I dont know how it works with the "synthetic" patina .
if you have patinated an barrel , will it still develope the real patina by age ??
or have you "sealed" it with the chemicals ??
it would be nice to know as I probably not will live long enough to find out the result
but most of all antique cannons have been more or less polished after they was cast .
for an average polished bronze casting it will take 30 - 40 years to develope an deep brown inside patina .
the deep green color you can have from keeping the piece outside close to the sea in 3 - 5 years .
here in the inland where I lives with approximately 200 miles to the closest salt water the bronze are more likely to turn black and it will take much longer time . so one possibility for me is to use battery acid
that will turn the bronze green .