Alum works and works well…sometimes. It will not attack all steel
alloys equally, and only works really well on simple carbon steel,
so cheap imported bits tend to dissolve while high quality HSS bits
just sit there. Buy aluminum alum from any drug store, simmer in a
glass pot with the piece submerged in the solution. For a small
diameter cheap bit, full dissolution takes 1/2 to 1 hour, there
should be no visible effect on the jewellery. Clean off all oil
residue first, as this can somewhat protect the drill. Good
luck…I have heard that this solution works well as a general gold
pickle as well, but have not tried it.
Danny