Tracy Shea on wed 26 nov 03
I have made some casseroles and shallow baking dishes from Standard Clay =
#225 and have had problems with 2 "crackpots" in the oven. They were =
glazed with Ron & John's "oatmeal" and "burnt sienna" glazes from =
mstring^6. The bottom and side walls of each piece were approx. 3/8 of =
an inch- one of the pots had about a 45 degree angled wall , the other =
was more gradual and wide open- both had flat, unglazed bottoms. The =
cracks in both cases start from about the middle/bottom and run =
straight up the side. Each dish was baking in my own 350 degree oven =
and I heard each crack when it happened. -Anyway, I emailed =
standard clay co. for some suggestions, and part of their answer was =
"Although we do not advertise the #225 clay as an ovenware body a lot of =
our customers use it as such. Since the clay does not have materials =
that are used in ovenware bodies, you will have occasional cracking. To =
help keep this to a minimum you can make your pots a little thicker if =
they have large bottoms, and kept your wall and bottom somewhat the same =
thickness." =20
Pray, tell, what are these elusive materials that are in the ovenware =
clay of which he speaks??? I don't really want to make casseroles that =
have to be removed from the oven with a towmotor...:) Is there anyone =
out there using Standard clay as ovenware successfully??? Is there a =
magic material I can just wedge in to improve my clay???? sand? grog? =
I read through the Clayart archives and saw such suggestions as perhaps =
underfiring, making thicker pots- I have not sold anything claiming =
it's ovenware yet- thank the kiln gods- maybe a footring and a glazed =
bottom is the answer??=20
Thanks for your kind consideration- have a warm and filling turkey day!!
Tracy Shea in OH -=20
Mert & Holly Kilpatrick on wed 26 nov 03
I am surprised at this, and I have similar questions. I just went to the
Standard Clay website, and looked at the listings for all their mid and high
fire stonewares. Not one of them says anything about "ovenware". They
don't even talk about end usage, they talk about what techniques they are
good for, like slab work or production throwing. What bodies do they
advertise as ovenware, I wonder, and where do they advertise them as such?
They won't tell us what is in their claybodies, and they certainly don't
make it clear in their brief descriptions whether or not they are
recommended as ovenware bodies. It is frustrating trying to find out
something about Standard clay bodies. I am interested to hear other
people's input on this.
We have had trouble with 112 and s-cracking, it seems to be much touchier
than some other claybodies, requiring much more TLC, and 225 is supposedly
the same recipe without manganese. But your situation (Tracy's) is even
beyond s cracks.
Holly
East Bangor, PA
----- Original Message -----
From: "Tracy Shea"
I have made some casseroles and shallow baking dishes from Standard Clay
#225 and have had problems with 2 "crackpots" in the oven.
.. -Anyway, I emailed standard clay co. for some suggestions, and part
of their answer was "Although we do not advertise the #225 clay as an
ovenware body a lot of our customers use it as such. Since the clay does
not have materials that are used in ovenware bodies, you will have
occasional cracking. To help keep this to a minimum you can make your pots
a little thicker if they have large bottoms, and kept your wall and bottom
somewhat the same thickness." ............Pray, tell, what are these
elusive materials that are in the ovenware clay of which he speaks???
Ron Roy on thu 27 nov 03
Hi Holly,
When you say Manganese - is it the granular kind that gives specks in
oxidation - or is the clay uniformly dark?
You are right - s cracks are drying and compression related problems.
Cracking after firing are expansion/contraction problems - completely
unrelated and requiring different solutions.
RR
>I am surprised at this, and I have similar questions. I just went to the
>Standard Clay website, and looked at the listings for all their mid and high
>fire stonewares. Not one of them says anything about "ovenware". They
>don't even talk about end usage, they talk about what techniques they are
>good for, like slab work or production throwing. What bodies do they
>advertise as ovenware, I wonder, and where do they advertise them as such?
>
>They won't tell us what is in their claybodies, and they certainly don't
>make it clear in their brief descriptions whether or not they are
>recommended as ovenware bodies. It is frustrating trying to find out
>something about Standard clay bodies. I am interested to hear other
>people's input on this.
>
>We have had trouble with 112 and s-cracking, it seems to be much touchier
>than some other claybodies, requiring much more TLC, and 225 is supposedly
>the same recipe without manganese. But your situation (Tracy's) is even
>beyond s cracks.
>
>Holly
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on thu 27 nov 03
Hi Tracy,
Both the glazes you used have a lower than normal expansion rate ( you said
oatmeal and burnt sienna but meant raw sienna - right?)
It could be the clay you used has a higher than normal expansion (high
expansion on heating - high contraction on cooling) so it is probably the
combination of those two factors that resulted in your problem.
