Why Can’t You Put Metal in the Microwave?

My roommate’s kid once left a spoon in his soup bowl and started the microwave. Wow, what a light show! Sparks were flying. Though I stopped it within seconds, the inside of the microwave was already blackened with soot. And while most of us know not to put metal in the microwave, sometimes it takes us by surprise.

“I learned this the hard way when I popped a dish towel in to cradle a hot bowl,” says chef Maricel Gentile. “It must have had a metal thread hiding inside because a minute later I saw sparks leap from the towel, then smoke and finally it burst into flame with a whoosh. It was really scary and taught me to keep anything with metal far away from the microwave.”

Here is the science behind why metal should not be put in the microwave, plus the best materials to use inside one.

Why Can’t You Put Metal in the Microwave?

Metal, including aluminum foil, bread ties and utensils, reflects microwave energy instead of letting it pass through. That builds up heat, causing sparks (aka, arcing). “It’s like shining a flashlight into a mirror; the light just shoots back at you,” says Gentile.

What Happens If You Put Metal In the Microwave?

You will damage the metal and the microwave, and possibly even create a fire inside the microwave oven. At first, you’ll probably see tiny blue or white sparks dancing around the edges of the metal, followed by a sharp popping or crackling sound, like little firecrackers.

“Soon you’ll smell something burning, like scorched cloth or melted plastic,” says Gentile. “If you keep going, gray smoke will curl up and you’ll hear a tense buzzing as the sparks arc back and forth.”

If you do accidentally put metal in, appliance repair expert Matthew Morrison recommends you immediately stop the microwave by unplugging it or pressing the stop button, then either let the item cool off before touching it or try to extinguish the fire inside.

What Makes Material Microwave Safe?

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Microwave-safe dishes are those that let waves pass through them, such as glass, ceramic and certain plastics. They send the microwave energy into your food, without absorbing or reflecting the radiation.

“Materials that will melt or release harmful chemicals into the food, such as styrofoam containers, are not considered safe,” says Morrison. “Even some microwave-safe cookware can still get very hot during the cooking process, so use oven mitts when handling bowls or plates.”

Are There Any Microwave-Safe Metals?

Yes, there are a few. For example, some metal trays are made specifically for microwave cooking, and the metal racks inside microwaves are made of thick, smooth and rounded stainless steel. But even with those, the protective finishes can become compromised.

“Thin metals or items with sharp edges are not safe at all in the microwave,” says Glenn Lewis from Mr. Appliance. “When possible, it is best to just keep all metals out of your microwave and avoid the potential of damaging the utensil or pan and the microwave.”

Best, Microwave-Safe Materials

The best materials to microwave food in include:

• Tempered glass, which has the benefit of being clear, so you can watch your food cook;
• Ceramic or stoneware, which holds heat well, to keep your food warmer for longer;
• Microwave‑safe plastic, which is designated by a wavy‑line symbol and a “microwave-safe” label.
• Silicone, which comes in flexible molds and mats that can conveniently go from the microwave to the freezer.

Especially with plastics, it is vital to your health to steer clear of any that aren’t labeled microwave-safe. “As a general rule, do not use the takeout container to reheat your food, but instead transfer what you have to a microwave-safe bowl or plate,” says Morrison.

FAQ

How does a microwave work?

A microwave oven produces electromagnetic waves, called microwaves, which cause the molecules in your food to move, or excite, creating friction. The heat from that friction warms food from the inside out.

“The invention itself was discovered by accident as scientists were experimenting with radar technology to detect enemy aircraft in 1945,” says Morrison.

Are all foods microwave safe?

No. Never put whole eggs or grapes in the microwave, as they can explode. Poke thick-skinned vegetables, like potatoes, so steam can escape while they’re cooking. Also avoid microwaving carrots, since their high mineral content can cause arching; hot peppers, as their capsaicin can become toxic; and hot dogs, because of their high concentration of salt and other additives.

“Although personally I have done hot dogs many times with no visible damage,” says Morrison.

When in doubt, check your manufacturer’s recommendations for specifics about your model, says Lewis. “Some great microwave cookbooks on the market can be helpful in this case,” he says.

About the Experts

• Maricel Gentile is a chef and cooking author. She also owns the cooking class destination Maricel’s Kitchen in East Brunswick, New Jersey.
• Matthew Morrison is an appliance repair expert on JustAnswer, the expert on-demand platform. His background includes 20 years of experience as appliance technician with Sears and GE factory service, and as a ground equipment mechanic in the Air Force.
• Glenn Lewis is President of Mr. Appliance, an appliance repair franchise and Neighborly company. He is responsible for the overall leadership and growth of Mr. Appliance throughout the US and Canada.