How Do I Kasher a Microwave?
Shailah of the Week by Rabbi Zvi Nussbaum
Rabbinic Coordinator, Kosher Hotline Administrator for the Orthodox Union
How can a microwave be kashered after it was used to heat non-kosher food?
The microwave oven should be cleaned well and left idle for 24 hours. Then a full cup of water should be heated in the microwave for ten minutes, allowing the water to overflow and the microwave to fill with steam. The cup should then be moved to another location in the microwave and again the water should be boiled for ten minutes.
However, the above kashering would be insufficient to kasher the glass turntable, which is assumed to have had direct contact with non-kosher food. Instead, the glass plate should be wrapped in two layers of plastic wrap.