This question about how to achieve a black patina seems to come up
frequently. Properly preparing the metal to receive the patina has a
critical influence on the outcome. I have found that borrowing at
least one technique from the Japanese masters of metal arts is one
the keys to success.
By using a horse hair brush specifically designed for the use with
an abrasive powder slurry of silicon carbide the metal will accept a
patina readily. There can be a little or a lot to this technique
depending on how refined you want the results, but in all cases the
outcome will be superior than not preparing the metal.
I suggest looking into the techniques of these masters and
determining if the preparation of the metals before applying the
patina could solve your problems with getting a durable black.
Patrick Hastings has a web site taganearts where he offers a kit
that will enable you to achieve good results preparing the metal. Go
to: Tools & Supplies for Japanese Metal Work and then select
Finishing/Patina to see the Polishing Powders (kit). I think the
price is very reasonable.
Jim Kelso offers some tips for using the brush and abrasive powder
on his site under Patination Tutorial. Look at the under
Polish. There you will find the process about use of the brush and
Avoid breathing the dry powdered silicon carbide dust. not good
stuff. As long as it is wet there should be no problem since it will
not go airborne.
All the best,