Liver of sulfur is Potassium Sulfide. Buy it in lump form from a jeweler’s supply
company. You can buy it pre-mixed, but since it has a shelf life, it’s much better to buy it
in lump form and mix it as needed.
Break off a pea sized lump and add it to 1/2 a cup of warm water and dissolve the liver of sulphur.
To work, either the liver of sulphur or the metal must be hot. Liver of sulphur smells pretty badly, so heating the metal reduces the odor.
I use the bottom part of an old air popcorn popper* to heat the metal. Have your liver of
sulphur ready, along with a paintbrush. Now you can paint the liquid onto the metal where
you would like it to be darkened.
If you would like the whole thing darkened, dunk the item. If it’s a bead, string it on a wire or string. Your piece should be polished before you apply the liver of sulphur, which is your last step. Rinse off the piece under running water. If you accidentally got any liver of sulfur in areas where you don’t want it, remove it with a 3M polishing pad (Microfine) or with a fine polishing wheel on your flexible shaft.
*These are getting rarer in the 2nd hand market. Another heat source that works is a free standing coffee pot warmer. But not a mug warmer, mug warmers do not get hot enough.
This information is available as a pdf here: http://www.creativetexturetools.com/resources.html