Glass and ceramic hot plates cannot be kashered. To kasher, clean microwave thoroughly and do not use for 24 hours.

What materials can be Kashered?

As a rule, materials such as metal, wood, stone, natural rubber, and fabric can be kashered. Surface must be heated to a dry temperature of approximately 850° (i.e. self-clean oven) or until it begins to glow. Surface should be completely cleaned with hot water and unused for 24 hours.

Can earthenware be Kashered?

The Gemara (Pesachim 30b) writes that earthenware utensils cannot be kashered with hagalah (by immersing in boiling water), since boiling water alone will not completely purge the absorbed flavor that is embedded inside the cheres.