I thought some people may find this tip useful. I used Alum to
dissolve away a broken drill bit in a brass part.
Alum works, though slowly. Boiling it speeds things up. But ordinary
pickle also does the job, and I think, faster than Alum. Mix up a bit
of fresh pickle, so it doesn't have dissolved copper in it, to avoid
copper plating your work.
Pickle will take out not just carbon or high speed steel drills, but
also carbide, tungsten steel, or cobalt steel. You can speed it up
greatly by putting the pickle and piece in a small jar that you can
then suspend in your ultrasonic cleaner. The combination of the acid
and the ultrasonic to keep fresh acid at the drill even at the bottom
of the partially drilled hole, makes it work much faster than just
soaking. When you're done, the bit of pickle you mixed up can go
right into your pickle pot. The tiny bit of iron it dissolved from
the bit doesn't hurt the pickle at all, since it's already dissolved
now, and no longer can cause copper plating.