But what do they call it in the stores?
This is what I used (usual non-affiliation disclaimer applies) to completely dissolve away about 0.2” of 1-72 tap broken off in bronze. It did take a few days, but the tap was completely gone and no damage to the threads at all. A 1-72 screw went right in, no problem, without any clean-up or chasing with another tap.
I didn't do any measuring out, just mixed in multiple teaspoons full of the powder. You want it strong, but a fully saturated solution, while fine, is not necessary. Watch for a stream of bubbles coming off the broken tap or out of the hole, or a single slowly growing bubble. That's the best “tell” that the solution is strong enough and hot enough and is working away (keeping it just short of boiling gives the best rate of reaction). I only heated it during the day, turned the heat off at bedtime.
