Microwave Info
It turns out that any flat metal sheet—the thicker the better (and we'll get to why in a second)—can be used safely in the microwave because they act just like the flat metal walls: they reflect microwaves. A sheet of flat metal can actually be used to shield parts of your dinner to prevent them from overcooking since they would prevent the microwave energy from actually striking the food. And when placed under, say, a DiGiorno's frozen pizza, a flat sheet of metal can actually help brown and crisp foods by reflecting additional energy against the bottom of the food.
Inorganic materials are a different matter entirely. Non-reactive, non-metal vessels of ceramic, plastic, or glass do not absorb radio waves at that frequency and consequently, don't heat up very much in the microwave. Metal, on the other hand, actually reflects microwave energy. That's why the inside of every microwave is essentially a secure metal box—they keep microwaves from spreading out across the kitchen and cooking your insides from across the room.
Sometimes the electromagnetic field within the microwave can get a little mixed up and generate small arcs of electrical discharge. This can be caused by innocuous items like carrots (when grown in mineral rich soil) and hot dogs (when the salt and additives aren't properly mixed). More moderate sparking occurs with the gold paint on your nice dishes and forgotten twist ties. But a big hunk of curved aluminum foil like on a big box of wine spells instant appliance death.
Unlike the sturdy walls of a microwave, small, thin, and pointed pieces of foil cannot withstand the flow of microwave energy over them and rapidly heat until they ignite. Any thin, crinkled edges allow current to run along them, arcing against the microwaves metal walls and setting fire to your meal.
