"Terry Sullivan/Nottingham Center for the Arts. San Marcos," on thu 19 nov 98
From Terry Sullivan
Nottintgham Center for the Arts
return personal response email to Go2tms@aol.com
OK here's one that's got me puzzled.
I was re-heating some tea in a white earthenware cup , from cost plus, and the
cup got to hot to touch while the tea warmed a little ( not even close to
boiling). Had it in there for about a minute.
I am of the impression that microwaves heat water and thus whatever it's in
but not inorganics that don't contain H2O.
So I put an empty cup of the same kind and an empty stoneware cup into the
microwave and ran it for three minutes.
Stoneware cup was room temp and the earthenware cup was blistering hot.
The earthenware cup was brand new, never used and still had sales label on the
bottom, and thouroughly glazed on all surfaces.
So what is heating the cup ???? Shouldn't be any H2O inside the body of the
cup.
Can any of you refer me to a ceramics technical source.
I am trying to get through to the ACS, email link didn't seem to go through,
and would like the line to a college(s) teaching ceramics tech.
As a related note: I used to heat my tea in a stoneware mug, my favorite, in
the microwave each morning. After a year or so the mug just seemed to
disintegrate.
I figured that water got into the matrix of the stoneware, yes stoneware does
have a minute porosity, and the repeated microwave use caused a high tech
version of "frost wedging" only from liquid to steam. In other words I very
slowly "blew" the piece up by rapidly converting minute quantities of water to
steam inside the stoneware body. Well; that's my theory anyhow.
This phenomenon brings up several safety issues:
Since rapid microwave heating can, apparently, destroy the integrity of cone
10 stoneware and cause earthenware to heat far more rapidly than it's contents
how does this affect the safety of glazes and clay bodies used.
What happens when someone takes a piping hot caserole out of the microwave in
their " oven safe" stoneware pot and it comes apart in their hands.
This puts a different spin on labeling work "Oven safe". It might explain the
experience of some who have the odd customer complaining of a piece that
"mysteriously" cracked or otherwise failed when it appeared completely sound
when sold ( I believe there was Clayart discussion on this recently).
While waiting/participating in the technical disscusion on this; I'm doing
some simple research. Taking various handmade and commercially made pieces and
running them through repeated cycles of microwave and dishwasher treatment on
an excelerated schedual. I'll keep identicle pieces for control. Also I'll run
before and after toxics leaching tests to see how porosity etc. are affected.
I know this isn't rigorously scientific but you don't need a degree in Physics
or rigorous tests to "know" something. If we did the human race would be long
gone.
Terry Sullivan
Nottingham Center for the Arts
email ( off list) Go2tms@aol.com
Christine Dubois on fri 20 nov 98
I think the more metal (iron?) particles in the cup the hotter the microwave
oven will make it and the more it will stop the microwaves from getting to the
liquid. This is why you dont put metal containers in microwave ovens.
Joanne Van Bezooyen on fri 20 nov 98
It is a material...chemical...I think it is a metal ....in the glaze that vibrat
from the microwave, the friction of which causes the heat. Microwaves vibrate
water, fats and oils, and sugar the most, therefore producing the most heat from
these ingredients. However, which ingredient in a glazee which also responds to
the microwaves. I know metals reflect microwaves and you must be careful not to
use them in the ovens because a microwave that is reflected back into the ray
thing that emits them can damage it. However.....have you ever put a wire-tie
from bread or so, into the microwave by mistake? the paper covering catches
fire.....therefore, I think it is because the metal used in the tie created an
intense heat.
HMMMMMMMMMMM
Terry Sullivan/Nottingham Center for the Arts. San Marcos, wrote:
> ----------------------------Original message----------------------------
> >From Terry Sullivan
> Nottintgham Center for the Arts
> return personal response email to Go2tms@aol.com
>
> OK here's one that's got me puzzled.
>
> I was re-heating some tea in a white earthenware cup , from cost plus, and the
> cup got to hot to touch while the tea warmed a little ( not even close to
> boiling). Had it in there for about a minute.
>
> I am of the impression that microwaves heat water and thus whatever it's in
> but not inorganics that don't contain H2O.
> So I put an empty cup of the same kind and an empty stoneware cup into the
> microwave and ran it for three minutes.
> Stoneware cup was room temp and the earthenware cup was blistering hot.
> The earthenware cup was brand new, never used and still had sales label on the
> bottom, and thouroughly glazed on all surfaces.
