Michael Wendt on sun 10 jul 05
Jeanette,
Refiring does sometimes work but without saggars, large plates are likely to
crack in refiring. Higher bisque firing will most likely not alter the final
warping.
My clay like most porcelains is so soft at cone 10 that it takes on the
shape of the shelf below. Before you refire, get a high quality straight
edge and a torpedo level. Check your kiln shelves for level with the torpedo
level and sag using the straight edge. Plates are tough to keep flat if the
shelf is either not level, not flat or worse, both.
Another cause of warping is the differential temperature that exists between
the edge of the kiln shelf and the middle. Depending on whether you are
rising or falling in temperature and how fast, the differential can be small
or large.
Since porcelain's fired shrinkage varies with how hot it gets, some parts of
a plate can shrink more than others in some kilns and this sets up the
needed stress to cause actual warping. Saggars often help solve this.
I fire larger plates and platters with protective saggars made of cut
firebricks to even out the firing. Be sure to fire taller layers above and
below the plates as this improves uniformity of heating by allowing a better
radiation path to the center of the load.
If the plates are wasters, they are ideal candidates for refire to develop
successful refire strategies, something that can save hours of work with
intricate plates.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Jeanette wrote:
Should porcelain be bisqued at a higher temperature than stoneware to
guard against warpage and sagging? And has anyone ever refired a
warped plate and had it flatten out?
--
Jeanette Harris
Still puzzling over my warped plates
Vince Pitelka on sun 10 jul 05
> Should porcelain be bisqued at a higher temperature than stoneware to
> guard against warpage and sagging? And has anyone ever refired a
> warped plate and had it flatten out?
Jeanette -
Question #1 - There are some good reasons to bisque-fire porcelain higher
than other claybodies, but it wouldn't guard against warping and sagging,
because those things happen in the glaze firing, and wouldn't be affected by
the bisque temperature. We bisque everything to cone 08, but I know that
lots of potters like to bisque fire porcelain to cone 06 or 04 in order to
get a stronger bisque, but that strength is to make the pots easier to glaze
with less risk of breakage, and to reduce the incidence of glaze flaws.
Question #2 - yes, as long as they are refired on a very flat shelf. But
it's gotta be a pretty special plate to be worth refiring. Also, remember
that wide flat forms like plates are very subject to cracking in refiring.
Be sure to put them in the middle of the set, away from the heat source, and
I'd also be a little cautious in heating the kiln.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Jeanette Harris on sun 10 jul 05
Should porcelain be bisqued at a higher temperature than stoneware to
guard against warpage and sagging? And has anyone ever refired a
warped plate and had it flatten out?
--
Jeanette Harris
Still puzzling over my warped plates
Poulsbo WA
Potter's Council member
Jeanette Harris on mon 11 jul 05
Thanks, Michael and Vince for your replies and advice.
Yesterday I fired another bunch of plates, but this time I used wide
stilts for support underneath and they came out beautifully. So, I
can now conclude that it was the stacking rim stilts that were the
cause of the warpage. (Beside some design things that could have
contributed to warping.)
So it looks like the bisque fire at 04 is okay for my particular
porcelain. The glaze is a good fit. The plates had underglaze
drawings on them that had been previously laid down when the plates
were leather hard. The bisqued plates got a quick dip in the water
bucket to remove any dust that might be on them, allowed to dry till
cool to the touch, then dipped in thickish transparent glaze that had
been seived and put into one of those Office Depot tall trash cans.
I use a computer program that takes about 14 hours to fire. I lowered
the top temp this time to 2080. I also had some very finicky
weathered bronze pieces in the kiln too and had previously
experienced problems with that glaze jumping off the pots and being
very runny. Those pieces came out well too. I'm firing in a Skutt
10/shorty (whatever that is--one of the little kilns).
The plates are visual jokes. The idea grew from a conversation I had
a few years ago with one of the faculty when I was working at Seattle
University. The springboard was, "What would Christo's cat look
like?" So, I drew a cat all wrapped up in cloth strips like a mummy.
That little sketch was rattling around in my stuff for a long time.
What brought it to mind was the combination of a show that comes up
in April with a dog/cat theme and that supports charity. I got to
thinking about what other artist's cats would look like. Here's the
result.
