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microwave safe???????

updated tue 6 jun 00

 

Dave Finkelnburg on sun 21 may 00


Anji,
Sorry to hear about the failure! :-(
I don't particularly want to test this theory myself, being quite timid,
:-), but is it possible the clay on this piece absorbed enough water to
have a steam explosion occur?
I am thinking that part of the reason ceramics don't heat up in the
microwave is they aren't absorbing any moisture. Only the moist things
inside the ceramic container are heated. Unless, of course, there's a
little water in the clay matrix, due to the clay being underfired. At least
that's one thought on this problem. You could probably test this safely by
checking the absorption of water (by weight) by a fired, unglazed test tile
made from your clay. I hope this helps.
Dave Finkelnburg in summery warm southern Idaho
-----Original Message-----
From: Anji Henderson
To: CLAYART@LSV.CERAMICS.ORG
Date: Sunday, May 21, 2000 11:17 AM
Subject: Microwave Safe???????


>Ok here is the deal … I have been told by a large hand
>full of educators that ceramic ware is microwave
>safe.. I have been told it is probably not a good idea
>to put low-fire ceramics in the microwave, but I have
>never had a problem with it in the microwave.. Well
>except for it getting quite warm….. Stone ware has
>always been fine (6 & 10) . … So now I put a piece in
>the microwave and it pops.. Then I took it out
>immediately and set it aside.. There were small hair
>line cracks in it.. Five minutes later it explodes..
>Thermal Shock????? There are a lot of guesses I can
>think of but none of the guesses, do I have a
>reasoning for.. Let me explain the piece..
>
>It is a pinch pot, and after it exploded I found that
>WOW I made it really really REALLY consistent.. I used
>Standard Ceramic Brown #306 stoneware. Bisqued to ^06.
>I glazed it with a glaze that was called Temuku, but
>looks for all the world like Ohata Kaki..
>
>Now, the recipe that I was told was Temuku, I compared
>to the recipe that Glen Echo Pottery in Maryland has
>for Ohata Kaki and the recipe that U.C.L.A. has for
>Ohata Kaki, and they are basically the same just a few
>grams off on the Feldspar, Whiting, and the Iron Ox.
>
>I only glazed the inside……
>
>So, I fired it in ^10.5 Gas Reduction.. It has been in
>use for about 6 months or more, now Kabluee…
>
>So what do you think??
>
>I am a little leary of stating anything is microwave
>safe to begin with… Tooooo many variables and I can't
>say I understand the machine so I wouldn't want to
>make any blanket statement any way, especially now….
>
>Thanks for any input,
>Anji
>
>
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Anji Henderson on sun 21 may 00


Ok here is the deal … I have been told by a large hand
full of educators that ceramic ware is microwave
safe.. I have been told it is probably not a good idea
to put low-fire ceramics in the microwave, but I have
never had a problem with it in the microwave.. Well
except for it getting quite warm….. Stone ware has
always been fine (6 & 10) . … So now I put a piece in
the microwave and it pops.. Then I took it out
immediately and set it aside.. There were small hair
line cracks in it.. Five minutes later it explodes..
Thermal Shock????? There are a lot of guesses I can
think of but none of the guesses, do I have a
reasoning for.. Let me explain the piece..

It is a pinch pot, and after it exploded I found that
WOW I made it really really REALLY consistent.. I used
Standard Ceramic Brown #306 stoneware. Bisqued to ^06.
I glazed it with a glaze that was called Temuku, but
looks for all the world like Ohata Kaki..

Now, the recipe that I was told was Temuku, I compared
to the recipe that Glen Echo Pottery in Maryland has
for Ohata Kaki and the recipe that U.C.L.A. has for
Ohata Kaki, and they are basically the same just a few
grams off on the Feldspar, Whiting, and the Iron Ox.

I only glazed the inside……

So, I fired it in ^10.5 Gas Reduction.. It has been in
use for about 6 months or more, now Kabluee…

So what do you think??

I am a little leary of stating anything is microwave
safe to begin with… Tooooo many variables and I can't
say I understand the machine so I wouldn't want to
make any blanket statement any way, especially now….

