If properly handled, it is entirely safe to use metal in a microwave. The traditional wisdom stems from the fact that when improperly handled, metal can arc and cause problems. While it takes some care, and should certainly not just be attempted randomly, there is no reason why everyone shouldn’t be able to use metal in a microwave.
Microwave ovens work by using a device called a magnetron that sends out microwave radiation — electromagnetic waves — at a frequency of about 2.45 GHz, meaning they vibrate at almost two and a half billion times a second. When these waves come into contact with metal, they excite its electrons, which then start moving rapidly, and bounce the microwaves bounce off them. For this reason, we can say that metal reflects microwaves.
Many other substances, most notably water and fats, don’t reflect microwaves. Instead, they absorb the energy, which in turn heats up whatever absorbed it, and that’s how food gets cooked.
When one puts metal in a microwave, and that metal is long and pointed, it essentially acts as an antenna to the microwave energy bouncing off of it. This can create arcs of energy, which can cause sparks that tend to scare people. The sparks can also arc to the edge of the microwave if they’re near enough, and may even cause an increase in temperature within the magnetron, reducing the life of the microwave. Because of the danger to the life of the machine, and because of the fear such arcing causes in consumers, most manufacturers recommend that people not use metal in one.
These small arcs actually are almost never cause for alarm. A microwave is closed while operating, and a small arc of this magnitude is little different from a static discharge when your finger touches a piece of metal if you’ve been walking across carpet. Only certain metal in a microwave, most notably thin metal stripping that is often used as decoration on some china or glass, is cause for alarm. This metal can actually heat up enough to catch fire, or at least damage the item it’s on.
In fact, one of the biggest dangers people have when putting metal in a microwave is that they choose some sort of metal that has plastic attached. Many metal pots and pans have plastic handles made of phenolic, which may explode if placed in the microwave. There are, however, pots and pans made specifically for use in the microwave that are made of metal. These are usually labeled as Microwave Safe, and can be used for a wide range of cooking.
Although not for everyone, cooking with metal in a microwave can actually give results closer to traditional convection cooking than any other form of microwave cooking. If the proper materials are used, and care is taken, cooking with metal can produce brown crusts, and perfectly evenly heated foods.