But I'm a chemist, not a craftsman. I forwarded your question to The Designary, a jeweler who specializes in patinated bronze (among other things). Her response is given below.
You can go the commercial route, which produces some very beautiful effects quickly or you can go the slower, home made route and let the piece age with time. You can also apply a "stopper" which, when reaching the desired color, will stop it at that point.
To go the commercial route, which I have used quite successfully, try www.chemtek.com. There is extensive material on their site, ordering info, on and offline, and an 800 number to call. I have had wonderfully helpful conversations with the owner, who incidentally usually answers the phone pre-working hours.
Another supplier of patinas is Rio Grande, Albuquerque, NM, who I have used for many years. They also have a site, www.riogrande.com, tho specific info regarding rusting and patinas is not available there. You must call them and ask for a catalog. They are a jewelry supply house and do not specialize in patinas, but the material is easily bought there.
Also, try www.artmetal.com. This site is devoted to metal and the trials and tribulations of all artists who are smart enough to know they need a community to share and repair. THey have a great mailing list and all the messages are saved and posted on the site and a search can be done for past articles on "rusting".
The homemade recipes can be obtained in various publications. One that I highly recommend is: Jeweler's Resource, by Bruce Knuth. It is a hard to find publication but can be ordered by calling 509-624-8565. Another book is The Colouring, Bronzing, & Patination of Metals by Richard Hughes and Michael Rowe.
Liver of Sulphur is a compound which you may want to order. One can use it on the various metals. (Liver of sulfur is a colloquial name for potassium sulfide. -Ed.)
Listed below are sample recipes:
Author: The Designary firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
Comments & questions to email@example.com
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/redox/faq/print-metal-patina.shtml