> So what is heating the cup ???? Shouldn't be any H2O inside the body of the
> cup.
>
> Can any of you refer me to a ceramics technical source.
> I am trying to get through to the ACS, email link didn't seem to go through,
> and would like the line to a college(s) teaching ceramics tech.
>
> As a related note: I used to heat my tea in a stoneware mug, my favorite, in
> the microwave each morning. After a year or so the mug just seemed to
> disintegrate.
> I figured that water got into the matrix of the stoneware, yes stoneware does
> have a minute porosity, and the repeated microwave use caused a high tech
> version of "frost wedging" only from liquid to steam. In other words I very
> slowly "blew" the piece up by rapidly converting minute quantities of water to
> steam inside the stoneware body. Well; that's my theory anyhow.
>
> This phenomenon brings up several safety issues:
> Since rapid microwave heating can, apparently, destroy the integrity of cone
> 10 stoneware and cause earthenware to heat far more rapidly than it's contents
> how does this affect the safety of glazes and clay bodies used.
> What happens when someone takes a piping hot caserole out of the microwave in
> their " oven safe" stoneware pot and it comes apart in their hands.
>
> This puts a different spin on labeling work "Oven safe". It might explain the
> experience of some who have the odd customer complaining of a piece that
> "mysteriously" cracked or otherwise failed when it appeared completely sound
> when sold ( I believe there was Clayart discussion on this recently).
>
> While waiting/participating in the technical disscusion on this; I'm doing
> some simple research. Taking various handmade and commercially made pieces and
> running them through repeated cycles of microwave and dishwasher treatment on
> an excelerated schedual. I'll keep identicle pieces for control. Also I'll run
> before and after toxics leaching tests to see how porosity etc. are affected.
> I know this isn't rigorously scientific but you don't need a degree in Physics
> or rigorous tests to "know" something. If we did the human race would be long
> gone.
>
> Terry Sullivan
> Nottingham Center for the Arts
> email ( off list) Go2tms@aol.com
Gavin Stairs on fri 20 nov 98
At 12:19 PM 11/19/98 EST, Terry Sullivan wrote:
....
>So I put an empty cup of the same kind and an empty stoneware cup into the
>microwave and ran it for three minutes.
>Stoneware cup was room temp and the earthenware cup was blistering hot.
>The earthenware cup was brand new, never used and still had sales label on
the
>bottom, and thouroughly glazed on all surfaces.
>So what is heating the cup ????
....
Hi Terry,
Microwaves will heat several kinds of substances:
1. Electrical conductors. This means the gold trim on some pots, and
metallic utensils. If the conductor is highly conductive, like most
metals, you will probably see arcing: visible sparks and sharp snapping
noises. This is not good for the article in question of for the microwave.
2. Magnetic dipoles. Like ferrites. This could be what is happening in
the earthenware, if there is any ferric iron in it. There are other,
weaker magnetic effects which can also induce heating.
3. Electrostatic dipoles. Water is such a dipole. So are many
dielectrics like ceramics, especially those with some iron in them.
In the first case, the heating comes from ordinary ohmic heating, like in
the elements of a kiln. In the other two, it comes directly, by the
flipping of the dipoles (e.g., water molecules) in the rapidly alternating
electromagnetic field of the microwaves. This rapid rotation couples to
other degrees of freedom in the substance, and we perceive this as heat.
So your earthenware is heating not because it contains water, but because
the iron oxides and silicates in the body are being flipped about by the
microwave field. The stoneware has less iron, and the ceramics in it are
not of the lossy polar dielectric type, so it doesn't heat up like the
earthenware.
Some stonewares can be heated in a microwave. This fact is used by
so-called browning stones to put some sizzle into cooking steak or bacon in
a microwave.
Part of being microwave safe is this business of not self heating in the
microwave. Most stonewares are safe, but many earthenwares, particularly
iron bearing ones, are not. I posted a test for this some time ago. Just
microwave a mug full of water to the boiling point. If the handle is
relatively cool while the water and bowl are hotter, the mug passes. If
the handle gets as hot or hotter than the bowl, then there is a possibility
that the handle may get hot enough to burn skin, and the test is failed.
Secondarily, there is the question of water absorption: absorbed water may
flash to steam, resulting in breakage, or, in the extreme, explosion.