Picasso's cat is angular with a face similar to the nudes in Les
Demoiselles d'Avignon.
Michelangelo's cat is the Sistine hand/finger of God reaching out as
the cat walks away.
Calder's cat is a mobile
Henry Moore's cat is a undulating sculpture on a plinth with holes
for eyes and a large hole in the middle.
Dali's is a landscape with pyramids for ears, a tree and leaves
forming the brows, eyes and nose and a tongue rippling down a set of
stairs.
Jackson Pollack's is a painting with cat tracks crossing over the
canvas and going off the plate.
Georgia O'Keefe's is a cat skull over a black cross.
I'm going to test market this morning's plates at a show in a couple of weeks.
>>Should porcelain be bisqued at a higher temperature than stoneware to
>>guard against warpage and sagging? And has anyone ever refired a
>>warped plate and had it flatten out?
--
Jeanette Harris
Poulsbo WA
Potter's Council member
Chris Campbell on mon 11 jul 05
Jeanette -
Porcelain is often fired in reverse of how you usually
fire other clay bodies. Hi fire bisque, lower fire glaze.
This is done so that the form can be supported during
the high heat when it is so soft.
Wide forms that are not self supporting are going to
warp if you don't give them help.
I fire my bowls in a matching bowl of a studier high fire
clay body. If any part of it hangs over the edge I support
it with a shelf support.
Another crucial factor is the cool down firing of the kiln.
I have stopped warping and cracks simply by programming
the kiln to fire down slowly.
Chris Campbell - in North Carolina - porcelain is the true diva
of clay bodies ... you better pay attention to her or she will punish
you.
Hoping that was not sexist but cannot think of what the p.c. term
is for spoiled brat.
Chris Campbell Pottery LLC
9417 Koupela Drive
Raleigh NC 27615-2233
Fine Colored Porcelain since 1989
1-800-652-1008
Fax : 919-676-2062
website: www.ccpottery.com
wholesale : www.wholesalecrafts.com
Jeanette Harris on mon 11 jul 05
Very interesting. I'm intrigued by the idea of firing one pot inside
another. I'm assuming you're talking about the bisque fire. Does the
pot support itself successfully in the glaze firing? Or do you do
only bisque? At what cones do you do the bisque and glaze? What's
your porcelain clay body range?
>Porcelain is often fired in reverse of how you usually
>fire other clay bodies. Hi fire bisque, lower fire glaze.
>
>This is done so that the form can be supported during
>the high heat when it is so soft.
>
>Wide forms that are not self supporting are going to
>warp if you don't give them help.
>
>I fire my bowls in a matching bowl of a studier high fire
>clay body. If any part of it hangs over the edge I support
>it with a shelf support.
One of the absolute joys of clay for me is working out these kinds of problems.
My computer crashed? Oh boy, how do I figure out how to get it running again.
A new version of MYST? lemme at it.
--
Jeanette Harris
Poulsbo WA
Potter's Council member
Chris Campbell on mon 11 jul 05
Jeanette wrote -
> I'm intrigued by the idea of firing one pot inside
>another. I'm assuming you're talking about the bisque fire. Does the
>pot support itself successfully in the glaze firing? Or do you do
>only bisque? At what cones do you do the bisque and glaze? What's
>your porcelain clay body range?
I use Southern Ice Porcelain which is
Cone 7 - 10.
My first firing goes to what I call
a hard Cone 8 ... almost 9.
My pots are wide and free form ... they
have no way of supporting themselves.
After years of experiments, reading and
good advice I go the long route by making
a form that holds my porcelain totally
supported during building and firing.
These forms can be used at least four
times. The only trouble is they shrink
so the work gets smaller as I go along.
When I glaze it is usually to Cone 04
( my work is not functional so I have
more leeway) and this is cool enough
that it does not seem to affect the
porcelain's shape.
I will need to glaze fire them to Cone 6
with the new stuff I am making so we will
see how well my theory holds at that temp
second firing.
Chris Campbell - in North Carolina - I am actually
looking forward to seeing what will happen ...
months away though!
Chris Campbell Pottery LLC
9417 Koupela Drive
Raleigh NC 27615-2233
Fine Colored Porcelain since 1989
1-800-652-1008
Fax : 919-676-2062
website: www.ccpottery.com
wholesale : www.wholesalecrafts.com
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