I have a darkroom microwave, vented as the sink is vented.
Very handy to heat previously boiled and stored R/O water when mixing a developer or fixer up up.
I have used this microwave to to heat up colur chems. I do it in 20 second bursts, and swirl the liquid in between.
You will find that blixes, with all the ferri in them, seem to heat very quickly; much faster than water it seems to me.