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Need to blacken brass, free from outgassing, for optical use

I wish to coat the inside of a 3 mm (1/8") brass tube in which a series of optical lenses will be inserted. The black should be as non-reflective as possible. I have read many letters and postings here, made phone calls and the biggest problems seem to be out-gassing, adhesion, and thickness. The chemicals used seem to continue to give of a gas even after the process is "complete" thus the optical lenses become coated or a protective coating must be added which inside a small tube would be difficult. In some cases the finish will rub off as a fine dust. It does not matter if the outside of the tube becomes black, what is most important is the inside. I would really appreciate a solution to this problem. Thanks for your time.

Ed Cannady
hobbyist - Lomita, California, U.S.A.
May 21, 2009

Try old copper nitrate ⇦this on eBay or Amazon [affil links] process; you can find exact description of it in download free old book on metal colouring by A.Hiorns,at internet archive website(www.archive.org).Hope it helps and good luck!G

Goran Budija
- Cerovski vrh Croatia
May 23, 2009