Vince Pitelka on tue 14 jun 05
David -
That's a very good question. I always sold my pottery as microwave safe,
and never encountered a problem. You might say "Microwave safe for
water-based foods and fluids. Do not use for heating liquid oils or for
melting sugar or candies, as the heat might cause the vessel to crack."
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
David Berg on tue 14 jun 05
Dear Functional Potters,
I have always touted my functional stoneware to be microwave safe and I
have used it my self to heat all kinds of different foods and liquids in
the microwave and never had any problems - except I burned my tongue once
when the coffee got too hot.
However, my neice recently tried to melt some caramels in the microwave in
one of my bowls and the bowl cracked. I assume it cracked from thermal
stresses caused by the super hot sugar in the bottom of the bowl compared
with the cooler upper part of the bowl that didn't have any caramel
touching it.
Now I think I might need to add a disclaimer to my advertised "Microwave
Safe." Something like: "The only exception to safe microwave use is to
avoid heating liquid oils and liquid sugars =96 they get too hot, leading to=
thermal stresses that may break the pottery.
Anyone else have any thoughts regarding this? Could the same thing happen
to Pyrex glass?
David
http://bergstoneware.com/
June on tue 14 jun 05
Yes, I have had the same thing happen to Pyrex. I was cooking candy in the
microwave and the pyrex bowl exploded!
-June
www.risingsunearthworks.com
> Anyone else have any thoughts regarding this? Could the same thing happen
> to Pyrex glass?
>
> David
> http://bergstoneware.com/
Tony Ferguson on tue 14 jun 05
David,
Your work should not crack in the microwave. I would examine how you fire and or cool your kiln as well as your claybody and glaze fit relationship. Even with a disclaimer, you are setting your self up for possible trouble. Perhaps others can chime in as you share information about your clay body, glaze, firing, etc.
Tony Ferguson
David Berg wrote:
Dear Functional Potters,
I have always touted my functional stoneware to be microwave safe and I
have used it my self to heat all kinds of different foods and liquids in
the microwave and never had any problems - except I burned my tongue once
when the coffee got too hot.
However, my neice recently tried to melt some caramels in the microwave in
one of my bowls and the bowl cracked. I assume it cracked from thermal
stresses caused by the super hot sugar in the bottom of the bowl compared
with the cooler upper part of the bowl that didn't have any caramel
touching it.
Now I think I might need to add a disclaimer to my advertised "Microwave
Safe." Something like: "The only exception to safe microwave use is to
avoid heating liquid oils and liquid sugars – they get too hot, leading to
thermal stresses that may break the pottery.
Anyone else have any thoughts regarding this? Could the same thing happen
to Pyrex glass?
David
http://bergstoneware.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.
Tony Ferguson
..where the sky meets the lake..
Artist & Educator
fergyart@yahoo.com
fergy@cpinternet.com
(218) 727-6339
http://www.aquariusartgallery.com
http://www.tonyferguson.net
__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com
Rog Coman on tue 14 jun 05
Sometime ago this subject came up on Clayart and it was suggested (don't
remember who) that one should not put any type of disclaimer sticker/message
on their pots. Don't know if that would help if one was to get sued because
of an accident involving one's ware. I don't put any disclaimers on mine,
but if asked, I tell them that because of the high temp to which the pot is
fired, it should be dishwasher, oven and microwave safe, but with the caveat
that it is not a guarantee. Have had no troubles - keeping my fingers
crossed.
Rog Coman
Fish Hook Pottery
Montrose, CO
Looking at the beautiful San Juan Mtns. which are still loaded with snow -
but it's 80 degrees here in the valley today. Love it!!!
----- Original Message -----
From: "David Berg"
To:
Sent: Tuesday, June 14, 2005 6:13 AM
Subject: Do I need to add a disclaimer to microwave safe?
Dear Functional Potters,
I have always touted my functional stoneware to be microwave safe and I
have used it my self to heat all kinds of different foods and liquids in
the microwave and never had any problems - except I burned my tongue once
when the coffee got too hot.
