in some cases, when i was in high school the college across the street
(where we often ate lunch) had an early heavy duty microwave for the
frozen vending foods.  if you put a pop can etc. in it after a while it
would arc to the inside of the oven!  i never did it but i have seen it.
 i don't know if the microwaves were rectified at a seem or dissimilar
(different alloy at least) junction and built up a charge or if there
was just an impedance mismatch and it acted like a transformer or if it
acted like a resonant cavity with energy feeding in through the hole at
the top and accumulating until there was enough stored to produce a high
field strength.  it was interesting and mildly amusing.  i've never been
certain what actually caused it though. =20

i do know one problem with solid metal in a microwave is that it
reflects microwaves, some of them back into the oscillator making it
dissipate more heat and run at a higher field strength.  of course if
there is some food etc. in there as well, particularly if it's on top of
the metal that will likely absorb enough of the energy to be ok.  it's
also hard on microwaves to run them empty or with very little in them,
particularly on the higher power settings for the same reason, i.e. the
microwaves are not being absorbed so the field strength increases and
more is reflected back to the oscillator and dissipated as heat there,
where you don't want it hot.  if i have very little in the microwave i
always run it on low, that way the average power dissipation stays
reasonable. =20

food wrapped on all sides with metal will of course keep the food from
warming and be hard on the microwave.  i have also seen the problem with
metal rings, on older plates that had a metallic paint in a circle
around the outer edge, it burns and discolors the metal and makes the
plate somewhat ugly, and of course you've probably also added some odd
metal vapor to your food along with whatever else was in the paint and
decomposed when it got hot.

Russell McMahon wrote:
>=20
> > BTW another dumb question: Can etchant be heated in a microwave?
> > I mean it does contain metal right?
>=20
> I'd be reasonably confident (3 standard deviations) that you'd have no
> problems.
>=20
> Metal per se is NOT a good microwave absorber. Metal smelting actually
> requires special absorbers which then transfer the thermal energy to th=
e
> metal. Problems with metal in microwave ovens usually arises from close=
d
> conductive loops which couple RF energy. You can (and I have) use flatt=
ish
> metal pie plates in microwaves with no problems. YMMV / Don't try this =
at
> home if you don't know what you are doing / standard caveats apply  ...=
 :-)
-------

--=20
Philip Stortz--"In Germany they came first for the Communists, and I
didn't speak up because I wasn't a Communist.=20
Then they came for the Jews, and I didn't speak up because I wasn't a
Jew.=20
Then they came for the trade unionists, and I didn't speak up because I
wasn't a trade unionist.=20
Then they came for the Catholics, and I didn't speak up because I was a
Protestant.=20
Then they came for me, and by that time no one was left to speak up."
-- Martin Niem=F6ller, 1892-1984 (German Lutheran Pastor), on the Nazi
Holocaust, Congressional Record 14th October 1968 p31636.



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