Claudia I Franco on mon 28 feb 05
For about one year now, I have been making some large plate-like
platters. They are round about 50 cm in diameter, and about 1.5 cm thick,
mostly flat, but curve upward, like a very very shallow bowl. They are
slab rolled and slump molded in plaster molds. Still to this day, they
CRACK. I am using stoneware clay, bisqued to cone 05 (they crack in the
bisque), and I have tried everything i can think of to keep them from
cracking.
Started slab rolling a thick piece and using large rolling pin to flatten
(like tortilla making), adding ceramic fiber, slab rolling the finished
thickness and piecing together by scoring and wetting, making them
thicker, making them thinner, slow firing, normal firing, new clay,
recycled clay, wedging the clay, not wedging the clay. AAAARRRGGG. Since
my slab roller is not big enough, i have to slab them and then piece them
together then place in plaster mold. They dry leather hard in the mold,
then they are removed and left to air dry on the shelves. They are fired
to cone 05, and stilted so that they dont sit directly on the shelf. Does
anyone have any other ideas to try to keep from cracking before I crack??
TIA
claudia
L. P. Skeen on tue 1 mar 05
Claudia,
You mention all the stuff you're doing to make the platters, but I don't
understand from what you wrote if they have feet of any kind? You also did
not mention if the cracks are occurring along the join. If you have a wide,
shallow disc form like that without a foot ring, (I"m picturing a
parenthesis shape or thereabouts) and you place it on its bottom, then the
weight of all the other clay is pressing down on that bottom and can cause a
crack because the clay will sag a bit during the firing. I don't know if
I"m explaining that well or not, but I shall press on. Have you tried
firing them on their rims instead of on their bottoms? Have you tried
firing them on their bottoms, but with support around the rim to prevent the
sagging?
L
----- Original Message -----
From: "Claudia I Franco"
Subject: opinions for building large flat platters please please
> For about one year now, I have been making some large plate-like platters.
> They are round about 50 cm in diameter, and about 1.5 cm thick, mostly
> flat, but curve upward, like a very very shallow bowl. They are
> slab rolled and slump molded in plaster molds. Still to this day, they
> CRACK.
Victoria E. Hamilton on tue 1 mar 05
Claudia -
Sounds like you might need to dry your slabs slower. Cracking sometimes
begins to occurs as/when the edges of your piece begin to dry faster than
the center, but may not be evident until after the bisque. Try leaving the
slabs in the slump mold a day longer, covered, than you currently do. As
soon as you take the slab out of the mold, it no longer has the support it
needs. Also, try applying some wax resist to the edges to retard drying.
Best of luck,
Victoria Hamilton
Millennia Antica Pottery
Seattle, WA
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Claudia I
Franco
Sent: Monday, February 28, 2005 19:38
To: CLAYART@LSV.CERAMICS.ORG
Subject: opinions for building large flat platters please please
For about one year now, I have been making some large plate-like
platters. They are round about 50 cm in diameter, and about 1.5 cm thick,
mostly flat, but curve upward, like a very very shallow bowl. They are
slab rolled and slump molded in plaster molds. Still to this day, they
CRACK. I am using stoneware clay, bisqued to cone 05 (they crack in the
bisque), and I have tried everything i can think of to keep them from
cracking.
Started slab rolling a thick piece and using large rolling pin to flatten
(like tortilla making), adding ceramic fiber, slab rolling the finished
thickness and piecing together by scoring and wetting, making them
thicker, making them thinner, slow firing, normal firing, new clay,
recycled clay, wedging the clay, not wedging the clay. AAAARRRGGG. Since
my slab roller is not big enough, i have to slab them and then piece them
together then place in plaster mold. They dry leather hard in the mold,
then they are removed and left to air dry on the shelves. They are fired
to cone 05, and stilted so that they dont sit directly on the shelf. Does
anyone have any other ideas to try to keep from cracking before I crack??
TIA
claudia
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Mayssan Shora Farra on tue 1 mar 05
Hello Claudia;
I make some flat platters but modestly sized to fit my kiln 15", I have not
had a crack in over 2 years. The only new thing I changed is .I started
drying them on wire shelves with plastic on top of them so they dry
uniformly as the water settles down in the clay and evaporate from the
bottom, at least that is my theory and I stand by it:)
you might want to try and see if you can prove it or shoot it down:)
Good Luck
Mayssan, in Charleston WV that feels like Minnisota:)
http://www.clayvillepottery.com
Mark Knott on tue 1 mar 05
you could try firing them on one single round shelf which will allow for even heating and cooling and will eliminate the space between the two shelves. or another option which works very well is to stand the platter on edge on the bottom of the kiln and place a small piece of kaowool between the platter and the wall. then simply continue this for each of the next platters. by doing this you will allow the heat to pass evenly across the platter during cooling. which is when most platters crack. this has worked extermly well for me with platters to the 30" - 35'' range. good luck, m. knott
>
> From: Claudia I Franco
> Date: 2005/02/28 Mon PM 10:37:59 EST
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: opinions for building large flat platters please please
>
> For about one year now, I have been making some large plate-like
> platters. They are round about 50 cm in diameter, and about 1.5 cm thick,
> mostly flat, but curve upward, like a very very shallow bowl. They are
> slab rolled and slump molded in plaster molds. Still to this day, they
> CRACK. I am using stoneware clay, bisqued to cone 05 (they crack in the
> bisque), and I have tried everything i can think of to keep them from
> cracking.