This is why we included the set of expansion glazes in chapter 5 - so you
can test the expansion of clays - this is especially important with any
oven ware - less important for tea pots and mugs - but still an important
factor.
Bodies used for oven ware should be low expansion type - so that - when one
side gets heated faster you don't get cracking - which may be part of the
problem here - the low expansion glaze makes it more likely to happen.
You may still be able to use that combination but you will need to do some
things differently.
Our glossy clear liner glaze (page 97) has a higher expansion rate and
would be a better choice for the inside glaze for instance.
Flat bottoms will tend to concentrate any fit problems at the inside
corners - to make matters worse you usually get extra glaze in that corner
which compounds the problem - Keep that area as round as possible - no
sharp corners - a rounded form is much better for casseroles by the way -
easier to clean as well.
Keeping the clay thicker and the inside glaze thinner is also a wise
precaution - lessens the influence of the glaze expansion and the thicker
clay makes the pot stronger.
If you decide to try other bodies - mix up our chapter five fit testing
series and find out where each of them crazes - you will have a much better
idea of which clay is most suitable for oven ware.
I'll be happy to help you with this - If you are willing to fire a sample
of that clay and send it to me I will measure the expansion and then be
able to compare it with other clays - a useful exercise. Pity clay makers
don't have this kind of info on their products - think of how much easier
it would be to choose a clay for oven ware.
If you are willing to do the sample just lest me know and I will send
instructions - it is very easy - just a few coils of clay fired to glaze
temperature - no glaze.
RR.
>I have made some casseroles and shallow baking dishes from Standard Clay
>#225 and have had problems with 2 "crackpots" in the oven. They were
>glazed with Ron & John's "oatmeal" and "burnt sienna" glazes from
>mstring^6. The bottom and side walls of each piece were approx. 3/8 of
>an inch- one of the pots had about a 45 degree angled wall , the other was
>more gradual and wide open- both had flat, unglazed bottoms. The cracks
>in both cases start from about the middle/bottom and run straight up the
>side. Each dish was baking in my own 350 degree oven and I heard each
>crack when it happened. -Anyway, I emailed standard clay co. for some
>suggestions, and part of their answer was "Although we do not advertise
>the #225 clay as an ovenware body a lot of our customers use it as such.
>Since the clay does not have materials that are used in ovenware bodies,
>you will have occasional cracking. To help keep this to a minimum you can
>make your pots a little thicker if they have large bottoms, and kept your
>wall and bottom somewhat the same thickness."
>Pray, tell, what are these elusive materials that are in the ovenware
>clay of which he speaks??? I don't really want to make casseroles that
>have to be removed from the oven with a towmotor...:) Is there anyone out
>there using Standard clay as ovenware successfully??? Is there a magic
>material I can just wedge in to improve my clay???? sand? grog? I read
>through the Clayart archives and saw such suggestions as perhaps
>underfiring, making thicker pots- I have not sold anything claiming it's
>ovenware yet- thank the kiln gods- maybe a footring and a glazed bottom
>is the answer??
>Thanks for your kind consideration- have a warm and filling turkey day!!
>
>Tracy Shea in OH -
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Mert & Holly Kilpatrick on fri 28 nov 03
From: "Ron Roy"
Ron wrote:
> Bodies used for oven ware should be low expansion type - so that - when
one
> side gets heated faster you don't get cracking - which may be part of the
> problem here - the low expansion glaze makes it more likely to happen.
Ron,
Why would the low expansion glaze make cracking more likely to happen on a
low expansion claybody?
Thanks, Holly
East Bangor, PA
Ron Roy on fri 28 nov 03
Hi Holly,
You have it right - I did not express that in the best way possible.
Ovenware bodies should be of the lower expansion type.
If they are high expansion - when one side gets heated faster - it expands
faster and can crack because of that - add in a low expansion glaze and you
have an even worse situation.
In industry they try for a body with very low or no expansion - the problem
then becomes finding a glaze that will not craze.
So low expansion for both body and glaze is the right way to go.
I don't know of any clay manufacture that will provide expansion rates for
their clays although I have diled many of Tuckers bodies.
Porcelains are usually harder to fit glazes to that will not craze - so we
can think of them as "lower expanding" bodies. They are not the best for
ovenware however - because they are so vitrified (or should be) that they
have little "flexibility".
Studio potters should find clays that have lower expansion rates and no
cristobalite and match the body and glaze - having the glaze low enough so
that it does not craze but no lower - and understand about how and where
stress is concentrated by the form and glaze application.
RR
>> Bodies used for oven ware should be low expansion type - so that - when
>one
>> side gets heated faster you don't get cracking - which may be part of the
>> problem here - the low expansion glaze makes it more likely to happen.
>
>Ron,
>Why would the low expansion glaze make cracking more likely to happen on a
>low expansion claybody?
>
>Thanks, Holly
>East Bangor, PA
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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