You've watched the video, you saw the inferno. That was caused by double-whammy of not paying attention and using inappropriate materials being used. Using insulation is sound, provided it's rated to be used in high-heat situations. In my effort to keep things accessible and open I chose to use a rigid-type foam building insulation. Bad idea. The heat from the crucible in combination with the duration I had set caused a corner to the foam to ignite. I shut off the microwave and waited to the flame to die out, but it was only getting worse. Fearing a backdraft if I opened the microwave door, I risked it anyway. Yup, huge fireball.
We were able to get our cameras running just when the flames died down from reentry temperature to just immolation inferno.
The lesson here is to use just bricks to create the hearth and wait the amount of time required to to the job effectively, without trying to accelerate the process with insulation.
Science can be messy and dangerous, so be safe and have fun!