Sue Hintz on sat 24 may 97
I've been making platters for awhile now and have had little problems. So the
other day I fired what I thought was my best platter yet.
It came out of the bisque with a crack all of the way around the bottom edge of
the platter where the bottom ends and the rim begins. Here are somethings I did
differently, do you think any of these things caused this to happen?
In order to prevent the rim to warp during drying I placed a piece of drywall on
the rim once it could support the weight. I alway dry my platters on drywall,
right side up but the rims tended to be a littlet warped in the past. It was dr
completely by the time I fired it. I did not put the piece back on the wheel to
be trimmed. I just sanded it down a bit.
When I fire large platters I always stack things on it and did so again. Howeve
I do believe I had more weight/more pots on it than I usually do.
I do not believe that the bottom was warped becasue it dried flat on the drywall
Any ideas about why this happened?
Cindy on sun 25 may 97
Sue,
As you know, drywall is kind of heavy. I suspect the weight may have
stressed your green pot at its weakest point--the change of direction. I've
heard (never tried it) that placing a sheet of newspaper across the pot
before removing it from the wheel will tend to discourage warping. My own
solution is to 1.) Throw the platter on a plaster bat and not cut it loose
until it can be lifted off. 2.) Turn the platter upside down and allow it
to dry on a flat, absorbent surface (such as your drywall). Since you don't
trim your bottoms, there shouldn't be major problems with the bottom
sagging, but if there is, you'll have to flip it back and forth--upside
down, rightside up, etc. Warping is one problem, at least, that I haven't
had with platters. Finding a big enough container to dip them in glaze
evenly, now . . . that's another matter. Good luck with your platters.
Cindy Strnad
Vince Pitelka on sun 25 may 97
>When I fire large platters I always stack things on it and did so again.
Howeve
>I do believe I had more weight/more pots on it than I usually do.
Sue -
There are several things I can see right off which could explain the
cracking. One is your habit of putting other things on top of platters when
bisque-firing them. I used to do this a lot, and I lost lots of platters.
Mine generally cracked from the outside towards the center, because the
pieces stacked on top of the platter would insulate the center from the
heat. The outer rim would heat up faster, and would expand, causing a crack
from the rim towards the center. Yours are cracking in a very different
way, but it could also be because of the way you are drying them. I think
that placing any weight on the rim of a platter as it dries is risky. I
have had best luck drying platters slowly with absolutely no air currents
around them. I often have a very slight waviness to the rims, but that is
the price I pay to get good platters with no cracks. And you can only spot
this minor amount of waviness if you really look for it. If you are drying
your platters too fast, the rims dry first, since they are drying from the
edge and from both sides, and in doing so the rim usually raises up. Then
the bottom dries, drawing the rim back down, setting up consiiderable
stresses in the zone between the bottom and the rim. This would be further
exacerbated by stacking things on the platters in the bisque.
If I get a platter thrown, dried to leather hard, trimmed, and dried to bone
dry, without significant warpage, then I can be reasonably sure that it will
not warp in the bisque-fire, even if the foot is not fully supported. I
place a platter directly on the kiln shelf, and then I place three softbrick
shims (1/4" x 1" x 1"), evenly spaced around the inside bottom of the
platter. I place another platter so that it rests on these shims, and place
three more shims directly over the previous ones, as you would place posts
beneath and above kiln shelves. The shims allow air to circulate freely
around the platters during heating and cooling, eliminating cracking
problems. With large platters I do not stack more than two or three. With
dinner-plate size and smaller I often stack four to six. I have always done
this with stoneware clays and have never had any problem with warpage at
bisque temperatures.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166
C.T. Wagoner on mon 26 may 97
Dick Hay at I.S.U. suggested that to keep platters from cracking in drying
or later because of stresses in firing to cut plastic with a hole in the
middle and allow the large pieces to dry from the center first allowing the
clay to "shrink down on itself" as opposed to the outside drying first and
than the center dries from the "inside out and cracks". I have found this
to work really well, especially on things like chip and dips that really
have a nasty habit of cracking. Seems like turning the pot upside before
the whole thing is completely dry is similiar as it allows the center to
catch up?
Anyway it works for us.
BTW Today when I was throwing I did a lot of "pressing" on the clay to keep
it from cracking. It was fun to think about all this discussion while I
worked.
Wagoner Pottery
"Made to be used!"
http://abcs.com/cwag/
D. Rush Tucker on mon 26 may 97
platters definately crack more in bisque fire when u load em down with
stuff........alot of my cracking problems were solved by bisqueing on a layer
on sand.........2cents worth
rush
Suzanne Storer on tue 27 may 97
Sue,
I dry my platters upside down with foam rubber supporting the bottom from
underneath - little warping. I never fire with anything on the platter.
The large flat bottomed platter form is a very weak one and many factors can
contribute to cracking.
Suzanne Storer
Ron Roy on wed 28 may 97
>It came out of the bisque with a crack all of the way around the bottom edge of
>the platter where the bottom ends and the rim begins. Here are somethings
>I did
>differently, do you think any of these things caused this to happen?
Hi Sue,
I'm still not sure where the crack was - inside the foot or outside? All
the way through - did the bottom fall out?
It would help if I knew if the crack is open or closed. If it is still
"open" then it happened between the throwing and dry state and "appeared"
during the bisque.
If the crack is "closed" then it probably happened around 570C (quartz
inversion) as your kiln cooled. I never stack my platters and I make sure
my kiln is closed as well as possible to slow down cooling. I fire in a gas
kiln and I use porcelain - Porcelain has much more free quartz than most
clays and will dunt easily if it gets a draft.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
ronroy@astral.magic.ca
Ray Carlton on fri 30 may 97
At 08:53 AM 27/05/97 EDT, you wrote:
>----------------------------Original message----------------------------
>Sue,
>I dry my platters upside down with foam rubber supporting the bottom from
>underneath - little warping. I never fire with anything on the platter.
>The large flat bottomed platter form is a very weak one and many factors can
>contribute to cracking.
>Suzanne Storer
>
>Hi suzanne I suggest you could try a different body in my not
inconsiderable experience I have found there are plenty of good commercial
bodies out there that don't crack up at the slightest provocation I use two
great bodies that never give me any problem I dont know where you are but
in australia i would recomend keanes white raku as a medium body [fires up
to ^13] or clayworks jb3 porcelaineous stone ware also ^13. they never
crack you can make platters up 750 mm [large] and you can jump on them [not
really but they are very good]
ray carlton mcmahons creek victoria australia
Ray Carlton McMahons Creek Victoria Australia
raycarlt@ozonline.com.au
Marni Turkel on sat 31 may 97
I know someone already mentioned dunting cracks and cooling the kiln
slowly. I would like to add that dunting in the bisque can also occur in
the heating cycle. I found that elevating the pieces a 1/4 inch or so from
the shelf on scatter pins (commercially available, 3" long triangular clay
pieces) allowed me to bisque platters and other wide bottomed pieces
stacked as tightly as I could have wanted without dunting. I found that I
had to go slowly through the 1000 degree F range on the heating cycle, but
with the pins, reduced a major dunting problem to an occassional nuisance.
Marni Turkel
Stony Point Ceramic Design
Santa Rosa, California
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