I have not used a microwave kiln, so my offerings are based on my understanding of glass as a material (composition, physics, etc.), my fusing knowledge and experience, and my experience with microwave ovens.
As others said, you really have no way of knowing what temp your kiln is at at any point in the process – a real problem for troubleshooting, understanding how glass behaves, or having much control over the process. That said, we do exercise some control over the temperature and rate of heating in a microwave every time we use the defrost function, or key in less than full power.
Glass, even small pieces, do have a couple of really touchy temperature points. One is around 350F – if you heat too rapidly up to and through this temp, you can get breakage from thermal shock. The second is the strain point – at about 850F for COE 90 glass – what the Fuseworks products are and their kilns are designed to use. Opening your kiln around this point is a good way to get broken glass and, when cooling the glass, to have bad stresses put into it if it doesn’t break. This is why all the basic fusing books tell you, when cooling down and annealing your glass, not to open the kiln from about 1000 – 1100 F and below, until room temp. You can cheat on this a bit, but it really is not a good idea to open the kiln until it is at most, about 200F.
You do need to make sure your kiln is very clean. You could try heating at about 50% power or on defrost for a portion of the time, the quickly shifting to full power for the time necessary to fuse the piece. If even the cost of a new small kiln is too much, keep your eyes on ebay (where I got my first one) for a real kiln, and have fun!
Fused Glass, Wood, Beads, PMC, Ceramics
Usable and Wearable Art
Art for the Sake of Art