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clay and thermal shock

updated sun 30 jun 96


Darlene Yarnetsky on fri 31 may 96

Hello mud lovers-
There's been some talk lately about clay bodies for oven use, thermal
shock etc. I've never mixed my own clay body (would love to experiment
when I get a bigger space someday) and am using clay from Standard
Ceramic Supply. My question is this: How does one determine whether an
already manufactured clay is good in the oven? I know Highwater Clay
advertises one of their clays as having good thermal qualities - what
exactly does this mean, or to be more specific, why would this certain
clay be less prone to thermal shock? While I'm not mixing my own clay, I
am curious as to what the theories are behind this as well as suggestions
on what already prepared clay I should use for casseroles and pie
plates. Perhaps someone can point me to a good text to satisfy my
curiosity! Any suggestions? Thanks everyone!

Darlene Y. in Madison, where the sun is finally shining!

Ron Roy on sun 2 jun 96

Hi Darlene,

Hi Darlene,

Low expansion/contraction bodies are preferred for oven use because, when
heated and cooled they "move" less than normal clay bodies. Most commercial
clay bodies advertised for oven use have materials in them which lower
expansion on heating and contraction on cooling. One of those materials
would be Kyanite.

The theory is - if a container is heated - say in an oven - and one or two
sides get heated faster than the others - then if the body has a low
expansion - those parts which are heated faster will not expand as much as
an ordinary clay body and will be less likely to crack the pot trying to
adjust to the slower heated clay.

Such bodies must be designed so that cristobalite is kept to a minimum
because it goes through dramatic, reversible volume changes between 200C
and 250C (392F - 482F) like in ovens, and will have the opposite affect.
What cone does cristobalite start to form? It depends, I suppose, but 1100C
seems to be the consensus. The main cristobalite inhibitors are sodium and
potassium - trouble is they have high expansion rates. Lithium does the
same kind of job as sodium and potash but has a lower expansion rate.
Bodies made with Spodumene are sometimes sold as stove top or flame proof

True low expansion bodies are difficult to fit glazes on without crazing
because of their low contraction on cooling.

So what to do: Cardew says add granular material like grog, mulite, kyanite
and mica. His theory is keep the body open and use a non crazing glaze to
keep the food in. I am leery about being able to do that - if the body is
open water can get in - the clay will partially rehydrate - expand and the
glaze will craze over time. Could there be a danger of explosion if there
is water in the clay (under the glaze) when put into a hot oven - I don't
know - has anyone ever had that happen? I feel sure it could in a microwave
given the right (wrong) set of circumstances.

What do I say. This is a tough problem and you better apply as much science
as possible and test test test. I have known potters who made ovenware for
years with little problems and then almost everything started coming back.
We use variable materials so we can't expect to be able to have complete
control. I say beware.

If a company advertises an ovenware body they should be willing to
recommend a glaze that fits. They should also recommend type and duration
of firing.

Testing is the key. Whatever combination you use - you could make some
large casseroles and use them for awhile. They should at least be able to
survive starting out in the oven with cold food in them. Better be ready to
catch the contents in case they crack.

It is also a good strategy to use the same glaze inside and out to minimize
glaze stresses.

I am sure there are many potters who make casseroles and have little
trouble. I am also sure there are no potters making casseroles with clay
and glaze that have no problems. Having a proven combination would be a
definite advantage but be sure you duplicate the firing as well. Make sure
you provide instructions - reheating food in a casserole straight from the
fridge is just asking for trouble. Might be a good idea to have some
liability insurance in case.

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849