You can do it chemically with almost any iron salt, but I use a permanent
black marker. Does a beautiful job if the brass is clean, and if you
overspray with a matte lacquer or varnish, can't be distinguished from the
chemical treatment. Lot easier and less messy, too.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net
Visit http://www.psa-photo.org/