Andie on thu 13 apr 00
I am finally throwing large platters and bowls, ranging from 19" - 24" in
diameter. However, I am losing about 3/4 of them in firings to warpage. I
did find that stilting (with those porcelain cone 10 three point stilts) in
the glaze firing seems to contribute, but I have had some warp in the bisque
and some that weren't stilted warped in the glaze. They are thrown from two
clays (not mixed together, obviously - some from one, some from the other):
Standard 108 and 181. I let them dry slowly and completely upside down
(which alleviated a rim cracking problem I used to have), and then bisque to
04 with a soak at 5 (medium). I glaze fire them to cone 7. The bottoms and
walls are the same thickness, and I do trim and burnish the very bottoms.
The warpage is in the bottoms of them, so when they go in they sit flat and
when they come out they don't. Occasionally one will also warp across the
top, so it goes in round and comes out oval. I have already tried varying my
bisque schedule, and also have tried firing lower in the glaze, to a cone
5/6 (6 conesitter), but nothing has made any difference. Any ideas? I don't
want to keep wasting clay until I can experiment with some possible
solutions, and would absolutely appreciate any ideas. I love making and
glazing them, and it's so hard to keep losing them over and over. I'm
willing to try new clays, new firing schedules, anything.
Thank so much in advance for any ideas -
Andie
www.andie.net
Kent / Pat on fri 14 apr 00
Andie,
Although I,m not the best to respond to this I feel the information that my
instructors pass on may be of use. One was a production potter and the
othe has a MFA in ceramics and has done his share of decorative plates.
They both agree that the largest cause for warping plates may be lack of
compression when throwing. Collar and push. Collar and push. Up and down.
Up and down. then when opening the plate, continue to compress. Back and
forth. Back and Forth. Then undercut it from the batt, but leave it there.
Dry very slowly. I left mine in the damp room for 2 weeks and trimmed it
when leather hard, but did not rush it getting to that stage. Then dried it
to bone dry before bisque. For Hi Fire, you might use wads or dust the
shelves with alumina so it slides easily.
Thought this might help a little.
Pat Porter
pporter@4dv.net
http://www.geocities.com/windy _pines_2000/index.html
Aurora CO USA
----- Original Message -----
From: "Andie"
To:
Sent: Thursday, April 13, 2000 10:10 AM
Subject: Please Help with Large Warping Pieces
> ----------------------------Original message----------------------------
>
> I am finally throwing large platters and bowls, ranging from 19" - 24" in
> diameter. However, I am losing about 3/4 of them in firings to warpage. I
> did find that stilting (with those porcelain cone 10 three point stilts)
in
> the glaze firing seems to contribute, but I have had some warp in the
bisque
> and some that weren't stilted warped in the glaze. They are thrown from
two
> clays (not mixed together, obviously - some from one, some from the
other):
> Standard 108 and 181. I let them dry slowly and completely upside down
> (which alleviated a rim cracking problem I used to have), and then bisque
to
> 04 with a soak at 5 (medium). I glaze fire them to cone 7. The bottoms and
> walls are the same thickness, and I do trim and burnish the very bottoms.
> The warpage is in the bottoms of them, so when they go in they sit flat
and
> when they come out they don't. Occasionally one will also warp across the
> top, so it goes in round and comes out oval. I have already tried varying
my
> bisque schedule, and also have tried firing lower in the glaze, to a cone
> 5/6 (6 conesitter), but nothing has made any difference. Any ideas? I
don't
> want to keep wasting clay until I can experiment with some possible
> solutions, and would absolutely appreciate any ideas. I love making and
> glazing them, and it's so hard to keep losing them over and over. I'm
> willing to try new clays, new firing schedules, anything.
>
>
> Thank so much in advance for any ideas -
>
> Andie
> www.andie.net
Mark Mondloch on fri 14 apr 00
Andie,
I'm not familiar with commercial mixes but wonder if you couldn't use a more
open clay body.
I don't know if you have the capability to mix your own clay, but I've been
adding 35 mesh kyanite (10%), grog(5%) and a small amount of paper pulp(1 roll
toilet paper to 100 lb batch) to my mix for bird baths and sinks- has pretty
much taken care of cracking and warping problems. I double these amounts in my
clay for large tile work. The kyanite seems to add green strength and cuts down
on drying and firing shrinkage. My understanding is that it adds strength to the
fired clay too. The paper is a small enough amount that it doesn't become a pain
in the butt when you have to trim but I think it still really helps.
good luck,
Sylvia Mondloch
Andie wrote:----------------------------Original
message----------------------------
I am finally throwing large platters and bowls, ranging from 19" - 24"
indiameter. However, I am losing about 3/4 of them in firings to warpage...
--
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake, WI 53075
silvercreek@execpc.com
http://www.execpc.com/silvercreek
DeBorah Goletz on sat 15 apr 00
Andie - I'd like to know more about how you are handling those big platters.
A lot of warping can be minimalized by never touching the rim. I have
hot-glued round pieces of foam (from old cushions - about 3" thick) onto
standard bats. I use various diameters to fit INSIDE the rim of the
platter. I place the foam-bat upside down inside the platter - which is
still on it's original bat from throwing - and flip the whole works over
like a sandwich - then remove the original bat (wire it off if necessary)
from the now up-side-down platter. The foam supports the center of the
platter as I trim without disturbing the rim (which hangs out over the
foam). I then place a board on the trimmed foot and flip the whole set-up
over again - then remove the foam-bat. To keep those rims from drying too
fast, I wrap a piece of approx, 8" wide plastic around the rim (covering the
top, edge and bottom) which leaves the center of the platter to air dry (a
second sheet of plastic could be draped over the top to slow drying).
Ideally the center of the platter dries a little faster than the rim to
minimize chance of rim cracks.
Best,
DeBorah Goletz
Cindy Strnad on sat 15 apr 00
Andie,
Not having seen or touched your platters, I can't really help all that much,
I'm afraid. Keep in mind that it is very, very important that they be of
uniform and appropriate thickness all the way through. Larger platters need
to be thicker than smaller plates. Break some of those warped platters and
check them. The tendency is often to get them too thin at the very center or
at the spot where you begin the rim.
When you cut off that large a platter from the bat, you almost can't help
having the wire bow upward towards the center (pull it taut, taut!), and
unless you're using plaster bats, you absolutely have to cut them off or
they'll crack during drying. Therefore, it's a good idea to throw them
significantly thicker than you want them to finish up, even if you don't
intend to trim a foot ring. You can even them up during trimming.
Speaking of foot rings, make sure to make enough of them if you make any at
all. A large platter needs some support toward the center, too, not just at
the edge.
Finally, you need to make certain you're firing the platter on a flat,
unwarped shelf. No plate fired on a bowed shelf will come out flat--not even
a 10", let alone a 24".
I hope this is of some help--some things to check on, anyway.
Best wishes,
Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
Mike Gordon on sun 16 apr 00
Good advice! I would add. Always place a board on the lip of the plate
when turning it over to trim. Let it dry that way, lip down. Mike
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