I have recently been doing some research into the working principle of a microwave kiln, and have run into a few things that continue to confound me. The first of which being that the devices evidently focus microwaves - which supposedly have a wavelength of ~12.2cm - through a tiny hole that is often no more than 1/2" in diameter, and achieve temperatures high enough to fuse glass. How can this be?
Microwave kilns are made of a white insulating material. It is generally composed of a porous ceramic body with an inside coating of high temperature susceptor material. A susceptor material is the one which absorb microwave energy. Some of these can reach temperatures upto 1000 degree celcius. The actual composition of these materials are not known since they are patented trade secrets. The insulation of the kiln is the main reason of it reaching high temperature. The susceptor material also plays an important role.
The small hole helps it in increasing the temperature because once the microwave gets in it is a-kind-of-impossible to get out.