David Beumee on tue 12 oct 04
Ron wrote,
"> I remember explaining - on ClayArt - about the freeze/boil test for
> spotting this problem - some believed that pouring boiling water into a
> frozen pot was too extreme a test and that no pottery could survive that.
>
> Gives you some idea about the state of understanding by some potters
> producing ware out there."
Given the best possible situation, a fired body of high enough expansion/contraction to put a slight squeeze on a glaze that has been adjusted to expand/contract slightly less than the body, and that has been applied evenly inside and out of a pot, what is a fair test to determine if the pot will survive the rigors of the kitchen? I'm talking about someone taking a casserole out of a 375 degree oven and setting it hot on a wet counter, or taking a frozen casserole out of the freezer and putting it in a preheated oven, filled with frozen food.
Would you be willing to determine the coefficient of expansion of the porcelain body I use and adjust a base glaze to produce the strongest possible utilitarian ware? Do guidelines exist that could tell you just how much lower expansion the glaze needs to be in comparison to the body to produse the strongest possible ware?
David Beumee
Earth Alchemy Pottery
Lafayette, CO
> Hi Ivor,
>
> I agree - classic example of what a bad mismatch of glaze and clay can do -
> in a way better to see it right away instead of it happening on a pot in
> use in somebodies kitchen.
>
> I remember explaining - on ClayArt - about the freeze/boil test for
> spotting this problem - some believed that pouring boiling water into a
> frozen pot was too extreme a test and that no pottery could survive that.
>
> Gives you some idea about the state of understanding by some potters
> producing ware out there.
>
> RR
>
>
> >We have differing images in mind. You have put a glaze on the outside
> >of your example. I restricted my comment to glaze on the interior
> >surfaces only. In the latter example the glaze will contract during
> >cooling a lesser amount than the clay ceramic.
> >Thinking about the former example with identical glaze on the interior
> >and exterior surfaces. I have no argument with your conclusion. But in
> >extreme cases I believe the stage is set for the glaze on the outside
> >to spall, or as you say, shiver.
> >I am sending you a jpeg image by second post showing what I believe is
> >the extreme of having a low CoE on the interior and a high CoE n the
> >exterior.
> >I thought these were classic examples. Came out of the kiln like this.
> >Best regards,
> >Ivor Lewis.
>
> 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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Ron Roy on thu 14 oct 04
Hi David,
There are other factors that are involved - mainly the amount of
expansion/contraction of the clay itself. The lower that is the better for
starters. If a clay has no expansion/contraction on heating and cooling -
problem solved - Pyrex is an example.
So keeping the body expansion low is the fist step - I have never done the
research on that but I do have data that would allow me to come to some
conclusions. Kyanite and Spodumene come to mind. The problem then becomes
finding glazes that do not craze on such bodies.
To answer directly - there is in Hamer - a line that says - potters who
produce functional ware feel safer if their glazes craze now and then. So
that is real close to crazing - how much further can you go? I don't know
the answer to that David - and to some extent it depends on the body
expansion and contraction.
Out of the freezer into a hot oven is going to be very difficult with clay
and glaze layered - notice that ovenware sold is not laminated - solid all
the way through.
You will have to do the research yourself - but always add - you take no
responsibility - someone will find a way to make em crack.
Also - the form is very important - flat bottomed forms are going to be
problematic.
If you need help with this I will consider it - but my usual advice is to
make sure there is NO cristobalite and strict control on the clay body -
like test each batch to make sure it has the same CoE as the last batch -
and fire to the same temperature each time - top and bottom of kiln - and
randomly test the product hard to make sure it does what you say it does.
There is another strategy I see used - price em so high that the owners
will be loath to use em except as serving dishes.
RR
>Ron wrote,
> "> I remember explaining - on ClayArt - about the freeze/boil test for
>> spotting this problem - some believed that pouring boiling water into a
>> frozen pot was too extreme a test and that no pottery could survive that.
>>
>> Gives you some idea about the state of understanding by some potters
>> producing ware out there."
>
> Given the best possible situation, a fired body of high enough
>expansion/contraction to put a slight squeeze on a glaze that has been
>adjusted to expand/contract slightly less than the body, and that has been
>applied evenly inside and out of a pot, what is a fair test to determine
>if the pot will survive the rigors of the kitchen? I'm talking about
>someone taking a casserole out of a 375 degree oven and setting it hot on
>a wet counter, or taking a frozen casserole out of the freezer and putting
>it in a preheated oven, filled with frozen food.
> Would you be willing to determine the coefficient of expansion of the
>porcelain body I use and adjust a base glaze to produce the strongest
>possible utilitarian ware? Do guidelines exist that could tell you just
>how much lower expansion the glaze needs to be in comparison to the body
>to produse the strongest possible ware?
>
>David Beumee
>Earth Alchemy Pottery
>Lafayette, CO
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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