And finally there is the gold trim problem. Not allowed.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419 stairs@stairs.on.ca
Cameron Harman on fri 20 nov 98
There is a difference between 'oven safe' and 'microwave safe'.
Certain metal oxides have resonance frequencies near enough to
microwave frequencies that hey will heat up as well. You may have
noticed that the microwave popcorn bag as a metal foil section in
it. The microwave heats the metal very quickly and that in turn
helps to heat the popcorn. The microwave alone is not enough to
get the popcorn popped in the time required.
Ceramics should be certified as microwave safe before placing them
there as those certain metal oxides will make them very hot.
as to your stoneware cup disintegrating, you were in effect
performing a standard moisture expansion test. In order to be
frost free or to be utilitarian in high moisture locations,
ceramics should pass the moisture expansion test. A stoneware that
is zero porosity will pass, however, one with some porosity will ,
by definition, absorb some moisture. many clay crystals are
"platy" , that is they are flat crystals that are closely
compacted together. Even after firing the structure often remains.
The water will get between the plates and slowly cause the ceramic
to expand.
That is the cause of crazing in wall tiles, for example, years
after they were installed. The British worked all of this out many
years ago and established wall tile formulations that would resist
moisture expansion. The common white ceramic body used by studios
in the United States is almost the same composition, for that
reason, those porous cups actually hold up better than cups of
some other formulations. The stoneware may have had moisture
expansion or may not, but it did allow the moisture to get into
the structure.
The test for moisture expansion is to take the ceramic and immerse
it in boiling water for two to 24 hours, or to take the ceramic
and place it in an autoclave ( a steam chamber) for two hours.
After those tests the ceramic is tested for microcracks or the
glazed ceramic is observed for surface cracks.
Continuous micro waving of the cup was similar to the boiling
test. There are industrial standards for all of this,
unfortunately some makes are not knowledgeable of the industry
standards and some developing countries ignore the standards. I
always look for "microwave safe" on anything I put into a
microwave.
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place for total kiln and drier support
**********************************************************
David Hendley on fri 20 nov 98
Terry, I had a similar experience a few months ago with a ^6 stoneware
mug I had traded another potter for (Not a Clayart trade).
I was boiling a cup of water to make some tea, as I often do with
my mugs. When the bell rang at 3 minutes, I reached in and grabbed the
mug by the handle, as I always do.
Well, it burned the #$&* out of me. Serious, blistering burn.
After the appropriate yelling, cursing, and trip to the first aid kit, I
took a close look at the mug. The weirdest thing I've seen:
There was a network of cracks all over the mug (crazing, that
was never noticeable before), with a dark grey foam coming from
the cracks.
My theory is that the clay is under-vitrified and porous. This,
combined with the crazed glaze, allowed the cup to absorb water
during use and washing. During microwaving, the absorbed
water in the handle reached the boiling point faster than the water
I was heating, and became super-heated.
The funny thing is, after the mug was allowed to cool, and was washed
and dried, it looked fine once again. I also know that the clay used
to make the cup was a standard claybody made by a big company and
advertised as a ^6 clay. It is also a "spotted" clay, that I assume uses
manganese for the spotted effect. Makes you wonder....
If this happens with under-vitrified stoneware, it must be even more
of a problem with earthenware. I can even imagine earthenware
absorbing moisture from a humid atmosphere?
I've been heating my ^10 stoneware mugs in the microwave for
as long as microwaves have been around, and never had problems
of any kind.
David Hendley
Maydelle, Texas
At 12:19 PM 11/19/98 EST, you wrote:
>----------------------------Original message----------------------------
>From Terry Sullivan
>Nottintgham Center for the Arts
>return personal response email to Go2tms@aol.com
>
>OK here's one that's got me puzzled.
>
>I was re-heating some tea in a white earthenware cup , from cost plus, and
the
>cup got to hot to touch while the tea warmed a little ( not even close to
>boiling). Had it in there for about a minute.
>
>I am of the impression that microwaves heat water and thus whatever it's in
>but not inorganics that don't contain H2O.
>So I put an empty cup of the same kind and an empty stoneware cup into the
>microwave and ran it for three minutes.
>Stoneware cup was room temp and the earthenware cup was blistering hot.
>The earthenware cup was brand new, never used and still had sales label on
the
>bottom, and thouroughly glazed on all surfaces.
>So what is heating the cup ???? Shouldn't be any H2O inside the body of the
>cup.
>
>Can any of you refer me to a ceramics technical source.
