Can I heat the mask in the microwave?
No. You must never heat the mask in the microwave. The mask contains two metal plates. Sometimes metal objects are shaped in such a way that they reflect the microwave radiation to one point like a concave mirror. Therefore, there is the possibility that almost the entire power of the microwave becomes concentrated in a small volume. This results in very strong local heating or sparking.
Heating the mask in the microwave will damage it.