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I have seen formulas for blackening brass. The problem will be that when
we speak of brass, there is no assurance that it will indeed be the same
brass as the brass that works with the formula.
These are adapted from Wall and Jordan, _Photographic Facts and
Formulas_*:
1)
Soln. A: copper nitrate................208g.
Water.................................500cc
Soln. B: silver nitrate.....................208g.
Water..................................500cc.
It appears they want us to use equal parts of the two solutions.
Clean the metal thoroughly (this means degrease, for which lye works very
well), immerse in the mixture, and then heat; repeat if needed.
They give another but it needs lacquering so it wouldn't work well for
us. They also suggest boiling the metal in a solution of pot. sulphide (
...ide, NOT sulphite!) until black enough, rinse and dry, then smear oil
on it. As my late old Grandma would have said, "Pee-Ewe!" Think about
boiling some sepia toner. Sounds great, doesn't it? But they say it
works better than the silver formula above.
As I recall, this is why I've never pursued these brass formulas. They
all seem to require lacquer, or oil, or something. Having fooled around
with stuff like this quite a bit, the silver appeared to me to be the
best of all possible worlds.
*If you aren't familiar with this wonderful book, it was published in
several editions (the last, re-edited by John S. Carroll in 1975) since I
think about 1921, by Amphoto. It is the most indispensible book in my
library. So much so, that even though it is somewhat hard to find, I've
got some 4 copies, just in case one is stolen -- no idle fear, it has
happened.