However, my neice recently tried to melt some caramels in the microwave in
one of my bowls and the bowl cracked. I assume it cracked from thermal
stresses caused by the super hot sugar in the bottom of the bowl compared
with the cooler upper part of the bowl that didn't have any caramel
touching it.
Now I think I might need to add a disclaimer to my advertised "Microwave
Safe." Something like: "The only exception to safe microwave use is to
avoid heating liquid oils and liquid sugars - they get too hot, leading to
thermal stresses that may break the pottery.
Anyone else have any thoughts regarding this? Could the same thing happen
to Pyrex glass?
David
http://bergstoneware.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.
Tom at Hutchtel on tue 14 jun 05
David...
Echoing Tony's post.....
No, even under the stress of a hot sugar mix, your clay body should not
crack in the oven. We switched to Highwater's Hestia a couple of years
ago, and immediately had problems with our chicken bakers ( sorry Tony
C...but we make chicken bakers.....it helps keep us alive to make other
pots). Highwater was immediately concerned and made changes in the body (we
were never specifically told what) and to some degree it ameliorated the
problem, but we still had 2 out of 20 crack and thus couldn't trust the
change...and for bakers only, went back to Continental's Tableware.
The point of all this is, don't trust the clay maker's words that
such-and-such body is oven safe. I would guess that even Tuckers doesn't do
regular destructive thermal shock testing (Ron can you comment?). I would
understand that if a body works for thermal shock as formulated, and that if
the mixer tests regularly for variations by batch, the theory would suggest
that the body is still resistant to thermal shock. However, you will not be
notified of changes made in the body.
After I notified Highwater that we had this problem (we replaced 42 bakers
for free), we later found that they had changed the recipe to become their
standard Hestia. Bet no one who regularly uses Hestia got a notice that
there had been a recipe change and to check their glaze reactions,
etc....indeed,, I would like to post a survey....has anyone on the list ever
been notified of a recipe change by a clay manufacturer?
Anyway, don not assume that changes aren't made and do not assume that your
clay manufacturer actually tests for thermal shock...heck, how many of you
actually do?
Tom
Subject: Re: Do I need to add a disclaimer to microwave safe?
> David,
>
> Your work should not crack in the microwave. I would examine how you fire
> and or cool your kiln as well as your claybody and glaze fit relationship.
> Even with a disclaimer, you are setting your self up for possible trouble.
Vince Pitelka on tue 14 jun 05
Tony Ferguson wrote"
> Your work should not crack in the microwave."
I don't think you can say that definitively. This is something not many of
us have encountered or thought about. Almost everything that gets heated in
the microwave is water-based, and the water turning to steam prevents the
food-pottery contact surface from rising above 212 degrees F. If you heat
oil or melt sugar in the microwave in a pottery vessel, the contact surface
can get much hotter, and the piece may not be able to withstand the
expansion discrepancy. I think that any studio ceramics would be
vulnerable to that extreme-case situation.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Mike Martino on wed 15 jun 05
Hello David,
Have you experimented by doing the same thing to some of your other
bowls? That bowl might have been the exception. You might still be
microwave safe. I've had porcelain 'everything safe' Mikasa ware crack
in the microwave, but only one so I'm assuming it was the exception to
the rule.
Mike
in Taku
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
mike martino
in taku, japan
muchimi@potteryofjapan.com
www.potteryofjapan.com
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Gretchen Morton on wed 15 jun 05
I think most people who use a microwave know that you must have liquid in the plate, bowl, or cup. When you heat something up in the pyrex or what ever ceramic or glass you use. When the liquid is gone or the liquid is what you are trying to make ----- melting sugar etc. that is not normal use. It comes under the direction of don't put an empty container, that means it is empty of liquid. If this is just a repeat
message, sorry. Mel just delete--thanks
Gretchen in NY
Vince Pitelka on wed 15 jun 05
> No, even under the stress of a hot sugar mix, your clay body should not
> crack in the oven.
