Fritted glass is a pain. If reverse flow and plain acid (both mentioned by the text) don’t solve it, piranha solution (the text calls it “acidic peroxide solution”; it’s sulphuric acid with some hydrogen peroxide) does the trick, without screwing with the pores. Just go easy on the hydrogen peroxide, to avoid explosions. (They can do worse than just kill you: they can ruin the glassware.)

In fact piranha solution [dis]solves almost everything. There’s also “basic piranha” (NH₃ + H₂O₂), good for obnoxious ionic compounds (the ammonia chelates them and tells them to fuck off).

I wouldn’t bother with chromic acid, aqua regia or HF, they aren’t worth the trouble.