One of the first things people learn about microwaves ovens is to never put any metal objects in it. So I was surprised to see a microwave oven with a metal rack in the center, like those you find in regular ovens. Naturally I tried to find out how the manufacturers had achieved this and under what conditions metal could be used.
But the explanations on the internet were all over the place and few of them really convincing. For example, this article says: “These are actually safe because the metal used is tuned to the metal in the cavity of the oven. The racks attach to plastic or rubber clips on the sides of the oven, and as long as the rack does not touch the walls or floor of the microwave, no arcing or warping will occur.”
It is true that microwave ovens work by sending out waves in the microwave region of the electromagnetic spectrum, with wavelengths in the range of 0.1 cm to 100cm. These waves are in the range of frequencies that cause water molecules in the food to vibrate and thus heat the food. But what does it mean to ‘tune’ the metal in the rack? And why does the lack of direct contact of the rack with the sides of the oven matter?
The Wikipedia article suggests that the problem with metal objects is that with any sharp edges that most objects have, that when they in an environment where currents flow, one can get very electric high electric fields at the edges and points that can cause arcing and sparks, and that as long as the metal object, like the rack, is thick and does not have sharp edges it might be safe to use in a microwave.
But people are definitely discouraged from experimenting themselves. Leave it to the professionals and oven makers because it could be dangerous.