Marc points us to this site with "detailed instructions on how to melt 'up to a quarter of a kilo of bronze, silver, white metal or iron.' with some simple mods to your existing microwave oven."
Domestic microwave appliances are based on the magnatron; an electronic device which converts electrical energy to microwave energy, which is fed via a waveguide to the cooking chamber. Since the conversion is somewhat less than perfectly efficient, the magnatron has to be cooled by a stream of air from a fan. This air is then led to the oven to help remove steam produced during cooking. Once in the chamber, the microwaves are reflected by the metal walls until they are absorbed, (usually by water-containing food), their energy being converted to heat. Should absorption not take place – if, for example, the oven is activated when empty, some energy will re-enter the waveguide and cause over-heating of the magnetron. Usually a safety switch turns the machine off when this happens. Note that the reflecting walls and the constant frequency of the microwaves set up standing waves in the chamber. This results in some areas being much more active than others and is the reason why food must be rotated through the varying field to cook evenly.
To be of use for metal casting, a domestic microwave oven rated D or E (850W or 1000W) needs two slight modifications: the rotating glass plate must be removed and the holes which admit air to the cooking chamber must be taped over (masking tape works reasonably well). The air from the magnetron cooling will then be re-directed to the exterior. No other modifications should be made. Microwaves are potentially dangerous and the uninitiated should treat the oven with respect.