You’re going to hate this one: pickle. Works great for dissolving
drills & taps.
Mix up some fresh pickle, a bit on the strong side, get it hot, then
dunk the piece in it. Go get a long lunch, and you’ll come back to
black sludge instead of a solid drill bit. It may or may not be
totally gone yet, but it should be gone enough to rinse out of the
The reason for mixing up fresh pickle is that way it doesn’t have
any dissolved copper to plate out all over your piece.