Jib -
Household ammonia.
To control ye project use a clear plastic container with screw cap like a large mayo or pb jar.
Place ye parts inside along with an open, upright, pill vial filled 3/4 with ammonia, cap the jar, set it aside and watch the brass age.
Don't spill the ammonia on the parts let the vapor do the work.
Depending on brass quality it may take over night to couple o days.
I've found this process best to age from dull to cruddy. You control the look you want.
Oh, and as Michael said above be careful as ye be dealing with chemicals and processes.
Jas. Hook