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.
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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
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Thank you for reading.
And I look forward to receiving your answers.

-June-