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

1. 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-