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It's practically common knowledge that microwaving metal objects can damage the microwave. I've heard this multiple times, mostly from my mom, and I've always believed it to be true. But is it? Would the microwave really be damaged by just microwaving a fork or an aluminum foil?

Gabe12
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  • Same here, my parents have always warned me against this, however, I've always heated my meal with a spoon inside and there was no problem whatsoever. – Haider Feb 14 '16 at 17:31
  • It entirely depends on the construction of the microwave. You are pumping 1kW of energy into 25 liters of space, it must go somewhere, various ways for it to chose from. If you are intrested in more visual explanations, google what stuff people microwave on youtube... – PlasmaHH Feb 14 '16 at 23:03
  • a friend of mine (who was well on the way to earning a 1st class honours in Physics at the time) once asked a housemate how to heat baked beans. "decant half into a bowl, put them in the microwave and heat on full power until suitably hot" was the reply. He dutifully decanted half, then placed the now half full can in the microwave. The result was spectacular, and the microwave was never the same again. – Joseph Rogers Feb 15 '16 at 09:52
  • Some Microwaves come with metal racks and shelves in them, so it depends on the metal, its shape, thickness etc... So the answer is sometimes, but not always. It depends on many factors. – superphonic Feb 18 '16 at 13:57
  • The user manual of my microwave oven suggests placing a metal spoon to the container when heating water. The reason for this is that water can become somehow overheated so that it's hotter than boiling temperature. I've seen this happen, a friend took a mug of water out from the oven and poured in the quick coffee powder, the water exploded out of the mug like a geysir. Apparently the spoon will prevent this from happening. . – Kimmo Lehto Feb 24 '16 at 22:19

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It depends on the metal and the microwave.

This video shows what happens with a crumpled sheet of aluminium foil:

https://www.youtube.com/watch?v=XKQ9vC_DK_c

Gold banding on cups is also notorious for causing arcing.

However, I was mildly surprised to learn that metal can be placed in a microwave - the USDA says that smooth aluminium foil should be okay (but make sure you read your microwave's instructions). However, you should also be watchful for arcing and stop the microwave as soon as you see it.

The danger to the microwave itself comes from the spark - there is a probability that the current from the electric arc will pass through the magnetron and damage it. Trying to find a reputable source - but it seems to be an obvious conclusion: electric arc through electric device equals damage.

Finally - and slightly anecdotally - a Scout unit I was involved with held a "mythbuster" night, and grabbed a bunch of old unwanted microwaves and tried various things like metal spoons and such. I can assure you, there were sparks.

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    But does it damage the microwave? That is the question, and I don't think this answers it. – Reinstate Monica -- notmaynard Feb 16 '16 at 16:03
  • Fair point - I'm reading up on that now... I'm pretty sure some of those microwave ovens the Scouts got their hands on were unusable after, but they were old already. –  Feb 16 '16 at 21:47
  • _Is_ there actually a (significant) probability that the arc will hit the magnetron? The cooking chamber is made of metal (possibly grounded, though I'm not sure), and it seems very unlikely that an arc would pass through that and contact the magnetron. – Reinstate Monica -- notmaynard Feb 17 '16 at 18:27
  • As far as I'm concerned - non-zero is significant enough... end of the day, why do you need to risk it? –  Feb 17 '16 at 21:24
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    I've seen burns on the inside surfaces. Does that count as damage? And I've seen food catch fire due to the sparks igniting it, but that's sort of secondary. – Sean Duggan Feb 19 '16 at 20:53
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Last month I'd uncrated a Rival brand microwave oven and found, on page 13 of the manual, under the category "Shielding":

  • Shield food with narrow strips of aluminum foil to prevent overcooking.

  • Areas that need shielding include poultry wing tips, the ends of poultry legs, and corners of square baking dishes.

  • Use only small amounts of aluminum foil. Larger amounts can damage your oven.

I called the 800 support number in the manual. They had nothing substantive to add--- merely: "Well, just be sure you don't use a lot of foil. Just what you'd need to keep the tips and corners from overcooking or burning."

So, including small small amounts of metal is actually recommended in some circumstances.

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Fred Kerns
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