Although we know that metals inside a microwave can cause some serious damage, the inorganic material is a different thing altogether. These non-metallic, not reactive vessels of glass, ceramic or plastic do not absorb the radio waves at the frequency delivered by the microwave and hence do not heat up that much in it. There is something weird however, it seems that you can use any flat sheet of metal, usually the thicker the better, safely inside any microwave because they act just like the metal walls.[irp]
These metal sheets then reflect the waves instead of absorbing them a flat sheet of metal can actually shield your food from being heated too much and can even reflect the waves on to those areas which you want to be heated more than the rest of the food such as the base of the pizza.
Sometimes though, the electromagnetic field inside the microwave can get varied away and generate small minuscule arcs of electrical discharge. Innocuous items such as carrots or hot dogs can cause such a phenomenon. A higher level of sparks can be detected on the gold rim of your fancy cutlery too. However, a sizeable chunk of foil which is curved will spell chaos and will surely kill your kitchen appliance as well as damage the surrounding area.
Different to the proper walls of a microwave, the thin, pointed and small pieces of foil will not be able to withstand the rapid heating in a microwave and will subsequently ignite. Any of the edges which are crinkly or thin will allow the current to run along them arching them up against the metal walls of the microwave and as a result setting your food ablaze.
If you tend to the problem immediately you can avoid any serious damage to your kitchen appliance or its surroundings, by simply removing the irritant metal piece. However, a long time in the microwave or a large chunk of aluminum foil will very likely cause damage which is irreparable. It is also possible that it can cause a fire in the surroundings of your microwave. The USDA advises the following on using foil in microwaves;[irp]
- Use smooth, new foil only. Wrinkled foil can cause sparks.
- Cover only a quarter of the food with foil.
- Shape it smoothly.
- The foil should not be closer than 1 inch to the walls of the microwave.
- The shiny or dull side of the foil does not make any difference.
- If the microwave has a metal shelf or metal turntable, do not use any metal pans or foil containers, and prevent the foil from touching the metal turntable or shelf.
- If you see any sparks remove the foil immediately and transfer the cold food from the foil container into any container which is microwave friendly.
Hence it can be concluded that if you keep the metal thick and flat, and cover the least amount of space possible, you can heat up even your gourmet meal.