Microwave oven was arcing, so after reading the repair faq I learned
that the wave guide cover was a possible source. Sure enough the cover
had food material on it that had carbonized. I tried soap and water
but that wasn't removing it. I was able to scrape some of it off using
my finger nail but there was still material that didn't want to come
off the cover. So then I tried a scotch brite scrubbing sponge and
gave it a good scrub. That did the trick and the microwave now works.
However, during the cleaning I noticed that a shiny material was being
removed from the guide in addition to the carbonized food particles.
I'm guessing this was the material the waveguide is made from which
appears to be mica. Are these small flakes toxic or otherwise
dangerous to health?