>I am trying to get through to the ACS, email link didn't seem to go through,
>and would like the line to a college(s) teaching ceramics tech.
>
>As a related note: I used to heat my tea in a stoneware mug, my favorite, in
>the microwave each morning. After a year or so the mug just seemed to
>disintegrate.
>I figured that water got into the matrix of the stoneware, yes stoneware does
>have a minute porosity, and the repeated microwave use caused a high tech
>version of "frost wedging" only from liquid to steam. In other words I very
>slowly "blew" the piece up by rapidly converting minute quantities of
water to
>steam inside the stoneware body. Well; that's my theory anyhow.
>
>This phenomenon brings up several safety issues:
>Since rapid microwave heating can, apparently, destroy the integrity of cone
>10 stoneware and cause earthenware to heat far more rapidly than it's
contents
>how does this affect the safety of glazes and clay bodies used.
>What happens when someone takes a piping hot caserole out of the microwave in
>their " oven safe" stoneware pot and it comes apart in their hands.
>
>This puts a different spin on labeling work "Oven safe". It might explain
the
>experience of some who have the odd customer complaining of a piece that
>"mysteriously" cracked or otherwise failed when it appeared completely sound
>when sold ( I believe there was Clayart discussion on this recently).
>
>While waiting/participating in the technical disscusion on this; I'm doing
>some simple research. Taking various handmade and commercially made pieces
and
>running them through repeated cycles of microwave and dishwasher treatment on
>an excelerated schedual. I'll keep identicle pieces for control. Also I'll
run
>before and after toxics leaching tests to see how porosity etc. are affected.
>I know this isn't rigorously scientific but you don't need a degree in
Physics
>or rigorous tests to "know" something. If we did the human race would be long
>gone.
>
>Terry Sullivan
>Nottingham Center for the Arts
>email ( off list) Go2tms@aol.com
>
Cynthia Spencer on sat 21 nov 98
Similarly, I've purchased majolica from a local craftsperson that
must've had enough chrome in their glaze to have the decoration actually
burn me. The water in the mug was only slightly warm, the rest of the mug
not hot at all, but the green glaze, HOT.
Another argument for knowing what all is in the stuff we use . . . or
possibly an argument against microwave use.
Cynthia
cynthia.spencer@cmug.com
541-753-4606
Corvallis, OR
Tim Stowell on sat 21 nov 98
I tell my customers that I don't recommend using any ceramic items that
you like in a
microwave oven, even store bought mega-mass produced items. Anything you
put in a
microwave oven should be considered disposable.
I have had Corningware break on me (they replaced it for shipping) after
using one of
their bowls as my favorite soup heater upper.
The following is OPINION not science.
There appear to be two things at work. One is a thermal differential
between the rim of
the item and the center spot or concentrated heat area. Even newer
microwaves with
turntables can spot heat the food and there fore the item to a very high
temperature. Over
time and repeated usage this temperature difference can weaken the
structure.
The second one is perhaps the, just my folksy distrust of new fangled
gadgetry. I think
that the microwave energy itself over time can weaken the structural
integrity of the items
in it.
I can't prove any of this and if called into a court of law I would say I
made it up. Even if I'm
wrong, it is better to err on the side of caution then to tell someone
not to worry about it.
-Tim
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the cat lady on sat 21 nov 98
>As a related note: I used to heat my tea in a stoneware mug, my favorite, in
>the microwave each morning. After a year or so the mug just seemed to
>disintegrate.
>I figured that water got into the matrix of the stoneware, yes stoneware does
>have a minute porosity, and the repeated microwave use caused a high tech
>version of "frost wedging" only from liquid to steam. In other words I very
>slowly "blew" the piece up by rapidly converting minute quantities of water to
>steam inside the stoneware body. Well; that's my theory anyhow.
>
Whoa! Sounds like you have a problem with your clay/glaze fit or the
vitrification temp of your clay!!!
I have been using the same mug for 7 years now. I make one large pot of
tea in the morning, and heat each cup to drinking temp (and gosh, but I
like it burning hot) and also re-heat each cup a *minimum* of 2 times while
drinking. As I said, I like my tea HOT. This cup gets into the dishwasher
2-3 times weekly.
While this mug shows mild crazing 7 years later, there is minimal staining
and NO degradation at all. The shiny part is still shiny, etc. (it is an
overlap glaze combo which can be seen on my homepage on the cream & sugar
set).