Thanks for any input,
Anji


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Wade Blocker on sun 21 may 00


----------
> From: Anji Henderson
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Microwave Safe???????
> Date: Sunday, May 21, 2000 10:08 AM
>
> Ok here is the deal … I have been told by a large hand
> full of educators that ceramic ware is microwave
> safe.. I have been told it is probably not a good idea
> to put low-fire ceramics in the microwave, but I have
> never had a problem with it in the microwave.. Well
> except for it getting quite warm….. Stone ware has
> always been fine (6 & 10) . … So now I put a piece in
> the microwave and it pops.. Then I took it out
> immediately and set it aside.. There were small hair
> line cracks in it.. Five minutes later it explodes..


Anji,You are aware that one cannot put anything metallic into a microwave.
Your glaze recipe and clay might contain a large amount of iron, which
contributed to your disaster. Mia in ABQ











> Thermal Shock????? There are a lot of guesses I can
> think of but none of the guesses, do I have a
> reasoning for.. Let me explain the piece..
>
> It is a pinch pot, and after it exploded I found that
> WOW I made it really really REALLY consistent.. I used
> Standard Ceramic Brown #306 stoneware. Bisqued to ^06.
> I glazed it with a glaze that was called Temuku, but
> looks for all the world like Ohata Kaki..
>
> Now, the recipe that I was told was Temuku, I compared
> to the recipe that Glen Echo Pottery in Maryland has
> for Ohata Kaki and the recipe that U.C.L.A. has for
> Ohata Kaki, and they are basically the same just a few
> grams off on the Feldspar, Whiting, and the Iron Ox.
>
> I only glazed the inside……
>
> So, I fired it in ^10.5 Gas Reduction.. It has been in
> use for about 6 months or more, now Kabluee…
>
> So what do you think??
>
> I am a little leary of stating anything is microwave
> safe to begin with… Tooooo many variables and I can't
> say I understand the machine so I wouldn't want to
> make any blanket statement any way, especially now….
>
> Thanks for any input,
> Anji
>
>
> __________________________________________________
> Do You Yahoo!?
> Send instant messages & get email alerts with Yahoo! Messenger.
> http://im.yahoo.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 Buck on sun 21 may 00


Anji:
Do you have another pot (vase is best) from the same claybody at
the same period you made the cooking pot for the microwave, glazed only in
the inside and fired to Cone 10+ in reduction? If so, fill it with water
and let it stand overnight on paper toweling. If it weeps/seeps, that is
the likely cause of failure when heated in the microwave...moisture
absorbed by the pot went to steam in the microwave. and as you said
!blooey. But if the pot is water-tight , fine go to the next paragraph.
As friend Ron Roy has said here before, you risk fast cracking if
you glaze only on the inside, or one type of liner glaze inside and a dry
matt type on the outside. Please note: a well-fitting glaze always has a
slightly higher expansion/contraction rate than does the claybody under
the glaze's surface. If the glaze contracts too much, it crazes/cracks; if
contracts too little, it likely will bulge up and fall off ("shiver").
Hence, if your pot's glaze didn't shiver or craze, then the fit is in the
ballpark. Yet, the claybody can still be under tension (force to crack it)
because the glaze is pushing out on the inside and there is no
corresponding compressive force on the outside. Since claybody doesn't
heat up as fast as glass (the glaze), this outward push is made worse by
heating in the microwave.
First the tiny cracks, then blooey, as the glaze litterally forced
the body to crack.
Remember clay bakers? No glaze in or out, and the bakers survive
in a hot oven. A well-made stoneware or porcelain casserole will survive
in such an oven if the same glaze is used on both sides, and the pot is
put in a cold oven and heated slowly.
til later. Peace. Tom B.

Tom Buck )
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada

Jan McQueary on mon 22 may 00


Information you get with your microwave usually suggests testing ware for
suitability. Remember that the answer in ceramics is usually 'it
depends.' A lot of ceramic ware is microwave safe - but a glaze that is
very high in iron may not be suitable for the microwave.

Several microwave problems I have had:

I noticed that mugs glazed with a cone 10 Persimmon glaze - (high iron,
looks like our Ohata Kaki) got abnormally hot in the microwave. I don't
put it on mugs any more for that reason.

A separation crack developed at the bottom attachment of the handle of a
different cone 10 mug after several trips through the microwave. This
was a very nicely made mug from a local production potter.

Also years ago I had the handles come off some cone 6 mugs which were
made of a cone 6-10 stoneware. Don't remember which one it was - but I
think that the ware was porous enough for water to be trapped under the
glaze and cause problems.

