Blackening of Zinc, Copper, and Brass strip

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I have a customer requesting blackening of Zinc, Copper, and Brass strips. He had this process done at another plater, but apparently they went out of business. It was done continuously, and the samples did not look like plating to me, they looked like some sort of immersion deposit that wat wiped or brushed in continuous process after deposit. I found a solution to immersion deposit on the Zinc, although it is kind of slow, however the same solution does not transfer to the copper or brass substrates without electrolysis, so does anyone have an idea that does not involve the use of chromates or cyanides?

Michael Berrier
plating - addison, IL, USA