03.
You can also stack plates lip to lip and make what is known as a
"box". The technique of stacking lip to lip or rim to rim is actually
known as "boxing". I learned this from Mike Tyson. :>) Anyway, this
makes a closed form out of two or more pieces and a closed form is stronger
and has a better chance of surviving the quartz inversion. When you nest
plates, as in stacking them as you would in a cupboard, you make a dense
center that will heat slower than the rims. This can be deadly.
regards, Craig Martell Hopewell, Oregon
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