> Started slab rolling a thick piece and using large rolling pin to flatten
> (like tortilla making), adding ceramic fiber, slab rolling the finished
> thickness and piecing together by scoring and wetting, making them
> thicker, making them thinner, slow firing, normal firing, new clay,
> recycled clay, wedging the clay, not wedging the clay. AAAARRRGGG. Since
> my slab roller is not big enough, i have to slab them and then piece them
> together then place in plaster mold. They dry leather hard in the mold,
> then they are removed and left to air dry on the shelves. They are fired
> to cone 05, and stilted so that they dont sit directly on the shelf. Does
> anyone have any other ideas to try to keep from cracking before I crack??
>
> TIA
> claudia
>
> ______________________________________________________________________________
> 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.
>
ASHPOTS@AOL.COM on wed 2 mar 05
What clay are you using.. If they were mine id be using paper clay or a raku
clay,,
Mark
Vicki Hardin on wed 2 mar 05
I have seen a very similar problem solved by a potter who would stack an entire kiln with large plates. The outer rims were getting hotter than the centers. To solve this, he increased the space between the shelves by 2 inches to allow for more heat distribution and slowed his firing considerably. This solved the problem.
Vicki Hardin
http://vickihardin.com
Kathi LeSueur on wed 2 mar 05
Claudia I Franco wrote:
>For about one year now, I have been making some large plate-like
>platters. ....... Still to this day, they CRACK. I am using stoneware clay, bisqued to cone 05 (they crack in the bisque), ........ and stilted so that they don't sit directly on the shelf. Does anyone have any other ideas to try to keep from cracking before I crack??
>
>
>
>
Make a form that they can lean against ( I make a slab form inside of a
chimney flue). Fire them standing on their rim.
Kathi
gan8qj88@juno.com on wed 2 mar 05
Claudia,
There is much that you have not told. The suggestion I have is to use a Raku clay - my experience is Standard 239 is a very forgiving clay. One other thing that I see a question about is the joining of two pieces. Maybe someone on list can tell you more about the clay you are using and could this be part of the problem.
Best to You - Gretchen
June Perry on wed 2 mar 05
When rolling out your clay, are you only rolling it on one side? You might
try flipping it over and turning it about 45 degrees and roll again on the other
side.
Remove it from the mold as soon as it is firm enough to handle and dry upside
down, covered with thin plastic (the type you get from the drycleaners).
The cracking may be happening from lack of compression, if you are only
rolling it on one side. Also, it could be a drying problem, as has been mentioned,
or it could be a problem with your claybody if you have too much silica in
there and not enough spar.
Have you tried making these with any other claybodies?
Regards,
June Perry
http://www.angelfire.com/art2/shambhalapottery/
Claudia I Franco on fri 4 mar 05
>For about one year now, I have been making some large plate-like
>platters. They are round about 50 cm in diameter, and about 1.5 cm
>thick,
Thank you all for your replies. I have learned a great deal about what I
am not doing. I am using Cone 10 red stoneware, and ultimately glazed and
fired to cone 10. The problem may be in the drying as some of you have
said, and maybe the compression, I am going to try to put the molds on the
wheel and compress them a bit, to see if this helps, then dry them very
slow. The firing on their side scares me, since the edges are very
fragile. Its a good idea, though. Paper clay doesnt produce the same
desired effect, although I have tried that too. The platters have cracked
both at the seams, round cracks and mostly straight cracks from edge to
center.
Thanks again
claudia
Chris Trabka on tue 15 mar 05
Claudia,
Have you considered a form for forming and another smaller form for drying.
After the piece is leather hard, transfer the piece to the smaller form so
that it can remain inverted while drying. If this is not possible try
covering the form with plastic which has some holes near the center;
letting some air-flow occur while maintaining an even water content from
the rim to the center (if the rims dry sooner than the bottom, a great deal
of stress is forced into the clay - the center trying to move and the rims
unable to).
Chris
Eleanora Eden on wed 30 mar 05
Hi Claudia and all,
I have read all the replies and don't see the advice that Ron Roy
gave years ago that put my platter making on track. For years I had
been making gorgeous platters that would crack. Mine had feet. I
tried thrown ones, cast ones, slab on hump mold, made no difference.
Then Ron advised to make sure to fire the platters in the top half of
the kiln (to make sure they don't heat or cool too fast) and to
surround them with kiln posts to keep the heat differential down
between the rim and the center.
Presto, no more problems.
Eleanora
> >For about one year now, I have been making some large plate-like
>>platters. They are round about 50 cm in diameter, and about 1.5 cm
>>thick,
>
>Thank you all for your replies. I have learned a great deal about what I
>am not doing. I am using Cone 10 red stoneware, and ultimately glazed and
>fired to cone 10. The problem may be in the drying as some of you have
>said, and maybe the compression, I am going to try to put the molds on the
>wheel and compress them a bit, to see if this helps, then dry them very
>slow. The firing on their side scares me, since the edges are very
>fragile. Its a good idea, though. Paper clay doesnt produce the same
>desired effect, although I have tried that too. The platters have cracked
>both at the seams, round cracks and mostly straight cracks from edge to
>center.
>Thanks again
>claudia
>
>______________________________________________________________________________
>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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