Tom -
I agree, it shouldn't crack in the oven, but the microwave is a different
matter, since it only heats the food (unless there is water impacted in the
claybody). The melting sugar can heat one part of the pot to a temperature
much higher than the boiling point of water, and I can think of no other
circumstance where pots are subjected to this. This really might be worth a
special disclaimer. I have never encountered this problem myself, but our
society is becoming increasing litigious, and potters need to protect
themselves.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Jim Willett on wed 15 jun 05
I thought you might find interesting the warnings about Pyrex from the
company itself:
READ and SAVE THESE INSTRUCTIONS:
WARNINGS: Failure to follow these instructions can cause breakage
resulting in injury or property damage.
=95 NEVER USE ON TOP OF STOVE, under a broiler, in a toaster oven, or place
over oven vent or pilot light.
=95 AVOID SEVERE HOT TO COLD TEMPERATURE CHANGES and DO OT add liquid to hot=
dish, place hot dish or glass cover in sink, immerse in water or place on
cold or wet surfaces.
=95 HANDLE hot ovenware and glass covers with dry potholders.
=95 DO NOT use in microwave to hold or support popcorn bags, microwave
convenience foods with special browning wrappers, etc.
=95 DO NOT use to pop corn, carmelize sugar, or deep fat fry.
=95 DO NOT overheat oil or butter in microwave. Use minimum amount of
cooking time........
http://www.worldkitchen.com/FAQ/faq3.asp?id=3D212
Seems even the big companies don't want you carmelizing sugar or heating
oil.
Jim Willett
Out of the Fire Studio
Edmonton, Alberta, Canada
http://www.outofthefirestudio.com
Ron Roy on thu 16 jun 05
Hi David - I wish I knew what cone you are firing to - I could be more
specific in my answer.
What we are probably dealing with here is uneven heating - in this case the
bottom of the pot gets hot before the top and sides. Bottom expands faster
than the top and sides and dunting is the result.
If it happens with one pot - with a specific clay glaze combination - you
should assume there are factors which should be corrected because it
probably will happen again.
I am assuming the clay is vitrified enough to not leak by the way.
Is it the clay, the glaze, or the form - or a combination of all three?
=46irst we should consider the clay. Many cone 10 clays are going to have
cristobalite in them - judging from the recipes I see in the literature and
what potters have told me they are mixing. I have even found cristobalite
in cone 6 bodies lately - some thing I did not think was possible.
If there is cristobalite present is any significant amount it raises the
expansion rate of the clay over all - and goes through a fast expansion
between 100C and 200C - which is in the range of cooking ovens.
To find out if there is cristobalite present the clay needs to be measured
by dilatometery - perhaps your clay maker can tell you that - not a common
practice for clay companies but worth a try.
So - clay expands differently because it is heated unevenly and can crack
because of that - even without glaze. What is best for oven ware is a clay
with a lower expansion rate - so if it is heated unevenly it will not tend
to crack - certainly cristobalite is not wanted.
Peter Sohngen got it exactly right in his article on the subject - see
Studio Potter - volume 28 #1 - I did the dilatometry for the article.
The glaze: I should calculate your glaze to see if the expansion rate is
low - if it is then we can alter the liner glaze to be more compatible and
apply some testing. If it has a "normal" amount of expansion we then have
to suspect the clay. Low expanding glaze - does not contract enough during
cooling and ends up too big for the clay. The glaze is actually trying to
push the pot apart - so there is residual strain waiting for the uneven
heating to help it break the pot.
The form: Flat bottoms and straight sides are the worst - especially if the
corners - where the bottom and sides meet - are sharp. Glaze winds up
thicker in that corner - and any residual strain or strain brought on by a
mis fit between clay and glaze - or a high expansion/contraction clay - is
consent rated in that corner. The result is the bottom drops out for
instance.
This can happen even with tea pots - glaze is pushing on the clay - hot
water is poured in - and for an instant - the glaze - heated by the hot
water - expands - and the pot breaks - even though the temperature does not
even get to 100C.
In that case it's simply a bad clay glaze fit - although it may have be
brought on because cristobalite was present and that helped put the glaze
under too much compression.
Anyway - having a rounded bottom has two advantages - the glaze does not
get consent rated in the corner - and - the strain is transferred in a much
more even way.