>This phenomenon brings up several safety issues:
>Since rapid microwave heating can, apparently, destroy the integrity of cone
>10 stoneware and cause earthenware to heat far more rapidly than it's contents
>how does this affect the safety of glazes and clay bodies used.
>What happens when someone takes a piping hot caserole out of the microwave in
>their " oven safe" stoneware pot and it comes apart in their hands.
>
>This puts a different spin on labeling work "Oven safe". It might explain the
>experience of some who have the odd customer complaining of a piece that
>"mysteriously" cracked or otherwise failed when it appeared completely sound
>when sold ( I believe there was Clayart discussion on this recently).
>
I can truly say that my ware with this particular glaze combo fired to
full temp is "safe" - or at least as safe as I can make it :)
I think what you need to do is try some different clay bodies and new
glazes.
Just my $0.02
sam - alias the cat lady
Melbourne, Ontario
SW Ontario CANADA
http://www.geocities.com/paris/3110
scuttell@odyssey.on.ca
"Thousands of years ago, cats were worshipped as gods.
Cats have never forgotten this."
Robert Yellin on sat 21 nov 98
David Hendley wrote:
If this happens with under-vitrified stoneware, it must be even more
of a problem with earthenware. I can even imagine earthenware
absorbing moisture from a humid atmosphere?
Hi David-
Yes, that true. here in Japan where the humidity is quite high during
the
summer, I sometimes get mildew growing on some of my guinomi(sake cups)
or tokkuri(sake flasks). I can still use them though after I rinse them
out
and dry them in the sun.
From Numazu,
Robert Yellin
Earl Brunner on sat 21 nov 98
We use a major supplier, cone 6 clay body at the art center where I teach classe
and I have taken it to cone 10 in my kiln at home several times with no apparent
ill affects to the body, looks like a normal cone ten vitrified body.
Earl Brunner
David Hendley wrote:
> ----------------------------Original message----------------------------
> Terry, I had a similar experience a few months ago with a ^6 stoneware
> mug I had traded another potter for (Not a Clayart trade).
>
> I was boiling a cup of water to make some tea, as I often do with
> my mugs. When the bell rang at 3 minutes, I reached in and grabbed the
> mug by the handle, as I always do.
> Well, it burned the #$&* out of me. Serious, blistering burn.
>
> After the appropriate yelling, cursing, and trip to the first aid kit, I
> took a close look at the mug. The weirdest thing I've seen:
> There was a network of cracks all over the mug (crazing, that
> was never noticeable before), with a dark grey foam coming from
> the cracks.
>
> My theory is that the clay is under-vitrified and porous. This,
> combined with the crazed glaze, allowed the cup to absorb water
> during use and washing. During microwaving, the absorbed
> water in the handle reached the boiling point faster than the water
> I was heating, and became super-heated.
>
> The funny thing is, after the mug was allowed to cool, and was washed
> and dried, it looked fine once again. I also know that the clay used
> to make the cup was a standard claybody made by a big company and
> advertised as a ^6 clay. It is also a "spotted" clay, that I assume uses
> manganese for the spotted effect. Makes you wonder....
>
> If this happens with under-vitrified stoneware, it must be even more
> of a problem with earthenware. I can even imagine earthenware
> absorbing moisture from a humid atmosphere?
> I've been heating my ^10 stoneware mugs in the microwave for
> as long as microwaves have been around, and never had problems
> of any kind.
>
> David Hendley
> Maydelle, Texas
>
> At 12:19 PM 11/19/98 EST, you wrote:
> >----------------------------Original message----------------------------
> >From Terry Sullivan
> >Nottintgham Center for the Arts
> >return personal response email to Go2tms@aol.com
> >
> >OK here's one that's got me puzzled.
> >
> >I was re-heating some tea in a white earthenware cup , from cost plus, and
> the
> >cup got to hot to touch while the tea warmed a little ( not even close to
> >boiling). Had it in there for about a minute.
> >
> >I am of the impression that microwaves heat water and thus whatever it's in
> >but not inorganics that don't contain H2O.
> >So I put an empty cup of the same kind and an empty stoneware cup into the
> >microwave and ran it for three minutes.
> >Stoneware cup was room temp and the earthenware cup was blistering hot.
> >The earthenware cup was brand new, never used and still had sales label on
> the
> >bottom, and thouroughly glazed on all surfaces.
