Liver of Sulphur is used to create a patina finish on finished pmc pieces.  Just add a small chip of sulfur (about 1/4 inch) to a hot cup of water.  You can either dip the whole piece into the liquid of paint it on where you'd like.   Once the piece has changed to the color you prefer, rinse the piece in cold water.  You can then polish the piece with your polishing cloth or brush to highlight certain areas on your piece.