I found this interesting instructables:
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considering the small size of the microwave, this may be a bad idea…
What does size matter here? I mean, I can imagine all sorts of fun ways for things to go wrong, but the size of the microwave doesn’t factor in to them. 
to take in and out the ceramic pot thru the small opening may be difficult.
the closeness of the heating element (microwave radiators) to the liquid surface may cause impurities to evaporate and react with the radiators
What liquid surface?
And… it has been actually working, so theoretical problems haven’t materialized for him.
molten glass despite high viscousity could be considered liquid…but good that he didnt have any problems
You can also use a microwave to melt small quantities of metal, using the same general system: a silicon carbide absorber (usually a crucible) with the metal inside and a cover on top.
I would strongly suggest you acquire someone else’s old unwanted microwave rather than using the one in your kitchen. People get reallllllly crabby with you when you screw up and melt a big hole or blow molten industrial compounds all over the inside of the microwave they use for food preparation.
One neat thing about the microwave system is that for small materials, you can make a single-unit mold and crucible, so you put it in, melt, then pull it out and just turn it over, and the melt pours into the already heated mold. Again, something you don’t want to do in your kitchen.
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