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This might be a more scientific question, but it relates to cooking and I thought it was interesting.

I just made my lunch which was a microwavable bowl of chunky soup.

The directions said:

Remove metal lid, remaining metal rim is microwavable.

How can this be?

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up vote 38 down vote accepted

Metal on its own doesn't necessarily cause electric discharge in a microwave.

What causes the sparking that you see when you put a fork in a microwave is due to the "sharp" edges of the fork. These edges concentrate the voltage at their tips which will cause a spark when it exceeds the dielectric breakdown of air.

Things like sheet pans (with rounded edges), or rounded metal racks are used frequently in microwaves with no ill effect. The absence of any pointed edges allows this. The rim of your bowl fits this requirement.

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Wow very interesting! – JD Isaacks Oct 4 '10 at 17:43
I would caution readers not to assume that any metal without sharp edges is safe to microwave. My parents have dinner plates with frilly gold plating and I once put one in the microwave without thinking - it was quite a light show. – Aaronut Oct 4 '10 at 19:46
Yup, same thing at my grandmother's house.. she keeps peanut butter in the fridge for some reason. So I nuked it. Foil still on the rim of the jar, it was like Star Wars in there. – daniel Oct 4 '10 at 20:16
The foil and gold leaf does have a sharp boundary. Because it is the bounds of the conductor that counts. – dmckee Oct 4 '10 at 21:40
@dmckee: Indeed, this wasn't gold leaf though; it was just a little bit off from a circular rim. As roux points out, you could get the same effect with a big sheet or tiny piece of tin foil. Maybe there's some "edgy" stuff happening at the molecular level but I'm just cautioning readers not to trust their eyes. – Aaronut Oct 4 '10 at 23:40

I think it also has to do with arcing. If the metal is close enough to other metal that electricity can leap the gap, it'll spark. I once stuck a metal bowl--very round on the bottom--into a microwave with a metal turn-table. The bowl got arc-welded to the turn table.

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