Dave Finkelnburg on mon 22 jan 01
Hi all!
I will respond to both Neil and Janet's posts at once here.
First, Janet, I think the important thing I learned is the most simple.
If you want to know if the piece you make is microwave safe, first try it as
the buyer would use it. I discovered :-( early on some mugs which got to
hot to handle when fired that way, full of water, and the water hadn't
boiled yet.
Wow, so many variables. My microwave has no turntable, but a "wave
stirrer," whatever that is.
I regret I did not pay attention to the exact orientation of the handle
of each test mug relative to the microwave. I was looking for serious
handle heating, not subtle differences, so I don't think it would matter,
but it could account for some of the variability in my results.
Janet, yes the mugs were pretty much the same and the handle thickness
was quite similar. These mugs came from a set of 100, all made at the same
time, which I think made for particularly uniform pieces (for me!).
No, I didn't soak any of the mugs, they were all dry, had never been
used. The intent was to measure the effect of 10 to 20% RIO in a glaze, not
moisture in the mug.
No, the mugs weren't in exactly the same spot, or oriented the same
way--good ideas now that you suggest them!
When I fired the mugs individually, with water in them, I had to boil
the water in them for a long time (more than 5 minutes) to get the handles
to heat up noticeably. Even then I think it could have been heat coming
through the clay causing the heating, rather than simple microwave energy.
Neil, with my early mugs no two were designed the same, so the ones
which had hot handles were a variety of designs.
Thanks for your interest in this subject! Yours for better functional
ware,
Dave Finkelnburg
Timakia@AOL.COM on mon 22 jan 01
Dave I spoke to a proffessor in chemistry with considerable knowledge of
physics. He told me that a microwave heat nothing other than moistures. So as
I said before,I think one need to make sure that you have a good glaze/clay
fit, no matter of what firing temparature one uses and that one must make
sure that the glaze mature. Like I told you previously I just mixed a
terracotta glaze for ^10 porcelain and put a bowl with water in my microwave.
The water boiled and the rim stayed cool. Of cause porcelain is an easy one
since the clay also become glassy.
Someone else mentioned about a commercial mug that got hot. I think it is
easy to have cracks that appear on commercial work. Commercial work is fired
in big kilns and it is easy to end up with uneven temperatures in such kilns.
So although they do things well in theory, there might be failures.
Cracks in fired glazes that is not visable to the eye and porious glazes, I
think, will be real singoats when it comes to hot handles.
Antoinette Badenhorst
PO Box 552
Saltillo,MS
38866
http://hometown.aol.com/timakia
Gavin Stairs on tue 23 jan 01
At 09:38 PM 1/22/01, Antoinette Badenhorst wrote:
>Dave I spoke to a proffessor in chemistry with considerable knowledge of
>physics. He told me that a microwave heat nothing other than moistures.
Oh, how dangerous to profess to knowledge. To profess anything. I have
made my share of errors, too.
As I stated in other posts, microwaves heat polar structures. Water is
polar, so water gets heated. But certain fats and proteins can be heated
as well, and lord knows what else. Even things which don't start out polar
may polarize in the intense fields of the oven. Electrons can get bounced
around pretty well, and that heats things. Anything that conducts
electricity even a bit will heat up, which is another way water and aqueous
solutions (like soup) get heated.
Induction heating is an older form of the same technique which is used to
heat metals for various industrial purposes. Also to dry wood, etc., etc.
So, almost anything will get hot in a microwave oven, if you leave it long
enough, and the oven doesn't break down. What we are really discussing is
the relative rates of heating of various substances and mixtures. If you
put a mug in the oven empty and let 'er rip, it will probably get hot all
over. If you fill the mug with water and do the same, the water will
probably get hotter than the mug. However, if the mug is made of one of
the polar ceramics previously discussed, like the so-called searing or
browning platters used to cook meat, than the handle will likely come out
hotter than the rest of the mug, since the water will cool the mug, but not
the handle. What is dangerous is this situation where the handle gets very
hot in the time it takes to heat the water to boiling. Then the poor
victim may reach for the nice, hot cup of coffee and receive a burn from
the hot handle, which we have been led to believe should be cool, or at
worst warm.
Microwaves do penetrate into the heart of the mass being heated. The more
efficiently the microwaves are absorbed by the mass, the faster they will
attenuate with depth, so the heating rate is not uniform all over. If the
mass is somewhat conductive, like a mass of salted food, there will be
concentrations in nooks and loops, so shape may play a role in determining
temperature. And the field of microwaves is not uniform in the cavity,
especially after you insert some food. Also, as the food becomes hot, it
radiates and convects heat away from its surface, so the outside may not
get as hot as the centre. And cold plates or bowls (or mugs) will take
heat away from the food, so once again the outside may be cooled. Or, if
the container is one of those polar materials, it may get hotter, even hot
enough to sear the food. I know from personal experience, as well as from
the experience of one close to me, that you can burn the inside of a cake
or pudding without seeing any evidence of heating at the outside. All
sorts of strange things are possible.
Test the safety of your ware by heating it while it contains something,
like water. If, when the water becomes hot, the handle is not hot enough
to be uncomfortable, you're ok. If you have a porous, glazed ware, it is
probably not wise to call it microwave safe. If the pot gets waterlogged,
and the water flashes to steam, it may fracture and damage the microwave,
or worse.
Positively my last post on microwaves, I promise. Until the next time.
