Dave Finkelnburg on fri 19 jan 01
Hi all!
There was some discussion yesterday about what causes some mug handles
to heat up in a microwave. Speculation was split between metal in the glaze
and porous clay as causes. At the suggestion of Ron Roy, the designer of
this test, I conducted a small experiment to study the first theory.
The result of my test was that I couldn't find a very significant effect
on the rate of handle heating of a mug in a microwave with up to 20% red
iron oxide in the glaze on the mug. Here's what was done.
Ron reformulated one of my base glazes which fits my clay, and to one
batch of it I added 10% red iron oxide (RIO) and to another batch 20% RIO.
I mixed a third glaze by blending half of each of these to get a glaze with
15% RIO. I dip-glazed the exteriors of six mugs as follows: 2 in the 10%
glaze, 2 in the 15% and 2 in the 20% RIO glaze. The mugs were, as close as
possible, identical, and made from a clay which I have tested and know that
at my firing temperature it absorbs less than 1% water by weight.
I split the mugs into two sets and fired one set in heavy reduction, and
one in oxidation.
Finally, I tested each mug in a microwave. At Ron's suggestion, I
placed a mug of water in the microwave, and one of the test mugs empty
beside it. I then turned on the microwave for a set period of time and
checked the handle of the empty mug to see how hot it was. I kept doing
this until I couldn't pick up the mug. This was obviously a quick,
partly-subjective test, but I think it provided useful information.
The amount of time needed to heat the mug handle hot varied some, but
the variation seemed to be unrelated to the iron content of the glaze. It
was pretty random. I can only conclude that other variables, like glaze
thickness, just where in the microwave the mug was, handle thickness, etc.
were as important as the iron content in the glaze. In other words, nothing
in this test was very important in causing the mug handles to get hot.
This is exactly NOT what I had expected. I was pretty certain the
high-iron glaze would heat up in the microwave. I still think that may be
possible, but now I realize it will take a lot of metal, probably more than
most reasonable potters would put in a glaze anyway.
By the way, I have a couple of mugs I made when I first started potting
that have handles which get unpleasantly hot while being microwaved. I
observe this when I pull the mug out full of water for tea and find the
handle burning my fingers! :-( I discovered by testing that the clay
body/firing regime used when making those mugs resulted in fired test tiles
which absorbed about 3.5% by weight of water. However, I have mugs made
from the same clay, fired the same way, glazed the same, which don't get hot
handles in the microwave. Hmmmmmmmm....
I am beginning to suspect that moisture in the clay is causing the
occasional hot mug handles, but I'd like to see some testing to prove that!
My thanks to Ron Roy for suggesting the testing above, and providing the
technical guidance. It was educational for me!
Dave Finkelnburg
Idaho Fire Pottery
mudlark on sat 20 jan 01
Dave, I really do think it's moisture in the clay wall. Also, if you bring the
water to a boil and further it causes the pot to heat up. By the way, I've never
had one explode.
Dave Finkelnburg wrote:
> Hi all!
> There was some discussion yesterday about what causes some mug handles
> to heat up in a microwave. Speculation was split between metal in the glaze
> and porous clay as causes. At the suggestion of Ron Roy, the designer of
> this test, I conducted a small experiment to study the first theory.
> The result of my test was that I couldn't find a very significant effect
> on the rate of handle heating of a mug in a microwave with up to 20% red
> iron oxide in the glaze on the mug. Here's what was done.
> Ron reformulated one of my base glazes which fits my clay, and to one
> batch of it I added 10% red iron oxide (RIO) and to another batch 20% RIO.
> I mixed a third glaze by blending half of each of these to get a glaze with
> 15% RIO. I dip-glazed the exteriors of six mugs as follows: 2 in the 10%
> glaze, 2 in the 15% and 2 in the 20% RIO glaze. The mugs were, as close as
> possible, identical, and made from a clay which I have tested and know that
> at my firing temperature it absorbs less than 1% water by weight.
> I split the mugs into two sets and fired one set in heavy reduction, and
> one in oxidation.