> >So what is heating the cup ???? Shouldn't be any H2O inside the body of the
> >cup.
> >
> >Can any of you refer me to a ceramics technical source.
> >I am trying to get through to the ACS, email link didn't seem to go through,
> >and would like the line to a college(s) teaching ceramics tech.
> >
> >As a related note: I used to heat my tea in a stoneware mug, my favorite, in
> >the microwave each morning. After a year or so the mug just seemed to
> >disintegrate.
> >I figured that water got into the matrix of the stoneware, yes stoneware does
> >have a minute porosity, and the repeated microwave use caused a high tech
> >version of "frost wedging" only from liquid to steam. In other words I very
> >slowly "blew" the piece up by rapidly converting minute quantities of
> water to
> >steam inside the stoneware body. Well; that's my theory anyhow.
> >
> >This phenomenon brings up several safety issues:
> >Since rapid microwave heating can, apparently, destroy the integrity of cone
> >10 stoneware and cause earthenware to heat far more rapidly than it's
> contents
> >how does this affect the safety of glazes and clay bodies used.
> >What happens when someone takes a piping hot caserole out of the microwave in
> >their " oven safe" stoneware pot and it comes apart in their hands.
> >
> >This puts a different spin on labeling work "Oven safe". It might explain
> the
> >experience of some who have the odd customer complaining of a piece that
> >"mysteriously" cracked or otherwise failed when it appeared completely sound
> >when sold ( I believe there was Clayart discussion on this recently).
> >
> >While waiting/participating in the technical disscusion on this; I'm doing
> >some simple research. Taking various handmade and commercially made pieces
> and
> >running them through repeated cycles of microwave and dishwasher treatment on
> >an excelerated schedual. I'll keep identicle pieces for control. Also I'll
> run
> >before and after toxics leaching tests to see how porosity etc. are affected.
> >I know this isn't rigorously scientific but you don't need a degree in
> Physics
> >or rigorous tests to "know" something. If we did the human race would be long
> >gone.
> >
> >Terry Sullivan
> >Nottingham Center for the Arts
> >email ( off list) Go2tms@aol.com
> >
John K. Dellow on sat 21 nov 98
> The test for moisture expansion is to take the ceramic and immerse
> it in boiling water for two to 24 hours, or to take the ceramic
> and place it in an autoclave ( a steam chamber) for two hours.
> After those tests the ceramic is tested for microcracks or the
> glazed ceramic is observed for surface cracks.
>
> Cameron , is it possible to do the moisture expansion test in a pressure
> cooker ?
John dellow the flower pot man
>
>
> --
Joseph Herbert on sun 22 nov 98
------------------
Cameron Harman mentions the resonance of metals with microwaves.
I saw a PBS (Nova?) program that showed researchers melting metal in a =
common
kitchen microwave. They had a crucible and some Magnesium Oxide bricks in =
the
microwave. Said it was lots cheaper than a muffle furnace and they could =
just
throw the Microwave away and get a new one from time to time.
DON=92T TRY THIS AT HOME=21=21
Joseph Herbert
JJHerb=40aol.com
Darrell Gargus on sun 22 nov 98
Hey guys! I am wondering... I had a mug explode on me quite a few years
ago in the microwave, when I looked at it(it split in half) there was
black in between the tan(highfire stoneware). Now, I know that in
reduction, the carbon will get trapped in between the clay walls, but
the clay walls on this mug seemed very thin and a lot of black. I
thought then that the ware was just too heavily reduced and that is way
it slit apart during the microwaving(I had microwaved this baby several
times before this had happened). Was I wrong in my thinking? Seeing
all these post about this subject makes me wonder...
Becky
Ron Roy on sun 22 nov 98
Hi Terry,
I would think there had to be water under the glaze- it is true that water
can be absorbed if the glaze is crazed or has some other opening from
moisture in the air.
Your point about stoneware - if stoneware is not vitrified properly then
water can get in under the clay - takes some time after firing - that would
certainly lead to it getting hot.
I regularly use my own ware in the microwave and have no problems - it is
vitrified porcelain with a Tenmoku glaze - it has 7.5 red iron in the
recipe - so iron - in my experience is not a problem if it is in solution
in the glass - can't say what would be if there was iron on the surface of
the glaze - like so much that it is right on the surface - like some
metallic glazes.
