Microwave firing of ceramics is said to improve their physical properties by creating an inverted temperature gradient: since the microwaves can penetrate deeply into the ceramic to heat it, the heat source is spread evenly throughout, while the heat sink is concentrated at the surface, where cool air cools the surface of the ceramic. Consequently, water is driven out of the ceramic before it hardens, avoiding the creation of steam bubbles that create microscopic cracks in the ceramic.
David Reid, an amazing hacker, recently figured out that he could cast aluminum and pot-metal in his kitchen microwave by making clay molds with graphite and magnetite mixed into the clay. Apparently graphite absorbs microwaves quite well at room temperature and until it gets fairly hot, while magnetite doesn't absorb them that well until it's already relatively hot --- but continues to absorb them until it's hot enough to melt the metal. (http://home.c2i.net/metaphor/mvpage.html) Unfortunately he didn't get it hot enough to cast steel or fire ceramics; the magnetite melted. He's still trying to solve this problem. I speculate that a small microwaveable vacuum chamber could help with home microwave-firing by diminishing the heat loss due to convection. (Reid used ceramic wool and taped over the ventilation port in the microwave to help with this.) Perhaps a simple bell jar, with a ceramic base, a non-microwave-absorbing rubber seal for the bell-jar to rest on, and a little nylon valve and hose fitting to let you pump the air out before you put the thing into the microwave. Used microwave ovens are available cheaply in the US: craigslist currently lists several for $35, $20, $50, $15, $20, $20, $35. By contrast, small electric kilns seem to start around $500 new and $200 used.
