I use first a Birchwood Casey product called "Brass Black". (It's sold in gun shops. They use it for brass gun sights.) Anyway, I dilute it heavily with distilled water to about 20 to 1 and brush on a number of coats cold.
On top of that I use a solution of cupric nitrate and distilled water, brushed on hot. I heat the bronze with a torch until the solution sizzles a little when I apply it. Then I apply wax with a Scotchbrite pad. That abrades away a little of the regularity. I use transparent shoe wax.
Here with another piece, on his face I used a more dilute solution of cupric nitrate and stopped when the green was just beginning to show up.
When I applied the wax it turned a very dark bronze green. On the helmet I used only the "Brass Black".
Richard