Perfect Black Coating method for Brass Zipper??

Hello? I am from South Korea.
----This is a question about Zipper coating.---------------------------------------------
A small zipper company where I've worked, is looking for a solution for effectly coating a brass zipper into black without any yellow remains. ----------------------------------------------------------------------------------------------------------
They burn Brass(=Copper + Zinc) zipper to make its color black. (They are producing black zippers)
---- The Problem is:-------------------------------------------------------------------------------
Many times the zipper is not entirely burnt
so the yellow surface of original brass appears, which should never happen to a black zipper.
-------The Question is:-----------------------------------------------------------------------------
1. A Knowhow to effectively & PERFECTLY coat the brass zipper black without any yellow remains
2. or any alloys with similar COST and STRENGTH but with BLACK color
------------------------------------------------------------------------------------------------------------
Thank you for reading. And I look forward to receiving your answers.
-June-
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sodium chlorite in concentrated sodium hydroxide solution at near boiling temperature. it took 30 seconds or so.
FWIW, i used a commercial Enthone product that comes premade as a powder. sorry. i can't recall the item number.
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