There is an addition to this process where the water and alum is bought to the boil and kept simmering. It is (was) detailed in a report at RWG.cc. Just went looking for the info, and it looks like its been lost over the years of changes.
So I searched my hard drive and look what came up-( this is about dissolving a broken tap, but the same principal works well for stems.)
The only issue i have ever had was on one watch where I left it in a bit long and after the stem was dissolved it started on the crown. It actually just made the crown thread a bit sloppy, and I used some solder to keep the new stem in the crown.
Recipe for Alum to dissolve stems/ broken taps etc
1 Obtain some Alum from the spice section of your local grocery store.
2 Bring about 1" of water to a boil in a glass, ceramic, or aluminum pan. Pyrex is perfect.
3. Add in about 1.5 ounces of Alum.
4. Bring the mixture down to a simmer (lowest heat). Drop in the case. Push the pot to the side of the burner, and the case to the side of the pot. (This to assure that the case won't be accidentally overheated and discolored.)
After a few seconds, a rapid stream of bubbles will begin to emerge from the tube hole.
5. Cover the pan to limit evaporation. Simmer for about 3 hours.
6. Remove the case, rinse, and allow to cool.
7. Use a paperclip to press out the dissolved remains of the tap.
Offshore
