We are often warned not to put anything metal in the microwave. Instructions that come with microwave ovens, and occasionally printed on the ovens themselves, say not to use metal containers or aluminum foil in them. Metal dishes may be stamped with the words, “Do not microwave.”
Yet what the potential hazards would be if we put metal in the microwave remain unclear. This warning has the flavor of an old wives’ tale: the admonition is often repeated, but not many people know the real reason for it.
Potentially, metal in the microwave could start a fire. However, this is not a common occurrence. Metal in the microwave causes sparks and popping sounds that make people think a fire is starting, but a full blown blaze caused by the use of a metal container is unlikely.
Even without a fire, the greatest risk of putting metal in the microwave is damage to the microwave itself. This damage is unlikely to show immediately, but each instance of metal being used in the microwave would shorten the life of the device.
The reason for this is the same reason why microwave ovens work as they do. Microwaves operate on a device called a magnetron, which generates a low frequency electric current. When this electric current hits a substance such as water or plastic, the substance absorbs it and gets warm. That is how a microwave heats food and beverages in minutes or seconds, and why microwaved dishes are usually hotter than the food when you take them out.
Anything that a microwave heats so thoroughly has very few free moving electrons. All substances have electrons, particles that react when the substance is touched by electricity. In water and plastic, the electrons are mostly bound up and cannot move about freely. When electric current hits them, the electrons absorb it. This results in the water or plastic warming up rapidly.
Metal is another story. The electrons in metal are not bound and move freely. When electric current hits them, the electrons start jumping around. This results in the electricity being bounced off the metal rather than absorbed. Inside a microwave oven, the space is so tight that the electric current has nowhere to go except back toward its source.
To use a visual image, metal is like a volleyball team where each player can move around and hit the ball, which is like the electric current. Water and plastic are what the same volleyball team would look like if they all stood in a circle, with arms linked, and couldn’t move. When the ball (electricity) lands in the middle of the circle, no one hits it out, so it stays there.
In a microwave, substances comparable to inept volleyball players are the safest. Its electric current (the ball in this analogy) is meant to stay where the magnetron throws it. A metal object in the microwave is a good volleyball player, and long, pointy metal objects are the best volleyball players of all. They bounce the electric current back so hard that it may arc and create sparks. This is what many people take as a sign of danger.
However, it is rare for the sparks to actually start a fire. The real risk is that the electric current will bounce back to the magnetron and overheat it, shortening the life of the microwave.
Certain metal objects do present a greater danger. Some glass and ceramic dishes have decorative metal strips. These strips, unlike the typical metal dish or cooking utensil, have the potential to heat up so much in the microwave that they would indeed catch on fire.
Metal pots and pans with plastic handles are also more risky to place in the microwave than dishes made entirely of metal. The handles are made with a material called phenolic, which can explode if microwaved. This is a much greater hazard than the metal itself presents.
While the risks of using metal in the microwave are not as dire as they are often made out to be, the potential for damaging the oven is reason enough not to do this. It is rare for metal in the microwave to cause a real fire. However, using metal in the microwave may greatly shorten the oven’s life.