Jan




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Cindy Strnad on mon 22 may 00


Dave,

Some clay does heat up in the microwave, and it's not necessarily because
it's absorbing water. My clay seems to be all right, but I know one of my
glazes tends to heat up much hotter than all the others. This glaze sports
something like 18% RIO (I'd have to check the recipe--I don't often use it),
and I think that may be the reason.

Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730

Ron Roy on fri 2 jun 00


Hi Cindy,

Just a quick comment on this.

My tenmoku glaze has 7.5% red iron in it and we use it constanly in the
microwave with no heat up - it's on porcelain.

This would make an excellent experiment - fire a glaze - on a completely
vitrified clay body - with increasing amounts of iron. 10, 15 and 20% iron
and microwave each to see when it starts to heat up.

I'll be glad to design the glazes if someone will do the glazes and firing.

RR


>Some clay does heat up in the microwave, and it's not necessarily because
>it's absorbing water. My clay seems to be all right, but I know one of my
>glazes tends to heat up much hotter than all the others. This glaze sports
>something like 18% RIO (I'd have to check the recipe--I don't often use it),
>and I think that may be the reason.
>
>Cindy Strnad
>earthenv@gwtc.net
>Earthen Vessels Pottery
>RR 1, Box 51
>Custer, SD 57730
>
>______________________________________________________________________________
>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
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849

Liz Gowen on sat 3 jun 00


I am not totally convinced it is the glaze on the pot that causes the heat
up. After getting out of college I had bought a popular commercially made
set of dark brown plates with a little white over the brown on some edges.
These plates get HOT but not on all sides and it is also where the white
glaze is. ( if it was the iron I would expect it to be hot all over and
maybe not as hot on the white) They are evenly poured or pressed, double
thickness glaze only where white drip is. I put frozen items on them and
nuke them
( since I care less if they crack than some others) The item gets modestly
warm but the plate gets very hot but not on all sides. I have noticed the
glaze now has some crazing. Is it perhaps water retained after washing in
the piece itself that is heating since microwave works best on the moisture
in an item?
Liz Gowen
-----Original Message-----
From: Ron Roy
To: CLAYART@LSV.CERAMICS.ORG
Date: Saturday, June 03, 2000 10:54 AM
Subject: Re: Microwave Safe???????


>Hi Cindy,
>
>Just a quick comment on this.
>
>My tenmoku glaze has 7.5% red iron in it and we use it constanly in the
>microwave with no heat up - it's on porcelain.
>
>This would make an excellent experiment - fire a glaze - on a completely
>vitrified clay body - with increasing amounts of iron. 10, 15 and 20% iron
>and microwave each to see when it starts to heat up.
>
>I'll be glad to design the glazes if someone will do the glazes and firing.
>
>RR
>
>
>>Some clay does heat up in the microwave, and it's not necessarily because
>>it's absorbing water. My clay seems to be all right, but I know one of my
>>glazes tends to heat up much hotter than all the others. This glaze sports
>>something like 18% RIO (I'd have to check the recipe--I don't often use
it),
>>and I think that may be the reason.
>>
>>Cindy Strnad
>>earthenv@gwtc.net
>>Earthen Vessels Pottery
>>RR 1, Box 51
>>Custer, SD 57730
>>
>>__________________________________________________________________________
____
>>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
>93 Pegasus Trail
>Scarborough
>Ontario, Canada
>M1G 3N8
>Evenings 416-439-2621
>Fax 416-438-7849
>
>___________________________________________________________________________
___
>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 mon 5 jun 00


Hi Liz,

I don't get this - the only reason I can come up with is the clay is not
vitrified enough and some water is getting in through the crazing. Is the
crazing only on the white glaze?

RR

>I am not totally convinced it is the glaze on the pot that causes the heat
>up. After getting out of college I had bought a popular commercially made
>set of dark brown plates with a little white over the brown on some edges.
>These plates get HOT but not on all sides and it is also where the white
>glaze is. ( if it was the iron I would expect it to be hot all over and
>maybe not as hot on the white) They are evenly poured or pressed, double
>thickness glaze only where white drip is. I put frozen items on them and
>nuke them
>( since I care less if they crack than some others) The item gets modestly
>warm but the plate gets very hot but not on all sides. I have noticed the
>glaze now has some crazing. Is it perhaps water retained after washing in
>the piece itself that is heating since microwave works best on the moisture
>in an item?
> Liz Gowen

Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849