The Scientific Reason Microwaving Metal Is A Problem

We often turn to microwaves to reheat our leftovers quickly, make a delicious mug cake, or perhaps warm up a frozen dinner when we are short on time. Most of us never purposefully put anything metal in a microwave, since plastic containers, despite being unsafe, are typically used to hold leftovers. Unless someone is hoping for a viral video, putting metal in a microwave is generally done by accident — such as leaving foil over your food.

While comedy shows and skits may show microwaves bursting into flames from metal, that may not always be the reality. Although your house is unlikely to burn down, there is still a risk of damage to the microwave itself. Microwaving metal can create sparks or even a flame. That is something you definitely do not want in your microwave. This occurs due to the way electrons move on the surface of metals — especially those with rigid edges.

Microwave ovens produce electromagnetic waves at a frequency of 2.5 gigahertz (GHz). They achieve this using an internal device called a magnetron, which generates microwaves by oscillating electrons in a magnetic field. The movement of these electrons in metal is critical when microwaving it.

Metal already conducts electricity very well. When placed in a microwave, metallic objects with sharp edges or uneven surfaces can experience a concentration of electric fields, causing currents to build up. If the electric field becomes strong enough, it can ionize nearby air molecules, causing arcing and sparks — which may damage the microwave or, in some cases, even start a fire.

The way metal reacts in a microwave is something that food manufacturers occasionally take advantage of. Thin metallic layers are sometimes used with microwavable instant meals, such as soups, on the underside of the lid — to help with cooking. However, these too are susceptible to fire if left in the microwave longer than the instructions say to cook the meal. If the layer is any thicker, the waves will bounce off it, and the food won't get hot. Technically, microwaving metal isn't just dangerous; it's also pointless.

As metal inside the microwave conducts electricity, the arcing could end up burning holes in the internal walls of the oven itself. It can also permanently damage internal components of the appliance. Shopping for a new microwave isn't usually at the top of someone's excitement list. When in doubt about how your microwave handles metal, it's always best to check the manufacturer's instruction manual that came with the product. It may have specific guidelines on how much metal the oven can handle before being damaged. 

Understanding the science behind microwaves helps not only to ensure safety in its operation, but also to prolong its life, so you are not saddled with an unexpected expense. While there may be nothing better than a home-cooked meal, microwaves are always there when we need them in a pinch. But when you decide to use it, know that keeping metal out of your microwave is always the best option to ensure the appliance is healthy and working properly.