If you get aluminum foil and line the outside of it with a thin layer of high heat resistant rubber, can you use that tin foil-rubber combination to heat up food in the microwave without creating sparks?
foil reflects microwaves, as I recall by having a current induced in it in antiphase or somethingrubber would not change that, the microwaves would penetrate the rubberif there's enough current density in the foil, it will get hot, but more likely is that you get enough voltage induced at the ends of the foil to break down the air, especially if there's a small gap to the wall of the microwave (also metal, designed to reflect the microwaves back inside), or an almost-closed loop of foil which could act as a resonatorIt's very dependant on the shape of the metal and the gaps whether you get arcing or notSince the RF field drops to zero at the surface of metal, you get no heating there, so there's not a whole lot of point unless as techno says you want to slow down cooking of a particular regionI sometimes cook meat pies in a foil dish, because I don't have a ceramic dish the same shape to move them to and it works OK - I'm too lazy to go buy one specifically for thatThere are some pie boxes which contain a material designed to absorb microwaves and get hot and brown the pie crust, but it's not pure metal and I'm pretty sure it has no rubber in it.
