[mikeash]

I don't think "most" microwaves have a metal rack, although they're not uncommon.

It's also not uncommon to see microwave-safe food containers that contain metal. I've seen grocery store deli soups, for example, that end up with a big ring of metal around the top when you open them, but can still be microwaved.

Still, "no metal" is a good approximation. "Unless it says you can use it" is probably OK to leave implied.

[larrys]

I've seen that as well. I'm wondering (from my own experiment) if it has something to do with angles vs. no angles [1] in terms of the metal in addition to the blocking by metal of the actual microwaves.

[1] Same as with stealth airplanes avoiding radar, right?

[ScottBurson]

You're on the right track. The strength of the electric field around a charged, curved metal surface is inversely proportional to the radius of the curve. Sharp points have a very small radius, so they generate a large electric field, which can easily ionize air and thus cause arcing.

So spoons are okay, as they normally have no sharp points. Forks can cause problems, though.

Actually there's another issue here. If you stick an unopened can of food in the microwave, the microwaves can't reach the food; you get the same effect as if you ran the oven empty, which is that the field becomes very intense and risks burning out the magnetron or other components. The oven is designed to have its output absorbed by something, not just reflected back.

[beachstartup]

microwaves induce a current in metal. if there's a gap for that current to jump, you'll see it jump, and it'll probably cause problems including fire, but if there's no gap to jump, all that voltage potential just stays in the metal.

put a table spoon in the microwave and turn it on - nothing happens. i do this all the time when reheating soup.

i've won money on small bets like this too. non-technical people have no idea WHY metal poses a problem in the microwave, so i just bet them $10 that i can put a spoon in there without problems and they never believe me. they just think i have special spoons.

[larrys]

Preface for anyone reading this: I don't have any technical degree only what I have picked up over time. That said:

So in terms of the instructions "wrap the chicken" assuming the chicken were wrapped in a way that there was no crunchiness that produced metal gaps then it would merely block the chicken from cooking in the area wrapped, right? So in theory a nice idea but in practice people slap on the tin foil and then you have sparking? Hence "no metal in microwave" is really simply not being able to rely that the general public using the product will know the nuances (which makes sense). (Human behavior is something that I do know quite a bit about..)

Maybe you've done this one: You take a glass of water in pyrex (or coffee cup) and heat it just until it is ready to boil. Then you put a spoon or other object in and it explodes. Because apparently (I think..) breaking the surface tension is the issue (which can be done with anything if you don't remove carefully and shake a bit it will also happen).

Another thing that I've noticed is that water obviously boils at different times depending on the humidity in the room (I'm non technical enough to think that I figured that one out but feel free to correct me..)

[prewett]

I think that water in a microwave can get super-heated (hotter than 100C) if the surface is smooth, or there are no more little air bubbles to start the boiling. Then, when you put something in, it gives a surface for bubbles that are desperate to form something to form on, and they all do it at once.

I had a spectacular explosion with my glass teapot one time. The microwave at work was in a different room, so I would heat up the water to boiling, and then not hear the bell, so some time later I'd remember, and do it again. Apparently reheating causes the bubbles to basically get used up so there wasn't anything to start the boiling. So the third time I was standing there waiting so I wouldn't forget, and suddenly I hear a BOOM and the pot was half empty with a lot of water outside. I think there was just one giant bubble that eventually formed and blew everything out. (The pot was unharmed)