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Black oxide on brass problem

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I'm trying to put a very dark black oxide finish on a 4" dia. brass disc. as part of an antique radio restoration project.

I tried a copper sulfate & sodium thiosulfate. It's not dark enough, nor is it very fast. The process leaves behind a yellow "smut".

1)Should I heat the bath?
2)Should the bath be agitated?
3)Is the "smut" typical?
4)How long should the item be in the bath?
4)Any better approaches or should I send the item out to a professional shop?

Brian Justin
hobbyist - Forest, Virginia

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Heating the object stimulates the molecular action thus speeding the patina process. We sell a patina which will work simply and quickly to acheive this look, Regards

Barry Feinman Restore It Yourself, Inc Carlsbad, California

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First of two simultaneous responses -- +++++++

My goal is not to add patina to make the knob look old, but rather to repeat the same process that would have been used in industry back in 1921 to create the very dark black oxide finish on the brass item when it was brand new. I have already stripped what was left of the original oxide with acid and
am down to the original raw brass.

Does your patina solution form a very black oxide layer?

Thanks!
-Brian

Brian Justin
- Forest, Virginia

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Second of two simultaneous responses -- +++++++

If you like to do it yourself try 200 gm copper carbonate/ 1 lit ammonia solution.Bad smelling but very effective-be careful ammonia fumes are very unpleasant ,use rubber protective gloves [linked by editor to product info at Amazon] and safety googles!Good luck!

Goran Budija
- Zagreb, Croatia