You've probably heard that putting metal in the microwave is a no-no, and while this is true only to a point, it's probably good advice to heed. Metals like tin, steel and stainless steel – which is just regular steel doped with chromium – reflect microwaves. That's not a problem by itself, but it can become one if the metal is crinkled, as in old tinfoil, or formed into complex shapes, such as forks. In such cases, the metal can reflect the microwaves back and forth, amplifying the energy and starting a fire. Flat, thick pieces of metal are usually okay, but if you nuke a stainless steel pan, the microwaves could bounce back and forth between the walls and the pan and ignite whatever you're cooking.
Metal in the Microwave
The working part of a microwave oven is the magnetron, which is a device originally developed by the military. It converts electrical energy to radio waves with a typical frequency of 2.5 gigahertz. These microwaves interact with the molecules in water, fats and sugar, causing them to vibrate at the same frequency. It's the vibrations of the molecules that heat your food, not some kind of stray electrical charge that's floating around inside a working microwave oven.
Technically, the microwaves shouldn't have any effect on metal items. Metal items don't get an electric charge from microwaves, and the molecules that make up a piece of metal are too closely bound to respond to microwaves by vibrating. The problem, however, is that metal reflects microwaves. Microwave oven manufacturers take advantage of this fact by constructing the walls of ovens out of metal to keep the radiation confined to the oven.
If two pieces of metal are close together inside a microwave, the reflection generates heat that can ignite anything in the oven and even ionize the air. Crinkled tin foil can actually burn, and energy passing between the tines of a fork can generate enough heat to produce sparks and crackles that resemble lightning. It's the air that is ionizing, but it looks like electrical arcing between the tines.
What About Stainless Steel?
Stainless steel has a layer of chromium oxide on its surface that prevents corrosion, but other than that, it isn't much different from regular steel as far as your microwave is concerned. It's just as reflective as regular steel or tinfoil – perhaps more so. A flat sheet of stainless steel sitting on the bottom of the oven will probably be unaffected by the microwaves, but a stainless steel fork will definitely crackle and pop.
Should You Do It?
If you're unsure about putting a stainless steel item in the microwave, the best advice is: Don't. If it isn't lying on the bottom of the oven, there's always the possibility that it could be too close to one of the walls and could amplify the radiation. This can happen with stainless steel pots and containers. Utensils other than forks are usually safe, but it's always best to err on the side of safety. You don't want to find out the hard way that the spoon you thought was safe can start a fire if part of it is sticking up.