john forstall on tue 4 nov 03
On page 47 of John & Ron's Cone 6 book under the heading of Testing for
Suitability for Use in a Microwave Oven, we read: "If the clay is not well
vitrified it will, more than likely, overheat and perhaps cause scalding or
burning of the person removing it from the microwave oven. It could even
crack or explode.
Sounds plausible, but have any of you out there ever had an incident like
this, or heard of anyone else having an incident? I've been using my own
pots in the microwave since they became available (about 24 years) and have
never had a problem. I've used all sorts of clays and have no idea what the
absorption was in the early days. Am I just lucky?
John Forstall
iandol on wed 5 nov 03
Dear john forstall,=20
Clay which has not vitrified remains porous. It adsorbs water which =
changes to steam under the influence of the microwaves. Since the pores =
are small and the volume of steam is large, like 22.4 litres plus for =
every 18 grams there is potential for a catastrophe. Please do not call =
this an "Accident". We know the potential for disaster.
Best regards,
Ivor Lewis. Redhill, South Australia.
Kenneth D. Westfall on wed 5 nov 03
Yes your just lucky. More that once from different potter I have heated a
cup of water and blistered a finger from a hot mug handle. Most of them
have been cone 6 or lower earthenware mugs.
Kenneth D. Westfall
Pine Hill Pottery
R.D. #2 Box 6AA
Harrisville, WV 26362
kenneth@pinehillpottery.com
http://www.pinehillpottery.com
Dave Finkelnburg on wed 5 nov 03
John,
Yes, you have been fortunate, and likely careful and have used good
pottery practices. I have not had a complete failure of any ware in an
oven, including a microwave oven, due to a steam explosion. I have to
confess, though, I have had the experience of making functional ware and not
firing it high enough to vitrify the clay body. I also blew up some work
through a steam explosion when refiring glazed ware that had been wet. It's
pretty ugly!
I bought some Grey Calico from Laguna, fired it to a small cone 5 in the
Dawson kiln sitter in my kiln. Vases eventually wet the table through the
foot, even though glazed inside. :-( Mug handles got blistering hot in
the microwave. More :-(
Fortunately I found Clayart, asked the newbie questions, got wonderful
help, followed Ron Roy's advice and installed witness cones, fired to cone 6
tip just touching the shelf (takes a small cone 7 in my sitter), fired
unglazed test tiles and found the % water absorption of my fired clay had
dropped from about 4.5% to around 2.25%. Problems all disappeared! Big :-)
Later I corresponded with several clayarters who sent me samples of ware
that exhibited the symptom of getting hot in a microwave. In every case the
problem turned out to be undervitrified clay. Apparently the ware absorbs
enough moisture, probably through the unglazed foot, to heat in the
microwave.
To study the hot mug handle in a microwave issue further, Ron Roy even
composed some glaze recipes using a lot of iron, 10, 15 and 20% red iron
oxide. I mixed the glazes, fired them on identical cups, and tested for
heating in a microwave to see if excessive iron in the glaze could make a
cup handle get hot. It did not. I could boil water in a cup glazed inside
and out with a glaze colored by 20% RIO and reach in the microwave and take
the cup out by the handle, bare-handed, no problem.
About this same time I refired one of my old, underfired bowls, one that
had sat outdoors with the foot inside another bowl filled with rain water.
The foot of the refired piece blew off entirely during the firing. The
remains looked like a clay "pigeon" that takes a direct hit in a skeet
shoot -- just little pieces left...stuck in the glaze and the insides of
everything on that level in the whole kiln. :-( I seldom refire anything
now, never refire something that has been wet!
Good potting!
Dave Finkelnburg, firing on a frosty morning in Idaho
Finally, ----- Original Message -----
From: "john forstall"
> On page 47 of John & Ron's Cone 6 book under the heading of Testing for
> Suitability for Use in a Microwave Oven, we read: "If the clay is not well
> vitrified it will, more than likely, overheat and perhaps cause scalding
or
> burning of the person removing it from the microwave oven. It could even
> crack or explode.
>
> Sounds plausible, but have any of you out there ever had an incident like
> this, or heard of anyone else having an incident? I've been using my own
> pots in the microwave since they became available (about 24 years) and
have
> never had a problem. I've used all sorts of clays and have no idea what
the
> absorption was in the early days. Am I just lucky?
Craig Dunn Clark on wed 5 nov 03
This may have already been said, but it is most likely a high iron
content in the clay body that is causing the heating problems in the
microwave. Beyond the immediate problem of heating associated with placing
earthenware in a microwave there is also the concern of drinking from or
eating off of a piece of pottery that is not made of a vitrified clay body
and covered by a stable, durable glaze as mssrs' "Ron and Jon" emphasize in
their book "Mastering Cone 6 Glazes."
As far as I know an earthenware clay body will actually melt before it
vitrifies. That is a problem that should preclude it's use as a body for
functional ware.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Kenneth D. Westfall"
To:
Sent: Tuesday, November 04, 2003 11:37 PM
Subject: Re: Microwave Safe Ware
> Yes your just lucky. More that once from different potter I have heated a
> cup of water and blistered a finger from a hot mug handle. Most of them
> have been cone 6 or lower earthenware mugs.
>
>
> Kenneth D. Westfall
> Pine Hill Pottery
> R.D. #2 Box 6AA
> Harrisville, WV 26362
> kenneth@pinehillpottery.com
> http://www.pinehillpottery.com
>
>
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Ron Roy on thu 6 nov 03
Hi Kenneth,
Thanks for posting this!
It can happen to clay fired at any temperature of course - it is the degree
of vitrification that is the factor. It is an epidemic at low fire
temperatures though.
RR
>Yes your just lucky. More that once from different potter I have heated a
>cup of water and blistered a finger from a hot mug handle. Most of them
>have been cone 6 or lower earthenware mugs.
>
>
>Kenneth D. Westfall
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on thu 6 nov 03
Hi John,
There is no doubt about this - if water can get into the clay - under the
glaze - the water will heat up during microwaving.
David Henley reported a bad burn from such a mug here on ClayArt.
Paper clay will do it because no matter what the vitrification of the clay
- water will get in through the pores left by the burnt out fiber.
Pots fresh from the kiln will not do it - if the clay is not properly
vitrified - because water will not have had a chance to get into the porous
body yet. You need to soak the pot in water - how long depends on how
porous the clay is.
We out line a test in the book by the way - put any mug in empty - along
with another full of water that you know is OK. Microwave it on high for 10
seconds - feel it (the empty one) - if it's getting warm the problem is
water in the clay - keep doing that and every 10 seconds feel the mug you
are testing - when the water in the other mug boils and you can still
comfortably touch the empty one you have a working mug.
I do think you have been lucky.
When you do get a shipment of clay it is easy to do an absorption test on
the clays - if the absorption is less than 3% you can safely assume they
will make microwave serviceable ware.
RR
>On page 47 of John & Ron's Cone 6 book under the heading of Testing for
>Suitability for Use in a Microwave Oven, we read: "If the clay is not well
>vitrified it will, more than likely, overheat and perhaps cause scalding or
>burning of the person removing it from the microwave oven. It could even
>crack or explode.
>
>Sounds plausible, but have any of you out there ever had an incident like
>this, or heard of anyone else having an incident? I've been using my own
>pots in the microwave since they became available (about 24 years) and have
>never had a problem. I've used all sorts of clays and have no idea what the
>absorption was in the early days. Am I just lucky?
>John Forstall
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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