Metal finishing Q&As since 1989

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In house tool blackening

Due to the location of my shop, and the small amount of tooling that requires blackening, it's not very cost effective to outsource my tool blackening. Is there an economical way of doing tool blackening in-house?

Ron Jones
precision machining - Shelby Twp., Michigan, USA
2004

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2004

Hot black oxide can certainly be done in house. It's not terribly complicated. Clean, acid dip, black oxide, and oil, with rinses in between. The issues are the implications, if any, the black oxide line has on wastewater discharge permits, the probable need for exhaust ventilation equipment, and the question of operating the line safely. Black oxide is an aqueous process that runs at about 285 degrees F, the boiling point being high because of the high concentration of dissolved salts. When water is introduced to make good evaporation losses, or the solution becomes stratified during a shutdown, the potential presence of relatively pure water (which flashes to steam at 212 degrees) encountering a 285 degree solution, can have explosive consequences. Proprietary cold blackening is similar but a bit safer in not involving a hot black oxide tank.

In general, though, it makes sense to move to in-house processing instead of outsourcing when the volume is very large, not when it is small.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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Cold black for iron and steel:

20 gm tannic acid [on eBay or Amazon]
15 gm phosphoric acid
1 lit water

Immerse object in solution and leave it to dry well. Only for grease free objects! You must lacquer or wax it!

Goran Budija
- Cerovski vrh Croatia
2004