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Black spots on chemically blackened brass
Q. We have a workshop building several batches of antique brass tubes(with lacquer after chemical black finish) every year. Recently, there have had some complaints from customers -- when we open the plastic bag, we found the black spots (dots, not film, not tarnish) were budding on the tube surface (not sure from the brass layer, or, from the lacquer itself). Is it caused by the lacquer? or by the non-completed water rinsing right after blacking reaction?
John Xu- Minneapolis
November 14, 2024
A. Hi John. You didn't mention plating, so I assume that these tubes are solid brass, not brass plated steel? -- cyanide spot-out could be a problem if there is brass plating.
Readers might be better able to offer promising theories if you send pics to mooney@finishing.com for posting here so we can actually see what you're trying to describe. From a verbal description it's quite difficult to even guess whether the problem is related to porosity or grain structure in the brass, a defective blackening solution, or something in the lacquer attacking the blackening.
Fortunately, Minneapolis is very close to several of the best known suppliers of brass blackening solutions, so local help should be readily available :-)
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Ted is available for instant help
or longer-term assistance.
adv.
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