kelty-huber on fri 1 dec 00
Dannon
as with all of these problems there are many variables.It is true that
with a low bisque there is no shinkage, and some suggest that there might
even be a slight expansion,but if the bisque is high enough that any
glasses form than there is shrinkage. One of the things that I found helped
my platter cracking problem was to spray my masonite bats with wd40 prior
to throwing the platter.This seemad to significantly lesson the tension in
the platters and all but eliminated my cracking problem. With regards to
the heating and cooling causing the cracking if there was no shrinkage (or
expansion) how would uneven heating or cooling cause cracking ?
Tom Huber
At 09:56 AM 12/1/00 -0800, you wrote:
>I remember that a while back someone, Mel I think, made a post observing
>> that clay does not shrink between bone dry and bisque. I didn't believe
>> it, but when I looked at the shrinkage rulers I had made out of several
>> clay bodies, I found this to indeed be true (at least on all the cone 6 &
>> cone 10 clays that I personally tried). If this is indeed the case, why
>> would there be any problems with platters cracking in the bisque if they
>> are not "loose" on the shelves? I could understand if they were cracking
>> in the glaze firing........
>>
>
>
>
>It is not the shrinkage that causes the cracking. It is the uneven
>heating/cooling between the rim and the center. The rim heats and
>cools much faster unless there is some mitigation.
>
>regards,
>
>Dannon Rhudy
>
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amy parker on fri 1 dec 00
I remember that a while back someone, Mel I think, made a post observing
that clay does not shrink between bone dry and bisque. I didn't believe it,
but when I looked at the shrinkage rulers I had made out of several clay
bodies, I found this to indeed be true (at least on all the cone 6 & cone
10 clays that I personally tried). If this is indeed the case, why would
there be any problems with platters cracking in the bisque if they are not
"loose" on the shelves? I could understand if they were cracking in the
glaze firing.
Amy
Amy Parker
Lithonia, GA
Dannon Rhudy on fri 1 dec 00
I remember that a while back someone, Mel I think, made a post observing
> that clay does not shrink between bone dry and bisque. I didn't believe
> it, but when I looked at the shrinkage rulers I had made out of several
> clay bodies, I found this to indeed be true (at least on all the cone 6 &
> cone 10 clays that I personally tried). If this is indeed the case, why
> would there be any problems with platters cracking in the bisque if they
> are not "loose" on the shelves? I could understand if they were cracking
> in the glaze firing........
>
It is not the shrinkage that causes the cracking. It is the uneven
heating/cooling between the rim and the center. The rim heats and
cools much faster unless there is some mitigation.
regards,
Dannon Rhudy
vince pitelka on sat 2 dec 00
> the platters and all but eliminated my cracking problem. With regards to
> the heating and cooling causing the cracking if there was no shrinkage
(or
> expansion) how would uneven heating or cooling cause cracking ?
Stop and think about this. In the bisque-firing this is never an issue of
firing shrinkage unless you are bisque-firing way too high. We are talking
about the expansion and contraction that always occurs when you heat and
cool things - uneven expansion in heating or uneven contraction in cooling.
If the rim of a platter heats up before the center does, the platter will
most likely crack. It has nothing to do with firing shrinkage.
Best wishes -
- Vince
Vince Pitelka
Home - vpitelka@dekalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
http://www.craftcenter.tntech.edu/
Linfield College on sat 2 dec 00
on 12/1/00 8:46 PM, kelty-huber at huber@CYBERTOURS.COM wrote:
> Dannon
> With regards to
> the heating and cooling causing the cracking if there was no shrinkage (or
> expansion) how would uneven heating or cooling cause cracking ?
> Tom Huber.......
>
>> It is not the shrinkage that causes the cracking. It is the uneven
>> heating/cooling between the rim and the center. The rim heats and
>> cools much faster unless there is some mitigation.
>>
>>Dannon Rhudy
Tom,
Whether or not there is shrinkage on firing, there is always expansion
and contraction DURING the firing. When this expansion and contraction
is markedly uneven (as when the wide thin rims of platters heat/cool
faster than the thicker foot-enclosed bottoms) THEN there can be, and
often is, cracking. These kinds of cracks run in a line from the rim
toward the center of the platter or bowl, typically. That was the kind
of crack described in the original post on this subject.
regards
Dannon Rhudy
Bob Hanlin on sun 3 dec 00
I have eliminated all my platter cracking problems by only bisquing them on
the top couple of shelves of my electric (have no experience bisquing in a
gas kiln) kiln. I do take them up slowly though
Bob
Connie Christensen on sun 3 dec 00
Dannon wrote:
....Whether or not there is shrinkage on firing, there is always
expansion and contraction DURING the firing. When this expansion and
contraction is markedly uneven (as when the wide thin rims of platters
heat/cool faster than the thicker foot-enclosed bottoms) THEN there can
be, and often is, cracking. These kinds of cracks run in a line from
the rim toward the center of the platter or bowl, typically....
I used to have problems with platters cracking like this until someone
on clayart (this was a couple of years ago) suggested laying kiln
furniture around the outside of the platter to hold the heat in so the
rims didn't cool faster than the rest of the platter. It worked great, I
haven't had any cracking problems since.
Connie Christensen
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