When is it ok to microwave metal?

December 14, 2004 1:19 PM

How come some microwavable containers contain metal? When is it okay to put metallic stuff in the nuker?

From the Campbell's Soup in Hand faq:

"Q: Even after pulling off the metal lid it seems like there is still metal on the rim of the cup. Is it safe to microwave?

A: YES, once both lids are removed, the Soup at Hand cup has been designed to be placed in the microwave. The soup inside the cup absorbs microwave energy, which prevents arcing (sparking)."

So soup is sufficient insulation? I've put less metal in the microwave than what's in the Campbell's can and have had big problems.

Please help me.

posted by mudpuppie to technology (18 comments total)

Alton Brown talks about this in the episode of Good Eats entitled "Ear Apparent." I don't have the transcript handy, but apprently as long as the pieces of metal are further away from each other than the length of the actual microwave (the energy, not the oven) then it won't arc.
posted by trey at 1:44 PM on December 14, 2004

Hmm. That makes sense to me if we're talking about two forks.

But what about a round can? How far away can the metal be from the metal when it's in a ring?

And how long is a microwave?
posted by mudpuppie at 1:47 PM on December 14, 2004

You can put flat aluminum foil in the micro as well. But if you crumple it, you're screwed. Mythbusters did an episode that showed this.
posted by u.n. owen at 1:51 PM on December 14, 2004

Here's some more about it.
posted by trey at 1:53 PM on December 14, 2004

Microwaves are much less sensitive to metals than they used to be. I recently defrosted a huge tin pan of food with tinfoil on it and a fork in it and as long as the metal didn't touch the walls it was fine, since then I put metal in whenever I have to, almost never arcs anymore.
posted by Cosine at 1:53 PM on December 14, 2004

Thanks for the link, trey. Backs up u.n. owen's theory.
posted by mudpuppie at 2:35 PM on December 14, 2004

My microwave has a big (metal) rack in the middle of it. Although I do remember (perhaps this is an issue from the past, as mentioned above) that if you took a stick of butter, removed *almost* all of the wrapper, and started to melt in the microwave, you were in trouble
posted by RustyBrooks at 2:46 PM on December 14, 2004

Here is a description of how the metal myth started, and a laymans/cooks description of how you can and can't use metals in the nuke.

In fact you can do all kinds of cool things with metal in a microwave. There is microwave smelting and sintering for example. It is just amazing what a little corporate myth making will do to the progress of science.

(This is taken from an old email I wrote, there might be lots of new stuff since then. A lot of links went dead too, sad...)
posted by Chuckles at 5:56 PM on December 14, 2004

If you cut a grape almost in half, leaving just a strip of skin connecting the two haves, and cook it in the microwave, it will act as a capacitor and build up a huge charge until it finally gets enough voltage to arc and the grape explodes in spectacular fashion. Its awesome. Cooking CDs in a microwave is pretty cool, too.
posted by ChasFile at 6:11 PM on December 14, 2004 [1 favorite has favorites]

Mythbusters did a piece on this. it took a ball of aluminum foil, if I remember correctly, to start a fire (not sure if it is the air that was getting trapped or what). otherwise, all ok. they even left a piece of silverware in there and nothing happened (again, going on memory).
posted by evening at 7:13 PM on December 14, 2004

Crap.

So many experiments, so little time.
posted by mudpuppie at 7:16 PM on December 14, 2004

I don't know why some things spark and others don't, but I do know microwaves operate at approximately 2.4Ghz; c/2.4Ghz gives approx 12.5 cm wavelength (in vacuum, but these numbers are so approximate it doesn't matter).
posted by fvw at 7:37 PM on December 14, 2004

I did my own experiment upon reading about RFIDs in $20 bills exploding when microwaved.

I took some regular printer paper, cut it into rougly dollar-sized pieces, put a pile of about 50 of them in my microwave, and turned it on. Aw hell. I lost the pictures. But you can try it yourself. Just make sure to STAY IN FRONT OF THE MICROWAVE because it only takes about 10 seconds.
posted by dirigibleman at 8:14 PM on December 14, 2004

I think it has to do with dissipating the static charge. The soup cup may have some conductivity (plastic can be made to conduct with the right additives) and the metal ring is then grounded to the cup, which is, in turn, grounded to the soup itself where the charge can dissipate in the soup? Or something like that.
posted by Doohickie at 8:56 PM on December 14, 2004

but . . doesn't that make the metal radioactive? More than one may want it to be?
posted by petebest at 9:45 AM on December 15, 2004

> apprently as long as the pieces of metal are further away
> from each other than the length of the actual microwave
> (the energy, not the oven) then it won't arc.

NOT CORRECT. The guy doesn't know what he's talking about.

The physics of electric arcs doesn't include a wavelength term.
Perhaps he's confused by that pattern of RF hotspots distributed
across the oven interior.

Arcing depends only on strength of the e-field at a particular
location. And the strength of the e-field depends on the sharpness
of the metal and the watts-per-square-cm where the sharp edge
is located THINK: if it depended on the RF wavelength, then
high voltage DC or 60Hz AC couldn't create sparks! Also, arcing
commonly occurs from a SINGLE sharp edge on a SINGLE object.

See my site:
UNWISE MICROWAVE OVEN EXPERIMENTS: FORK MYTH
posted by billb at 11:12 AM on December 15, 2004

Here's a simple, slightly dangerous experiment. Put an empty soup
can in a 1000-watt oven, placed off-center so it rides around on the
turntable, but placed so it doesn't approach any walls. Run the oven
for 30 seconds or so.

Chances are that you'll see a loud buzzing arc that leaps from a sharp
burr on the edge of the can. (If it doesn't work, try it with a soup can
which has the lid half-detached.)

EXPLANATION: With no water to absorb the RF energy, the energy
density in the chamber rises to very high levels (caused by 'cavity
resonance' where the waves bounce back and forth, growing
larger as more waves are added.) High RF energy is the same
as high voltage. The sharp edge of the can triggers a corona
discharge. Corona-plasma is a resistor: an energy absorber.
It swallows the entire 1000-watt oven output and grows as hot as
a blow-torch.

Arcs are "caused" by, first, not having enough food in the oven.
That sets up the high resonance and high voltage. Second,
you also need either a single sharp metal edge, or two metal
objects with a small gap between them (either you get a corona
discharge from the sharp edge, or you get a small spark that
leaps between two objects.)
posted by billb at 11:22 AM on December 15, 2004

doesn't that make the metal radioactive?

No, no, no, a thousand times no. The short explanation is that radioactive materials are radioactive because the nuclei of their atoms are unstable. Microwaves are orders of magnitude less energetic than what would be required to cause an otherwise stable atomic nucleus to become unstable.
posted by DevilsAdvocate at 1:36 PM on December 15, 2004 [1 favorite has favorites]