Glass has a strange property - it's mostly resistant to microwaves until it gets very hot - once molten it absorbs microwaves readily and a thermal run-away continues melting the glass bright yellow. I suspect a rotating turntable - or a microwave oven with a stir fan should prevent glass...
So I know it's generally not a good idea, but I wondered what actually is the limiting factor. Does the magnetron heat up much more quickly? - and if so what's the physics going on that's causing it to heat up - is it increased current in the resonant cavities ? Also is it bad for the cathode...