What Actually Happens to 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.)

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

Microwave 101

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 start bouncing around the positively charged walls of your oven. (Over)simply put, this activity produces light waves in the microwave range, and they start bouncing around, too. The foods you put in the oven are able to absorb these microwaves, thanks to the water they contain. That water is, in turn, magnetically attracted to the microwaves, and the pull of the attraction makes them vibrate till they heat up and transfer that heat to the molecules around them. Voilà! Hot food.

Where metal comes in

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 stuck in the oven. Some of it is reflected 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; only, without something to efficiently absorb the energy this attraction produces (i.e., food), it transfers it to whatever’s nearby—namely, the internal circuitry of your oven, resulting in overheating and possibly fire. By the way, these are the other ways you’re using your microwave all wrong.

The dangers of thin metal

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 start to 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.

Thick metal for the (safety) win

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, whose metal-lined pouches are designed to be microwaveable and to steer bouncing electrons toward food in order 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 are a matter of concern and should also be avoided. One of these is polystyrene foam, aka 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 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 9 microwave problems unless you want a big problem on your hands.

Not all foods!

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, these include edibles like hard-boiled eggs, since bouncing electrons can’t escape the shell or the egg whites; grapes, whose small size helps microwaves turn them to 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 in the microwave enough to kill off their spores, rendering them safe to eat.

Lela Nargi
Lela Nargi is a veteran journalist covering science, sustainability, climate, and agriculture for Readers Digest, Washington Post, Sierra, NPR, The Counter, JSTOR Daily, and many other outlets. She also writes about science for kids. You can follow her on Twitter @LelaNargi.