Irene Stephens on tue 5 sep 00
What determines whether a pot can be used for cooking in the oven. Is =
it the higher cone firing, the kind of clay used or what? Does the same =
apply for microwave cooking? I've asked around & no one seems to know =
the correct answer, lots of guesses.
Also, how do some of you do your year end inventories for taxes. My tax =
lady is after me.....help!
-Are you supposed to use the wholesale or resale value?=20
-Do you count all those 2nds that haven't been thrown out?
-Are you supposed to count all your unused clay?
Ron Roy on thu 7 sep 00
A complicated subject.
The best kind of clay body for oven ware is the type with a low expansion
on heating and cooling over oven temperatures. One reason for failure in
the oven is when the ware is closer to one side and gets heated faster. If
it is a body with high expansion it will expand more on that side -
especially if there is any cristobalite present.
Having the clays available to you measured for expansion would be the best
way to find those with lower expansion rates. Some clay manufactures do
provide that data - very few - and much depends how well maintained the
recipes for those bodies are monitored. The kinds of clay bodies we use are
made from materials that vary a lot - this makes it difficult to have the
control needed for such ware if it to have an extended life. I proper test
program is really the only way to get consistent results.
The second part of this is the clay/glaze fit. If the glaze on the inside
is of too low an expansion compared to the body - the body will cool faster
than the glaze - try to contract and cracking can result. If there is a
glaze on the outside of the ware that has a high expansion (crazed) then it
will weaken the ware and make the situation worse.
The third part is the form. A flat bottomed casserole is more prone to
cracking than a rounded one. If flat with straight sides the forces of fit
and expansion are concentrated at the inside bottom corner - sometime the
whole bottom will fall out. If rounded these forces are distributed more
evenly and the ware will last longer under the same forces.
The best situation would be a body with little or no cristobalite, lower
expansion with the same glaze inside and out with just enough compression
to keep the glaze from crazing. Certainly boron glazes are the most
forgiving and I would recommend them for this situation. I'm not saying all
boron glazes because they can have a low expansion but they are more
"flexible" than other glazes.
This is one of the areas I have some experience with and can help.
Those of you who fire at cone 10 reduction will do well to read the article
by Peter Sohngen on this subject - I can't find my copy of the Studio
Potter it is in but it was the last year. They have a web site so try a
library first or look it up at:
http://www.studiopotter.org/
The crucial factor in microwave cooking is the degree of vitrification - if
water can get into the body - under the glaze you can have a very hot mug
and I suppose even an explosive situation. Your microwave instruction
manual will probably have a way to test for this. Put in a mug of water and
the empty pot you want to test - it will test OK if it is just out of your
kiln because water will not have had a chance to get into it. Perhaps a
good long soak or boil on hot water would be appropriate - or just leave it
in the dish washer for a week to give the clay a chance to soak up some
water.
So - a mug full of water and the empty mug you want to test - start on high
for 10 seconds - feel the mug - repeat all the way up to when the other
water in the other mug is starting to boil. If the tested mug gets hot the
clay is not vitrified enough for a microwave.
Remember - no stoneware clay has a range of 3 or 4 cones - it will either
be underfired at the lower cone or overfired at the higher or both - you
can use this test to see if your latest batch of clay has to much
absorption.
RR
> What determines whether a pot can be used for cooking in the oven. Is it
>the higher cone firing, the kind of clay used or what? Does the same
>apply for microwave cooking? I've asked around & no one seems to know the
>correct answer, lots of guesses.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Victor Levin on sat 9 sep 00
In a related question, is pouring boiling water into a cup or teapot NOT
the way to test it for functional worthyness? Does it just seem
old-fashioned now that we have microwaves? thank you.
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John Hesselberth on mon 11 sep 00
Victor Levin wrote:
>In a related question, is pouring boiling water into a cup or teapot NOT
>the way to test it for functional worthyness? Does it just seem
>old-fashioned now that we have microwaves? thank you.
Hello Victor,
That is an excellent way to test clay/glaze fit and thermal shock
resistance. Put it in the freezer for a couple hours first to maximize
the test. And do it several times. Functional ware should be able to
withstand this kind of heat shock. Just do it in a sink in case it does
break or splash.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
Ron Roy on tue 12 sep 00
Hi Victor,
Two different tests. Pouring boiling water into a frozen cup is testing for
a low expansion glaze on a higher expansion clay - glaze under compression
from the body and - if enough - will crack the pot.
If a clay is not vitrified enough water can get (will get in) under the
glaze - if that happens - that water will get hot when microwaved. It's
really a test to see if the clay has been fired high enough to exclude
water.
RR
>In a related question, is pouring boiling water into a cup or teapot NOT
>the way to test it for functional worthyness? Does it just seem
>old-fashioned now that we have microwaves? thank you.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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