Gavin
Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 stairs@stairs.on.ca
Dewitt on tue 23 jan 01
At 21:38 1/22/01 -0500, you wrote:
>Dave I spoke to a proffessor in chemistry with considerable knowledge of
>physics. He told me that a microwave heat nothing other than moistures.
I don't believe that this is true. A couple of quick experiments:
1) Put a small amount of oil in a glass and heat for 5 or 10 seconds in
the microwave. Oil gets hot quickly, so microwaves do react with
substances other than water.
2) Put a wad of aluminium foil in the microwave and turn it on for a few
seconds. There will be a really impressive spark show. This is supposed
to be bad for the microwave oven, so don't do it for long.
3) Put a small florescent bulb AND a glass of water in a microwave and
turn it on for 15 seconds or so. (I would not use one of the incandescent
to florescent adapter bulbs) The florescent bulb will light up dimly. The
glass of water is supposed to avoid damage to the microwave oven.
I'm a proponent of the theory that some glazes do heat up in a microwave
oven. Several years ago, I was given a cheap import coffee mug that is
white with decoration in several colors of applied glaze. The only part of
the mug that gets hot is decorated with what looks like a chrome green
glaze. And based on the blister I got on my thumb, that particular glaze
heats up much faster and hotter than the water in the mug.
deg
Janet Kaiser on tue 23 jan 01
Thanks Dave. I am sure many share your interest
in functional ware being more "uniform" and
predictable. I admit my own main interest in
this subject, is naturally as the representative
of the final users... Because I cannot go around
testing other people's work, it is great hearing
how you work out the variables. It will help me
to be more confident when advising makers and
buyers on the issue.
On the whole, we (local makers and old-fashioned
me) have not taken microwave use into account
much in the past, but obviously that is
changing, just as dishwashers, ceramic hobs and
other appliances and technologies are becoming
more commonplace.
Also cooking methods are changing... For
example, pre-cooking and freezing food, then
taking it out of the freezer and putting it
straight into the microwave or oven. We tend to
forget that many folk work that way these days,
even if heating a mug in the microwave would
still be a pretty rare occurrence here... We
still tend to "brew up" tea and coffee by
putting the kettle on, not by heating a single
mug of water. Re-heating coffee this way would
be socially unacceptable! :-)
As a matter of interest, how many makers
actually state their ware is dishwasher,
microwave and/or oven-proof? Does anyone include
a sticker or piece of paper with "care of this
>item<" or "how to use this >item<"? Although we
are not as litigious as the US here in the UK,
it is universal bad press if buyers can blame
failure of a piece on its intrinsic qualities,
rather than on their inappropriate or mis-use of
it.
Another question: Is a glaze with a very high
iron content appropriate for microwaveable ware?
I only think of how lustres should not be used
in a microwave and metal will cause failure. Is
the amount of metal in a glaze or body always so
low (relatively) it would never matter? Or would
it affect the microwave over a long period of
time? Is there a rule of thumb? Is there a
maximum? Does anyone know?
Thanks
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 -----
> I think the important thing I learned is the
most simple.
> If you want to know if the piece you make is
microwave safe, first try it as
> the buyer would use it. I discovered
-( early on some mugs which got to
> hot to handle when fired that way, full of
water, and the water hadn't
> boiled yet.
> Wow, so many variables. My microwave has
no turntable, but a "wave
> stirrer," whatever that is.
> I regret I did not pay attention to the
exact orientation of the handle
> of each test mug relative to the microwave. I
was looking for serious
> handle heating, not subtle differences, so I
don't think it would matter,
> but it could account for some of the
variability in my results.
> Janet, yes the mugs were pretty much the
same and the handle thickness
> was quite similar. These mugs came from a set
of 100, all made at the same
> time, which I think made for particularly
uniform pieces (for me!).
> No, I didn't soak any of the mugs, they
were all dry, had never been
> used. The intent was to measure the effect of
10 to 20% RIO in a glaze, not
> moisture in the mug.
> No, the mugs weren't in exactly the same
spot, or oriented the same
> way--good ideas now that you suggest them!
> When I fired the mugs individually, with
water in them, I had to boil
> the water in them for a long time (more than 5
minutes) to get the handles
> to heat up noticeably. Even then I think it
could have been heat coming
> through the clay causing the heating, rather
than simple microwave energy.
> Neil, with my early mugs no two were
designed the same, so the ones
> which had hot handles were a variety of
designs.
> Thanks for your interest in this subject!
Yours for better functional
> ware,
> Dave Finkelnburg
Timakia@AOL.COM on wed 24 jan 01
Yes Gavin and so I understand microwave food causes cancer and people say
that of cellphones as well. LOL. BY the way, I meant with fluids any thing
that can become moist, including oils.
Whatever the cause might be, my concern are with the things that we as
potters can controll. One is not to sell your work as microwave safe unless
you are sure it is. Most people, think understand under "microwave safe" the
same as "oven safe" I came across people that except it that the mug or
casseroll becomes hot, as long as there are no sparks and blown up pots. We
should educate public beyond that.
Janet, I wonder about your question. If our glaze ingredients have a certain
melting point and are caught into fluxes that are safe, the microwave will
have to heat up high enough to melt the fluxes before it will release
anything; not true? Unless microwaves causes any chemical reaction that we do
not know of and if so would the original recearch not pointed that out? This
becomes more and more interesting.
Antoinette Badenhorst
PO Box 552
Saltillo,MS
38866
http://hometown.aol.com/timakia
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