What if I Put Aluminum Foil in the Microwave?

By: Marshall Brain  | 

This man may be smiling as he puts the aluminum foil in the microwave, but he won't be smiling for long.
This man may be smiling as he puts the aluminum foil in the microwave, but he won't be smiling for long.
Glowimages/Getty Images

The microwave oven is one of the great inventions of the 20th century -- you can find them in millions of homes and offices around the world. At one time or another, we've all been told not to use metal products, especially aluminum foil, when cooking­ with a microwave oven. Stories of incredible explosions and fires usually surround these ominous warnings. Why is that? Let's take a look at how microwave ovens work to find out.

As incredible as microwave ovens are, the technology behind them is fairly simple. Microwave ovens use microwaves to heat food. Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: Water, fats and sugars absorb them. When absorbed they're converted directly into atomic motion -- heat. Microwaves in this frequency range have another interesting property: Most plastics, glass or ceramics don't absorb them. But what about metal?

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The walls inside a microwave oven are actually made of metal. It turns out that a fairly thick piece of metal works a lot like a mirror. But instead of reflecting an image, it reflects microwaves. If you were to put food in a heavy metal pan and put it in the microwave, it wouldn't cook. The pan would shield the food from the microwaves, so the food would never heat up.

Other metals are a no-no in the microwave, too.
Other metals are a no-no in the microwave, too.
Tim Flach/Riser/Getty Images

Tiny sharp pieces and thin pieces of metal are a different story. The electric fields in microwaves cause currents of electricity to flow through metal. Substantial pieces of metal, like the walls of a microwave oven, can usually tolerate these currents without any problems. However, thin pieces of metal, like aluminum foil, are overwhelmed by these currents and heat up very quickly. So quickly in fact, that they can cause a fire. Plus, if the foil is crinkled so that it forms any sharp edges, the electrical current running through the foil will cause sparks. If these sparks hit something else in the oven, perhaps a piece of wax paper, you'll probably be reaching for the fire extinguisher.

While it's highly unlikely that a small piece of foil is going to cause your microwave oven to totally explode, it could cause a fire. So, it's a good idea to stick to plastic wrap, paper towels and any other non-metal kitchen aids.

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Originally Published: Mar 24, 2008

Foil In Microwaves FAQ

Is aluminum foil safe to put in the microwave?
Doing so is unlikely to cause an explosion, but the edges of the foil could potentially spark, leading to a fire.
What happens to aluminum foil in the microwave?
While the metallic walls that make up the inside of a microwave oven are also made of metal, they are not likely to present any real danger. Aluminum foil is thin, and when food wrapped in foil starts to heat up, water turns into steam, releasing energy. Because the food is covered with foil, this energy has no outlet. This makes the foil heat up fairly quickly at the risk of catching fire.
What will happen if I put foil in the microwave for only a short time?
Even a short time can damage the microwave and even start a fire.
Can I eat food that has been microwaved with foil?
While you should never put foil in the microwave, if your food somehow makes it out without any serious issues or damage to the microwave, it should be safe to eat.

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