[Speedslug]

You could ignite a piece of charcoal and stick it in the kiln as a half glowing ember.
That would burn up the oxygen in the microwave and give you a reducing atmosphere.

You would want some sort of metal dish of some sort small enough to fit under the dome with you piece or else use a few larger chunks in a metal dish and let them burn as an ember for a few minutes with the door closed to use up the oxygen inside the microwave.


You could light a candle flame and close the door to get some idea of how long it will take to use up the oxygen by timing when the flame goes out.

Metal can be used in a microwave as long as there are no points or sharp edges. I have had a metal tray that rotates in my thousand watt unit for the last 25 years.

A ceramic dish will do as well but the heat from the charcoal could cause it to break.