> Finally, I tested each mug in a microwave. At Ron's suggestion, I
> placed a mug of water in the microwave, and one of the test mugs empty
> beside it. I then turned on the microwave for a set period of time and
> checked the handle of the empty mug to see how hot it was. I kept doing
> this until I couldn't pick up the mug. This was obviously a quick,
> partly-subjective test, but I think it provided useful information.
> The amount of time needed to heat the mug handle hot varied some, but
> the variation seemed to be unrelated to the iron content of the glaze. It
> was pretty random. I can only conclude that other variables, like glaze
> thickness, just where in the microwave the mug was, handle thickness, etc.
> were as important as the iron content in the glaze. In other words, nothing
> in this test was very important in causing the mug handles to get hot.
> This is exactly NOT what I had expected. I was pretty certain the
> high-iron glaze would heat up in the microwave. I still think that may be
> possible, but now I realize it will take a lot of metal, probably more than
> most reasonable potters would put in a glaze anyway.
> By the way, I have a couple of mugs I made when I first started potting
> that have handles which get unpleasantly hot while being microwaved. I
> observe this when I pull the mug out full of water for tea and find the
> handle burning my fingers! :-( I discovered by testing that the clay
> body/firing regime used when making those mugs resulted in fired test tiles
> which absorbed about 3.5% by weight of water. However, I have mugs made
> from the same clay, fired the same way, glazed the same, which don't get hot
> handles in the microwave. Hmmmmmmmm....
> I am beginning to suspect that moisture in the clay is causing the
> occasional hot mug handles, but I'd like to see some testing to prove that!
> My thanks to Ron Roy for suggesting the testing above, and providing the
> technical guidance. It was educational for me!
> Dave Finkelnburg
> Idaho Fire Pottery
>
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Autumn Downey on sat 20 jan 01
Just to confuse the issue, the mug that heats up most in our microwave is a
commercial mug. A white clay, quite dense and the stamp on the bottom says
Otagiri Japan - Ivory stone.
It can be almost too hot to the touch, while the contents are average or a
little cooler.
Autumn Downey
Yellowknife, NWT
Janet Kaiser on sun 21 jan 01
That makes a good start Dave! Thanks for going
to the trouble and sharing with us. I have a
couple of questions though:
1. Is the thickness of the handles the same on
all your mugs, including those early ones? Where
they made in the same way? And do you consider
the clay is equally dense and/or was equally
compressed in the making?
2. Did you soak the mugs before your test, so
that the water absorbed in the body (if any)
would be the same?
3. Did you place the mugs in exactly the same
place each time?
4. What direction were the mugs being "zapped"
from in relation to the handles?
5. What wattage is your microwave and did the
water boil before the handles got too hot to
hold?
Hope you don't mind me asking, but I find these
sort of tests fascinating and a good place to
start for anyone wanting to find out whether
they are going to risk saying their ware is
microwave proof or not... Especially in
litigious countries.
Janet Kaiser
The Chapel of Art . Capel Celfyddyd
HOME OF THE INTERNATIONAL POTTERS' PATH
Criccieth LL52 0EA, GB-Wales Tel: (01766) 523570
E-mail: postbox@the-coa.org.uk
WEBSITE: http://www.the-coa.org.uk
----- Original Message -----
> There was some discussion yesterday about
what causes some mug handles to heat up in a
microwave. Speculation was split between metal
in the glaze and porous clay as causes.
NeilBerkowitz on sun 21 jan 01
I'm sure this has already been considered in the test but will ask anyway.
In what way might the directional nature of the microwaves be a factor? I
suspect that this goes beyond just whether or not the oven has a turntable
and extends to the placement and design of the handle. In Dave's post he
mentioned some early mugs with handles that got hot but others with the same
3.5% absortion body, glaze, and firing that did not. Are they the same
design?
Neil Berkowitz
In rainy Seattle, where I saw my first daffodil bloom of the year this
morning.
ferenc jakab on tue 23 jan 01
>
> It can be almost too hot to the touch, while the contents are average or a
> little cooler.
>
> Autumn Downey
> Yellowknife, NWT
Hi Autumn,
This could be because there is more mass in the body of the mug and its
contents than in the handle. ??
Feri.
Can't wait till the days get shorter and colder, but I wouldn't want the
days to be as short as yours!
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