MaryBeth Bishop on wed 12 dec 01
Jeremy,
I had a terrible time with some planters I made. They barely fit inside my
electric kiln and I wanted to bisque them in order to carry them from PA to
NC. One of three made it through and that one I had to fire on low for over
12 hours and still creep up from there. The kiln log isn't here so I can't
be more specific. To make a long story short, I decided not to make those
pieces unless I could put them in a gas or wood kiln without bisque firing
them. The edges just heated up so much faster than the bases because they
were near the elements. It didn't seem to make sense to spend all that
energy to get one pot firm enough to transport easily. I really think that
there is a major difference in the way heat moves through wide flat pieces in
an electric kiln vs an "atmospheric" one. To have any success in an electric
kiln I think the pieces have to be totally dry and then fired really slowly.
I'm not a great electric firer...not a great anything really, but that is
what I gleaned from my experience.
Mary Beth Bishop
Jeremy McLeod on wed 12 dec 01
Bleah!!
Two larger pieces (a 12" diameter cassarole and a 15" diameter
platter) didn't survive the bisque firing. They were thrown with Mile
High's ^6 "Ash" clay body, a porcelian-like substance. I compressed
the bottoms, I thought, quite well. Both cracked in a straight line
across the bottoms of the piece. Neither piece straddled across shelves.
Shelves are new and unwashed (kiln wash, that is). Fired to ^04.
Mugs, piggy banks, and 10" plates in the same loads came thru well.
Collective Wisdom, please?? If the replacement pieces I'm about to
throw are to be ready for Christmass, I don't have time to screw up
another set.
Jeremy McLeod
vince pitelka on wed 12 dec 01
> Two larger pieces (a 12" diameter cassarole and a 15" diameter
> platter) didn't survive the bisque firing. They were thrown with Mile
> High's ^6 "Ash" clay body, a porcelian-like substance. I compressed
> the bottoms, I thought, quite well. Both cracked in a straight line
> across the bottoms of the piece. Neither piece straddled across shelves.
> Shelves are new and unwashed (kiln wash, that is).
Jeremy -
In my experience this is almost always a drying problem. The crack may not
appear until the firing, but the stresses build up during drying. You
probably throw these on a bat, and run your cutoff wire under the piece
right after you throw it. But the clay can re-attach around the edges just
from the weight of the pot, and then it will not let the pot shrink
properly. The outer edge of the bottom dries without shrinking as much as
it wants to, because it cannot contract on the bat, and then the inner
surface dries, and inevitably cracks. The solution is to run your cutoff
wire under the pot again after it has stiffened slightly, and then as soon
as the walls and rim are stiff enough, very gently place another bat over
the rim, flip the pot and both bats over, and remove the bat from the
bottom. You can do this when the piece is still too soft to pick up, and
the the bottom will dry at the same rate as the walls, and will be free to
contract without building up any stresses.
Also make sure you remove every trace of water and slurry from the inside
bottom before you run the cutoff wire under the pot right after throwing.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Lexxy on thu 13 dec 01
Thanks for this question and answer.
I had just this same problem with 2 shallow, wide coffee table dishes only
it happened before the bisque fire. I was drying them on a plaster bat to
avoid just what you describe. The bottom was thinner than the ones that
didn't crack laterally. Could too thin bottoms make it risky here even when
care is taken drying on plaster? they seemed ok until they went from almost
dry to bone dry and then the crack appeared....and I have never had this
clay crack with anything so I'm sure I did something too risky.
Would you advise me to usually have the bottoms thicker than the sides which
are just 1 1/2-2"? I was trying to lighten up the pot an may have gone too
far trimming.
Lexxey
East GA, Pottery & Gardening
http://www.flummoxed.org/lexxey/index.htm
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of vince pitelka
Sent: Wednesday, December 12, 2001 8:19 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Cracks Radially Across Bottoms
> Two larger pieces (a 12" diameter cassarole and a 15" diameter
> platter) didn't survive the bisque firing. They were thrown with Mile
> High's ^6 "Ash" clay body, a porcelian-like substance. I compressed
> the bottoms, I thought, quite well. Both cracked in a straight line
> across the bottoms of the piece. Neither piece straddled across shelves.
> Shelves are new and unwashed (kiln wash, that is).
Jeremy -
In my experience this is almost always a drying problem. The crack may not
appear until the firing, but the stresses build up during drying. You
probably throw these on a bat, and run your cutoff wire under the piece
right after you throw it. But the clay can re-attach around the edges just
from the weight of the pot, and then it will not let the pot shrink
properly. The outer edge of the bottom dries without shrinking as much as
it wants to, because it cannot contract on the bat, and then the inner
surface dries, and inevitably cracks. The solution is to run your cutoff
wire under the pot again after it has stiffened slightly, and then as soon
as the walls and rim are stiff enough, very gently place another bat over
the rim, flip the pot and both bats over, and remove the bat from the
bottom. You can do this when the piece is still too soft to pick up, and
the the bottom will dry at the same rate as the walls, and will be free to
contract without building up any stresses.