If you are working at cone 6 there is a set of glazes in chapter five of
our book that will tell you the relative expansion of your clay. I can also
do a dilatometer test of the clay for you.
The first step is for me to calculate your glaze to see if the expansion is
in a normal range.
It also occurred to me that melting a few caramels would put more of a
strain on any ware but - I also think any ware should survive this kind of
situation. I'm sure it can be fixed.
RR
>I have always touted my functional stoneware to be microwave safe and I
>have used it my self to heat all kinds of different foods and liquids in
>the microwave and never had any problems - except I burned my tongue once
>when the coffee got too hot.
>
>However, my neice recently tried to melt some caramels in the microwave in
>one of my bowls and the bowl cracked. I assume it cracked from thermal
>stresses caused by the super hot sugar in the bottom of the bowl compared
>with the cooler upper part of the bowl that didn't have any caramel
>touching it.
>
>Now I think I might need to add a disclaimer to my advertised "Microwave
>Safe." Something like: "The only exception to safe microwave use is to
>avoid heating liquid oils and liquid sugars =96 they get too hot, leading t=
o
>thermal stresses that may break the pottery.
>
>Anyone else have any thoughts regarding this? Could the same thing happen
>to Pyrex glass?
>
>David
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
Earl Brunner on thu 16 jun 05
Ron, aren't we talking about a situation here, which could probably be
repeated with just about any clay and glaze combination? As has been
discussed, heating either oil or sugar in a container in a microwave (if I'm
reading it right) sets up a possible scenario where the uneven heating is
too great for the pot. If you can create the same condition with corning
ware, then I'm not sure it's something we can correct within our
limitations. Aren't we talking about something like the limitations for
flame ware here? And anymore, because of liability issues and technology
issues I thought that was pretty much outside of the range of studio
potters.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ron Roy
Sent: Wednesday, June 15, 2005 10:32 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Do I need to add a disclaimer to microwave safe?
Hi David - I wish I knew what cone you are firing to - I could be more
specific in my answer.
What we are probably dealing with here is uneven heating - in this case the
bottom of the pot gets hot before the top and sides. Bottom expands faster
than the top and sides and dunting is the result.
If it happens with one pot - with a specific clay glaze combination - you
should assume there are factors which should be corrected because it
probably will happen again.
Ron Roy on sat 18 jun 05
Hi Earl,
Yes - I think you are right - like spot heating any ceramic ware - hard to
make ware that will take that.
On the other hand - very few potters have a handle on clay and glaze
expansion and I think some testing would be useful for most of us who make
functional ware.
Since I have found some cristobalite in 4 different cone 6 bodies I would
like to find out just how prevalent the problem is - and in what material
the cristobalite is coming with.
Certainly it should not be present even in ware only used a microwave.
I did not know about the Pyrex warning when I wrote that - still - I am
curious to know if it is possible to adjust fit to handle the situation
better?
RR
>Ron, aren't we talking about a situation here, which could probably be
>repeated with just about any clay and glaze combination? As has been
>discussed, heating either oil or sugar in a container in a microwave (if I'm
>reading it right) sets up a possible scenario where the uneven heating is
>too great for the pot. If you can create the same condition with corning
>ware, then I'm not sure it's something we can correct within our
>limitations. Aren't we talking about something like the limitations for
>flame ware here? And anymore, because of liability issues and technology
>issues I thought that was pretty much outside of the range of studio
>potters.
>
>Earl Brunner
>Las Vegas, NV
>
>-----Original Message-----
>From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ron Roy
>Sent: Wednesday, June 15, 2005 10:32 PM
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: Do I need to add a disclaimer to microwave safe?
>
>Hi David - I wish I knew what cone you are firing to - I could be more
>specific in my answer.
>
>What we are probably dealing with here is uneven heating - in this case the
>bottom of the pot gets hot before the top and sides. Bottom expands faster
>than the top and sides and dunting is the result.
>
>If it happens with one pot - with a specific clay glaze combination - you
>should assume there are factors which should be corrected because it
>probably will happen again.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
| |
|
