Why does metal, like aluminum foil, spark when heated in the microwave?
This is about as in-depth as I could find. “Any object containing pointed metal can create an electric arc (cause sparks) when microwaved. This includes cutlery, aluminium foil, ceramics decorated with metal, and almost anything containing any type of metal. Forks are a good example. This is because the tines of the fork resonate with the microwave radiation and produce high voltage at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter (3×106 V/m). The air forms a conductive plasma, which is visible as a spark. The plasma and the tines may then form a conductive loop, which may be a more effective antenna, resulting in a longer lived spark. Any time dielectric breakdown occurs in air, some ozone and nitrogen oxides are formed, both of which are unhealthy in large quantities. Microwaving food containing an individual smooth metal object without pointed ends (for example, a spoon) usually does not produce sparking.” — http://en.wikipe
