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-----:Cold Bluing / "Room Temperature Blackening" Problems
Quickstart:
The 'black oxide' process is usually called 'bluing' when applied to firearms. Hot bluing/black oxiding is done in a tank of concentrated caustic soda plus oxidizing agents like nitrates and nitrites, operating at a slow boil. Depending on the concentration of salts in the solution, that boiling temperature will be between 265 °F and 300 °F.
A special, severe danger with this process, which has maimed and killed people, is 'eruption': water must be frequently added to make up for evaporation losses, but water flashes to steam at 212 °F, so there is the possibility of a slug of water encountering this much hotter solution and, instead of mixing with it, flashing to steam and blowing the contents of the tank onto an operator.
Cold bluing / room temperature blackening is a safer copper selenium coating process which simulates real hot bluing although it is not as consistent or corrosion resistant; although it's best suited to touch up, it has also been used industrially in some instances.
Q. I tried applying a black oxide (Cu-Selenium) at room temperature to a steel surface. However, the black layers started to peel off shortly after the black oxide process was completed. Why did this happen? Did I make a mistake in the treatment process? The steps I followed were: cleaning, rinsing with water, followed by activation in 5% HCl, rinsing with water, and then the black oxide process. However, afterward, the black layers appeared to fade or peel off.
Thanks for give some advice/solutions in this thread/topic.
Employee - Jakarta
April 12, 2026
A. Hi Rans,
We don't yet know whether your pretreatment process is inadequate, or your black oxide process itself is deficient. So, I think I would start by trying to black oxide a steel Hull Cell panel because your pretreatment process should be adequate for it.
If you can't successfully black oxide the test panel, your problem is probably an improper black oxide formulation. If you can black oxide the test panel, your pretreatment process is probably not adequate for the parts you are trying to process.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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⇩ Similar, related, Q&As -- oldest first ⇩
Q. I have recently introduced Room temperature blacking chemicals for Blacking of Iron & steel and find that after the last process of dipping in de-watering oil, the blacking is coming off if rubbed. I would be grateful to learn how this problem can be solved or where I could be going wrong.
Hemant Manek- Calcutta, India
1999
A. You don't say how aggressive the rubbing is, but I am assuming a relatively firm manual rub. You also don't say what oil is being used. That being said, you may be having an activation problem. Be sure to activate the parts in an acidic activator to ensure complete coating formation and adhesion. Remember though, even the best room temp black oxide can be rubbed off with a pencil eraser - these are decorative coatings, not necessarily protective or durable.
Dan Brewerchemical process supplier - Gurnee, Illinois
Q. Dear Sir, I want to know about steel blacking chemicals. If you have any information on this, please send me some. Corrosion, price, break away, etc. Your prompt reply will be highly appreciated. Best regards,
James Kim- Seoul, South Korea
2000
A. Hi, James. Please see our Black Oxide & Cold Blackening FAQ for a quick intro.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
Q. I see a company advertising in different machinist publications for their black oxide process. They say they use their product at about 200°F. Is this considered a cold process? I also have read in several posts that it's the "coating" or oil treatment that causes black oxide products to pass the salt spray tests. So if this black oxide process produces a uniform black color and is covered with a "good" oil would this be considered a good "black oxide" part? Has anyone had any experience with this product?
Gary Collinsshop employee - Alpena, Michigan
2007
A. Technically black oxide is just that, black iron oxide. It is obtained by the hot (290 °F) process and not by other means. The lower temperature processes produce a black coating, but is based on oxides of other metals such as selenium or copper. In either case the coating is very thin, and it's protective value is primarily from the wax, oil or other organic topcoat.
Can you substitute the low temperature processes? Depends. If it's for your own use, you may find it acceptable. If for a customer, you should get their approval. The low temp processes are sometimes not quite black - "there are different shades of black", as a salesman for a low temp process once told me. In addition, some of them will rub-off on hands.
Jeffrey Holmes, CEF
Spartanburg, South Carolina
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