I also use a friends mugs every day - many times a day and boil water in
them. The clay is a stoneware with 1 to 2% absorbency when boiled for 2
hours from perfectly dry. When doing this kind of test - weigh, boil 2
hours, weigh it is very important to cool the bars in the pot before
weighing - don't want hot bars loosing water between pot and scales. The
glazes on these mugs are a clear over one of those orange (bone ash iron
glazes) - they have never presented a problem and we have used some of them
for at least 10 years.
To test ware to see if there is a heating problem - put empty test piece in
microwave along with safe mug full of water. Microwave on high for 10
seconds at a time - checking the test piece every 10 seconds to feel if it
is getting hot. Ware right out of the kiln will not ever heat up unless for
some other reason - but after soaking for 24 hours or a run or two in a
dish washer it will probably start heating if moisture is getting in.
I for one applaud your testing program and hope you will publish your
finding to this list - it will be most helpful if you make some bars of
your clays and test the absorbency of them so you can draw meaningful
conclusions. If you need instructions about any aspects of this test just
let me know. The first step is to fire bars in different parts of your kiln
with a cone beside each bar - when unstacking mark the cone bend on each
bar with something that will survive boiling - weigh bars as soon as they
come out of the kiln so that there is no chance for atmospheric water to
get in.
Regards - RR
>----------------------------Original message----------------------------
>I am of the impression that microwaves heat water and thus whatever it's in
>but not inorganics that don't contain H2O.
>So I put an empty cup of the same kind and an empty stoneware cup into the
>microwave and ran it for three minutes.
>Stoneware cup was room temp and the earthenware cup was blistering hot.
>The earthenware cup was brand new, never used and still had sales label on the
>bottom, and thouroughly glazed on all surfaces.
>So what is heating the cup ???? Shouldn't be any H2O inside the body of the
>cup.
>
>Can any of you refer me to a ceramics technical source.
>I am trying to get through to the ACS, email link didn't seem to go through,
>and would like the line to a college(s) teaching ceramics tech.
>
>As a related note: I used to heat my tea in a stoneware mug, my favorite, in
>the microwave each morning. After a year or so the mug just seemed to
>disintegrate.
>I figured that water got into the matrix of the stoneware, yes stoneware does
>have a minute porosity, and the repeated microwave use caused a high tech
>version of "frost wedging" only from liquid to steam. In other words I very
>slowly "blew" the piece up by rapidly converting minute quantities of water to
>steam inside the stoneware body. Well; that's my theory anyhow.
>
>This phenomenon brings up several safety issues:
>Since rapid microwave heating can, apparently, destroy the integrity of cone
>10 stoneware and cause earthenware to heat far more rapidly than it's contents
>how does this affect the safety of glazes and clay bodies used.
>What happens when someone takes a piping hot caserole out of the microwave in
>their " oven safe" stoneware pot and it comes apart in their hands.
>
>This puts a different spin on labeling work "Oven safe". It might explain the
>experience of some who have the odd customer complaining of a piece that
>"mysteriously" cracked or otherwise failed when it appeared completely sound
>when sold ( I believe there was Clayart discussion on this recently).
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
eden@sover.net on thu 26 nov 98
Awhile back some folks will remember a similar thread. I think it was on
clayart I heard the following tale:
Somebody sold a plate as "microwave safe" and the purchaser put a piece of
bacon down the middle and put it in the microwave and the plate promptly
broke down the middle, the spot-heating from the bacon grease creating just
the extreme situation this potter didn't dream a reasonable person would
try....so after that she said her stuff was not microwaveable to save
herself replacing stuff for stupid people.......
Well so when people ask me if my stuff is microwaveable I tell them that
story. I explain that under any condition that I can dream up it would be
perfectly safe and if the customer can devise a use I can't think of they
are on their own. I also explain that the traces of metals in the
colorants will make the piece warm or hot depending on the time in the
oven. Personally I can't stand the feel of something coming out of an oven
cold and this increases the usefulness of the microwave for me as it heats
the container too.
BTW I am strictly earthernware, Miller lowfire white do both throwing and
casting bodies, and my stuff is fine in the microwave absolutely all the
way never any problems, certainly no degradation of any kind that I can see.
I wonder what uses of the microwave clever potters are making. Any
delicious hints?
Eleanora
.............
Eleanora Eden 802 869-2003
Paradise Hill
Bellows Falls, VT 05101 eden@sover.net
"Can love, through the exercise of art, overcome death?" ---SalmanRushdie
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