Remove any lacquer or varnish and clean the piece. First wipe the item with acetone; then dip the item into boiling water and let it boil for a few seconds. When the piece is cool enough to handle, clean it by wiping it all over with a clean dry cloth. Acetone is widely available as a nail polish remover.
Soak your item for 1 hour in a mixture of vinegar and salt. Use 1 tbsp. of salt for each cup of vinegar.
Bake the item in a 450 F oven for 20 minutes.
Soak the hot item in the vinegar solution until you are pleased with the color.
Shake off the excess vinegar and allow the brass to dry.