Looking for patina formula

Hi Michael: I do a lot of married metal with copper or bronze or brass
married to sterling silver. ( I have some images on my website:
Goss Design Studio Jewelry - Site Moved - if you’d like to see the effect). I
patinate the pieces with cupric nitrate and heat it until it turns
black. Cupric nitrate bonds to copper and its alloys but not to
sterling (or fine silver I’d guess) but it appears to cover it. I
then gently rub the piece with superfine steel wool under water to
remove the black patina from the sterling. It means you have a
brushed matte finish on the silver, but the black remains on the
copper, brass or bronze. You have to build up a good thick black
patina for this to work well. Usually at least 3 - 5 coats applied
and heated.

Perhaps this would work for you.
Sandra