Fact or Fiction: Why Can’t You Put Metal in the Microwave?

Can you really set your microwave on fire? It depends. (And no, you shouldn't try this at home to test it out.)

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Microwaving Metal

From the time countertop microwaves started to hit the market in the late 1960s, we’ve all known one thing: Metal is forbidden to go in the microwave!

However, as it turns out, that’s not completely accurate. Back in 2003, the MythBusters team actually dispelled the rumor that any metal placed in a microwave would lead to a certain explosion. But that doesn’t mean you should wrap a fork in tinfoil and stick it in on high. What’s the real scoop?

How a Microwave Works

First, let’s back up and consider how a microwave oven actually works. As Wired explains, when you press “Start,” the microwave begins producing negatively charged electrons that bounce around the positively charged walls of your oven.

The foods you put in the oven absorb these microwaves thanks to the water they contain. That water is, in turn, magnetically attracted to the microwaves. The pull of the attraction makes them vibrate until they heat up and transfer that heat to the molecules around them. Voilà! Hot food.

What Happens When You Put Metal In the Microwave

Metal is bad at absorbing microwaves. According to physicist David McCowan in The Takeout, since metal doesn’t contain any water, it has no way to effectively use those microwaves.

Some energy from the microwaves sort of dances around on the surface of whatever metal you stick in the oven. Some of it reflects off the surface of the metal and bounces off it, like a reflection in a mirror.

That doesn’t mean metal can’t get hot; it can. It contains electrons that are attracted to the microwaves, too. But without something to efficiently absorb the energy this attraction produces (i.e., food), the metal transfers it to whatever’s nearby — namely, the internal circuitry of your oven, resulting in overheating and possibly fire.

The Dangers of Thin Metal in the Microwave

The thinner and sharper your metal is, the more dangerous it is to stick it (alone) into your microwave, according to Spoon University. This goes for aluminum foil (especially if it’s bunched up), the delicate golden glaze on your grandmother’s tea set, and the tines of forks.

Basically, moving electrons either concentrate in the creases or along the sharp edges of the metal and build up a charge — like static electricity. Wired calls them “concentrated spots of negative change.”

This charge will bounce angrily around to find a place it would rather be. That’s what accounts for any spark you might see if you glance through the door of the oven. If there’s something flammable in the microwave with the metal (i.e., a piece of paper), you get a fire.

What About Thick Metal?

Why you might want to put metal (aside from the metal rack that comes with your appliance) into your microwave is a question only you can answer. But if you must, opt for a thick, smooth slab of the stuff.

Wired points out that thicker metal heats up much slower than, say, a thin sheet of aluminum foil. And smooth metal is not apt to cause sparks from bouncing, angry electrons looking to get to a better, less jagged location.

Other less dangerous metal-related microwave situations include covering the metal with food, and heating up items like Hot Pockets. With the latter, those metal-lined pouches are designed to be microwaveable and steer bouncing electrons toward food to crisp up its outsides.

Other Unsafe Materials

As multiple sources point out, it’s easy enough to avoid sticking metal in a microwave. But there are other food receptacles that should also be avoided as well.

One of these is polystyrene foam, a.k.a. styrofoam. This heat-unstable plastic material can warp or melt when exposed to microwaves. And it can also release harmful chemicals into your food when it gets hot, according to Real Simple. Non-microwave-safe plastic containers, like cold-storage tubs meant to hold foods like yogurt, share a similar problem. The Food and Drug Administration (FDA) recommends using only plastic that is labeled “microwave-safe.”

Food safety isn’t the only thing you need to worry about when it comes to your kitchen’s most convenient appliance. Don’t ignore these nine microwave problems unless you want a big problem on your hands.

Microwave-Safe Materials and Containers

It’s better to be safe than sorry and in order to avoid any microwave repair charges you must be careful about the materials you decide to put in. These materials are microwave-safe:

  • Glass and ceramic dishes;
  • Paper plates, towels and napkins;
  • Wax and parchment paper;
  • Plastics specifically labeled “microwave-safe.”

Beware! Not All Foods Are Microwave-Safe

There are plenty of foods you could be microwaving and aren’t. But there are also some foods that are an absolute microwave no-no.

According to the Atlanta Journal-Constitution, the no’s include edibles such as:

  • Hard-boiled eggs, since bouncing electrons can’t escape the shell or the egg whites;
  • Grapes, whose small size helps microwaves turn them into plasma before causing them to explode;
  • Hot chili peppers, which can release eye-searing, retina-burning chemicals when you open the oven to extract them;
  • And leftover potatoes, which may contain bacteria that will not heat up enough in the microwave to kill off their spores, rendering them safe to eat.

Reader's Digest
Originally Published on Reader's Digest

Lela Nargi
I am a veteran journalist covering food & food policy, science, sustainability, books, and parenting for a wide variety of outlets including NPR, Civil Eats, Sierra, and Publishers Weekly.