I'm not sure is this is what you mean but I used 99.9% pure sulphur and it gave a VERY even coat, I left some if for a few seconds/a few minutes and an hour. The ones left in for a few seconds (about 20-30 seconds) were a very light brownish grey , the ones left in for a few mins were almost black :/ and the one I left in for an hour had a thick coat of goo stuff all over it . All the coins I used were 92.5 sixpences. Also it Was VERY hot when I did it maybe 42-43C, so chances are it was acting much faster then normal.
Edit: Dynoman just reminded me that I cleaned a few with with isopropanol alcohol, a few with cloudy ammonia and a few just went it. The ones that I cleaned with alcohol turned out the best with an almost perfect coat, followed by the cloudy ammonia with a few streaks and splotches (it wasn't very pretty) and the ones I just put in were CRAP, I got some sulphur powder (you can get it on the bay, if memory serves right it was about $15 for 1kg with free post). One thing, if you do take this path is to make sure the sulphur powder is crushed up very nicely because it clumps up very easily.
Clean a coin with acetone, dilute some household bleach, dunk in a coin you don't care about you can create some interesting results this way. That's why you should'nt swim in a chlorinated swimming pool with your silver jewellery. It corrodes quite quickly. To remove the tarnish use household ammonia, it has the reverse effect.
I tried bleach once to shift some dirt, the coin went black, I wouldn't recommend it unless you have time to play around with different times, temperatures and concentrations, and plenty of sacrificial coins.