Laurence,
There is excellent book on bronze patinas is by Patrick Kipper...called
Patinas for Silicone Bronze.
Here is a classic opaque brown (old penny)
Chemical: medium ferric nitrate solution plus a few crystals of sodium thiosulfate.
sandblast the work piece to clean and etch the surface. Heat with a torch untill an amber coloration appears...then dab on the ferric solution. Continue heating and dabbing until desired color is reached. While still warm, seal with wax. Buff.
I like to do a cold base coat with liver of sulfur first....then buff of the high spots. Then apply the hot ferric patina. This adds more depth and color. The dark grey/black under-coat gives the antique look, and buffing the high spots gives a lightly used/worn appearance. The result is darker colors in the crevices that blend to lighter colors on the high spots on the surface.
These chemicals are available at art suppliers for bronze sculptor. Here is one online source that I've used:
Johnson atelier
They are on the east coast, however there are a few other suppliers on the west coast that handle patina chemicals.
Good luck and have fun with it,
Rob