David Beumee on fri 15 oct 04
Hi Ron,
> 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.
Here's a reliable porcelain body that is obviously of low expansion (because of the trouble I have fitting glazes to it):
Original BABU
55 Grolleg
20 Custer
13 Pyrax
12 200 mesh silica
2 Veegum T
As you have said, there's no reason why cristobalite should be a problem with a porcelain body because of the higher ratio of spar to clay and silica than with stoneware bodies. The Glaze Test Series has shown that no cristobalite exists at cone 10 with this body. But its very hard though to fit glazes to this low expansion body without crazing.
> 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.
I have a base glaze that I know to be of low expansion. If it turns out that it is not of low enough expansion to fit the Babu body, would you be willing to find out the CoE of the body and match the CoE of the base glaze to it so it won't craze?
I understand that if a glaze is of higher expansion than the body that it will craze. What I want is an uncrazed base glaze over a low expansion body, so that the base glaze is put into slight compression by the body to produce particularly strong ware.
My customers don't understand crazed glazes at all, because they're used to using bone china and they perceive crazing as a fault. I've printed a disclaimer about the use of my work in the oven, but I want to have done everything I can to know, for instance, that I can to put a room temperature platter or plate in a preheated oven without it breaking. We'll ofcourse make the success of this collaboration available on Clayart. Sound good?
Thanks very much for the advice about rounded versus 90 degree angles transferring shock. I see now that the design of the piece is definately part of the solution.
At $175 for a casserole and $375 for a platter, my customers are already loath to use them for anything but serving. Perhaps if I want to continue to eat I need to produce less expensive work, or find a different set of customers.
David Beumee
> 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
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy on sat 16 oct 04
Hi David,
First I have to express an opinion about using porcelain for ovenware - I
don't know exactly where it came from but - over the years I have heard
that a grogged stoneware is a better idea than porcelain. I believe it is
because porcelain cannot take heat shock as well as the more "flexible" and
more open stoneware. You might do some research in this area. Peter Sohngen
may be a good place to start.
You could also consider adding grog to your porcelain - to open it a bit -
but I don't know if molochite will work.
I think the theory is - if a crack comes to a piece of grog it stops - in
porcelain there is nothing to stop a crack so it continues - your are
dealing with a glass like - brittle body - I hope you can find out what
that is all about.
I don't think that cristobalite will be a problem with a clay that has 20%
spar - I have never seen any in any porcelain bodies I have measured.
I am interested to know - how would you use a glaze test series to confirm
cristobalite?
It would be easy to use a glaze that did not craze and one that does - to
do a line blend between the two - to find out when the crazing stopped. Or
I could reformulate the non crazing glaze - raising the expansion till it
did craze - that would give you the info you need. Using the dilatometer is
the hard way - calculation is the easy way.
I have no objection to publishing any such experiments on ClayArt - but I
think it will reach more potters if it is published in say CM. Up to you.
RR
> > 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.
>
>Here's a reliable porcelain body that is obviously of low expansion
>(because of the trouble I have fitting glazes to it):
>
>Original BABU
>55 Grolleg
>20 Custer
>13 Pyrax
>12 200 mesh silica
> 2 Veegum T
>
>As you have said, there's no reason why cristobalite should be a problem
>with a porcelain body because of the higher ratio of spar to clay and
>silica than with stoneware bodies. The Glaze Test Series has shown that no
>cristobalite exists at cone 10 with this body. But its very hard though to
>fit glazes to this low expansion body without crazing.
>
>> 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.
>
>I have a base glaze that I know to be of low expansion. If it turns out
>that it is not of low enough expansion to fit the Babu body, would you be
>willing to find out the CoE of the body and match the CoE of the base
>glaze to it so it won't craze?
>
>I understand that if a glaze is of higher expansion than the body that it
>will craze. What I want is an uncrazed base glaze over a low expansion
>body, so that the base glaze is put into slight compression by the body to
>produce particularly strong ware.
> My customers don't understand crazed glazes at all, because they're used
>to using bone china and they perceive crazing as a fault. I've printed a
>disclaimer about the use of my work in the oven, but I want to have done
>everything I can to know, for instance, that I can to put a room
>temperature platter or plate in a preheated oven without it breaking.
>We'll ofcourse make the success of this collaboration available on
>Clayart. Sound good?
> Thanks very much for the advice about rounded versus 90 degree angles
>transferring shock. I see now that the design of the piece is definately
>part of the solution.
> At $175 for a casserole and $375 for a platter, my customers are already
>loath to use them for anything but serving. Perhaps if I want to continue
>to eat I need to produce less expensive work, or find a different set of
>customers.
>
>David Beumee
>
>
>> 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
>>
>>
>>__________________________________________________________________________
>>____
>> Send postings to clayart@lsv.ceramics.org
>>
>> You may look at the archives for the list or change your subscription
>> settings from http://www.ceramics.org/clayart/
>>
>> Moderator of the list is Mel Jacobson who may be reached at
>>melpots@pclink.com.
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
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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