>>In our lab we use piranha solution (30% H2O2, 70% H2SO4) for cleaning
To clean fritted funnels, I'd set one up in the sink, add a few
millimeters of conc H2SO4 and trickle in 30% H2O2 -- a drop
or two for small funnels, more for larger funnels -- and let it
slowly drain thru the frit. Follow with copious water washes. I
have to admit that it didn't always work, probably because this
recipe isn't "real" piranha and some stains were too tough.
I do prefer alcoholic KOH but it can eat away frits and ground
glass surfaces so don't let stuff sit in it too long.