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topic 3746

Comparison Of Cold Blackening method Compared to Hot Blacking


Q. I would like to know about:

1. Room Temperature Blacking Method of Iron and Steel.
2. What are the Advantages and Dis advantages compared to Hot Blackening/Oxidization process.
3. Which web sites I can visit to know more about the process and the various manufacturers.

Hemant Manek
- Calcutta, India

Digital version

(No longer published, but Elsevier hasn't yet de-commissioned the online version of the Guidebook)
Download it before it disappears.

Metal Finishing Guidebook


A. The principal advantage of cold blackening is that it is far safer to operate than the hot process, which involves a boiling hot highly concentrated aqueous solution of caustic soda which, if not handled properly, can erupt and burn or kill people. The operating temperature of hot black oxide tanks is much higher than the boiling point of water, such that when water is added to make good the evaporation losses, it has the potential of instantly flashing to steam and blowing the contents of the tank all over the place.

Cold blacking chemicals are more expensive, do not have equivalent corrosion resistance or the shiny uniform appearance of hot black oxide, and can rub off as an abrasive grit which can be a problem in certain manufacturing processes.

We have a FAQ on Black Oxide & Cold Blackening which may be helpful, and if you can find a copy of the Metal Finishing Guidebook, it will have a chapter on the subject. Good luck.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

February 4, 2008

Q. If properly rinsed after the black oxide bath, should a hot-process black oxide leave smut residue when handled?

Parts I am receiving from a supplier leave a black, greasy substance, and since the parts will be handled by a customer, I'd like to minimize this. It seems to be after finished in oil. Rinsing in Isopropyl Alcohol [linked by editor to product info at Amazon] removes most of the smut but also some of the finish.


James Heisler
- Austin, Texas

February 23, 2009

A. Hi, Jimmy. You can have a different oil applied that is less smutty, or a wax instead. But a black oxide finish derives almost all of its limited corrosion resistance from the oil or wax. If you remove it, it will flash rust.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

Black oxide counter top

February 22, 2016

Q. I am investigating blackened steel kitchen bench top. Concerns from reading above would be reliance on a seal for corrosion prevention. Given only kitchen bench would a non sealed work. I got the impression blackening was done in first instance to stop corrosion.
Thanks in advance.

Darin Millar
- Christchurch, New Zealand

Butcher's Wax

February 2016

A. Hi Darin. Try a small and simple one first and let us know how it works out because I have no experience with it. A "dry" hot black oxide would probably be highly subject to rusting, but if you keep it waxed with butcher's wax or oiled with mineral oil, my guess is that it will be okay. Remember that petroleum oils (which would be the most commonly applied oils) are poisonous!


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Cold blackening toxicity?

January 28, 2017

Q. I'm looking at black oxide treatments for shop-made tooling--clamps, step blocks, boring bar adapters, that sort of thing. Typical materials are 1018 and 12L14 cold rolled steel, maybe some O-1, D-2, or S-7 tool steels on occasion.

The actual black oxide processes, even the ones at lower temperatures (220 vs 290) are wickedly dangerous due to the caustic sodium hydroxide, flash-over, etc.

Hence I'm looking at the cold black options. If I understand it correctly, these put a copper/selenium coating on the steel My concern with them is the selenium content. Selenium and its compounds are very toxic, with an LD50 on the order of 1-3 mg/kg body weight.

How safe is it to handle parts with a cold black treatment on a regular basis?

Paul Ste. Marie
hobbyist - Renton, Washington, US
  ^- Privately contact this inquirer -^

January 2017

A. Hi Paul. These are proprietary processes, so I think the suppliers' material safety data sheet may offer good info.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

February 7, 2017

Q. "Hi Paul. These are proprietary processes, so I think the suppliers' material safety data sheet may offer good info."

Unfortunately, they discuss the toxicity of the ingredients (selenous acid in particular--"may be fatal in small amounts"), but not the safety of the finished item.

Paul J. Ste. Marie [returning]
- Renton, Washington USA

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