Also make sure you remove every trace of water and slurry from the inside
bottom before you run the cutoff wire under the pot right after throwing.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
____________________________________________________________________________
__
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You may look at the archives for the list or change your subscription
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Dale Neese on thu 13 dec 01
Lexxy, the risky thing was drying the platters on plaster. Next time dry =
them slowly on several pages of newspaper under loose plastic. The =
plaster will suck moisture out of clay very quickly.=20
Dale Tex
Cindy Strnad on thu 13 dec 01
Dear Lexy,
The base should be about the same thickness as the
walls. If you still get cracks with slow drying,
and your base is of even thickness, try wedging
some grog into the clay.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
vince pitelka on thu 13 dec 01
> I had just this same problem with 2 shallow, wide coffee table dishes only
> it happened before the bisque fire. I was drying them on a plaster bat to
> avoid just what you describe. The bottom was thinner than the ones that
> didn't crack laterally. Could too thin bottoms make it risky here even
when
> care is taken drying on plaster? they seemed ok until they went from
almost
> dry to bone dry and then the crack appeared.
> Would you advise me to usually have the bottoms thicker than the sides
which
> are just 1 1/2-2"? I was trying to lighten up the pot an may have gone
too
> far trimming.
Lexxy -
I think that drying on a plaster bat may be too much of a shock. The outer
edge of the bottom will dry more slowly because of the moisture wicking down
from the walls, and the center will try to shrink, and will build up
stresses. When the clay approaches bone dry it is at its weakest, and that
is when the crack appears.
I would make the bottom exactly the same thickness as the walls, and I would
invert the pieces as soon as possible as described in my previous email, and
then let them finish drying upside-down.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Marsh Pottery on thu 13 dec 01
Lexxey,
Are the WALLS 1 1/2" thick?? If the wall is that thick - it is way too
thick.. 1/2" max. You have enough clay to make a pot three times that size
or height!
I've fired pots 12" across with 1/4" (or less) thick floors and no
cracking (walls 2" hi and about 3/8th thick - or less). Did you foot it?
A middle foot and a rim foot?? On wide pieces I always double foot for
strength. 2-3" out from center and near the rim for outer one...
Was it soft enough to depress when you footed?? Try putting a bat or two
under it next time to support the bottom as you press with a footing tool...
I do that. Sometimes I tend to push too hard and crack the bottoms.. Some
times they crack in drying - back to the reclaim.. worse when they crack in
the bisque or hi fire... :o( A bit thicker is better- but not bottom
heavy.. walls and floor should all be about the same thickness for a primo
pot. It should FEEL as good as it looks! I tend to err on the thin side..
My fish dishes only weigh about 8 or 9 oz glazed and they start out 8x11"
before I lay them in the wooden bowl to curve them.
Tried to add a pic, but Outlook refused me that option.. maybe because this
is a "reply"?
M
Lexxy on fri 14 dec 01
I don't mean to be dense....but let me get this straight...the plaster sucks
moisture faster than air? Is that right?
I put the wide bases on the plaster cuz I thought it would be comparable to
air drying on the sides and try to keep it at the same rate. I have learned
a lot about wrapping thinner rims [like plates] and spouts and other
attachments in plastic to slow the drying and make it dry more evenly so I
want to get this straight. It seems that throwing thinner is making for
more problems for me...sure didn't happen when I threw "doorstops" as I call
my earlier work!!
Lexxey...reading Clayart at 4:30 am....
East GA, Pottery & Gardening
http://www.flummoxed.org/lexxey/index.htm
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Dale Neese
Sent: Thursday, December 13, 2001 5:17 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Cracks Radially Across Bottoms
Lexxy, the risky thing was drying the platters on plaster. Next time dry
them slowly on several pages of newspaper under loose plastic. The plaster
will suck moisture out of clay very quickly.
Dale Tex
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Lexxy on fri 14 dec 01
That did sound funny...the walls were about 5/16" and the floor slightly
thinner like 1/4"....not that I'm that precise ...I was just trimming a lot
off the base to lighten the pot and must have done too much. And I did use a
bat under the dish while trimming....and I always put some nonskid spongy
shelf liner stuff between the pot and bat so it doesn't scratch. That
spongy liner is great for all sorts of studio stuff...like I have circles of
all sizes that I put under rims on the Giffen grip for non skidding and
scratch prevention.
Speaking of firing thick clay ...I was somewhat surprized recently when I
was firing some very thick sculptures that a friend's home schooled students
made....I could have sworn that 4" thick works wouldn't make it but I fired
them slowly and they all were fine...which was great cuz these kids did some
fine stuff!!! I would have had them hollow the pieces out, tho.. I did
learn something about pushing limits.
Lexxey
East GA, Pottery & Gardening
http://www.flummoxed.org/lexxey/index.htm
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Marsh Pottery
Sent: Thursday, December 13, 2001 7:46 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Cracks Radially Across Bottoms
Lexxey,
Are the WALLS 1 1/2" thick?? If the wall is that thick - it is way too
thick.. 1/2" max. You have enough clay to make a pot three times that size
or height!
I've fired pots 12" across with 1/4" (or less) thick floors and no
cracking (walls 2" hi and about 3/8th thick - or less). Did you foot it?
A middle foot and a rim foot?? On wide pieces I always double foot for
strength. 2-3" out from center and near the rim for outer one...
Was it soft enough to depress when you footed?? Try putting a bat or two
under it next time to support the bottom as you press with a footing tool...
I do that. Sometimes I tend to push too hard and crack the bottoms.. Some
times they crack in drying - back to the reclaim.. worse when they crack in
the bisque or hi fire... :o( A bit thicker is better- but not bottom
heavy.. walls and floor should all be about the same thickness for a primo
pot. It should FEEL as good as it looks! I tend to err on the thin side..
My fish dishes only weigh about 8 or 9 oz glazed and they start out 8x11"
before I lay them in the wooden bowl to curve them.
Tried to add a pic, but Outlook refused me that option.. maybe because this
is a "reply"?
M
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
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