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topic 1150p3

The black oxide process and gun bluing

A discussion started in 1998 but continuing through 2020

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April 25, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We regularly do blackodising on Mild Steel and High Tensile Steel Nuts and bolt. We use a ratio of mixing chemicals to prepare bath as follows:
(1) Caustic Soda 80%
(2) Sodium Nitrite 10% &
(3) Sodium Nitrate 10%.

WE ARE NOT ABLE TO ACHIEVE A CONSISTENT RESULT. Sometimes the fasteners are getting red rust like colour.
A) Is there a online chemical test where we can know the balance of the bath?
B)Can anyone specifically tell me what are the functions of all the above chemicals?
C) why are the nut bolts getting red in colour?

Any serious reply will be of great help to us.


Chintan Mehta
- Bhavnagar, Gujarat, India

May 2014

A. Hi Chintin. As you see, we appended your inquiry to a thread where experts like Rod Henrickson have explained the process in excellent detail. I urge you to carefully study his comments as my guess is that operational details rather than the precise ratio of chemicals is the origin of your inconsistencies. Good luck.


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

Black oxide boils at 260 ° no matter what we do

June 2, 2014

Q. Hi -

I was reading this thread, it's pretty interesting. We're having an issue with one of our blackening tanks: it won't get to 305 degrees, no matter how we adjust the solution; it's boiling around 260, tops. Does the water get deionized over time or something? There are also a few inches of sludge/slurry at the bottom that I'm going to have cleaned today but obviously want to know what the issue is. It's gas-fired and I don't think lack of BTU's is the issue.

Peter Brown
- Middletown Connecticut USA

simultaneous June 3, 2014

A. Peter,
Maybe you already tried these ideas: 1. Removing the sludge may fix the problem. 2. Check heat exchanger and burner for leaks. Also check flume if appropriate. Gas absorbing into the solution can reduce b.p. 3. I assume you already checked your temperature measuring device and location so you might try adding a little urea or similar additive to see if it helps but that is a more risky option.

blake kneedler
Blake Kneedler
Feather Hollow Eng. - Stockton, California

June 3, 2014

Boiling point is directly related to the concentration of dissolved chemicals.

If you cannot raise the boiling temperature above 260, then:

- Your temperature measurement is inaccurate, or
- You do not have enough heat input, possibly because of a layer of sludge or undersize burner(s), or
- Your temperature/water feed controller is inaccurate, or
- Your chemical concentrations are low, or
- You are using an improper mixture of home brew.

Since the first sentence above is accepted fact, check the above suggested 5 possible faults.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina

What blackening process do OEMs use

December 16, 2015

Q. Being heavily involved in vintage auto restoration, can anyone tell me the specific type blackening used by OEM suppliers for automotive bolts and parts? Thanks.

orrin tucker

December 2015

A. Hi Orrin. The hot black oxide process first described by Ken Lemke and Rod Henrickson is surely what OEMs specify for general "blackened" parts. But there are probably occasions where more corrosion resistance is required, so zinc plating with black chromate is specified.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

February 27, 2017

Q. My parts are not full blackened. There are spots on the surface. Guide me please.

Osama Farooq
- rawalpindi Pakistan

March 2017

? Hi Hi Osama,
Please post what kind of parts you are blackening, what they are made of, what your sequence of operation is, and the parameters of your black oxide tank (temperature, boiling?, time, concentrations, etc.) People can't tell you what you might be doing wrong until you tell them what you are doing. Thanks!


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

March 4, 2017

Q. Process sequence is as follows:
1. degreasing
2. pickling
3. blackening
4. chromic acid dipping
5. oiling

I am using 650 g/L of sodium hydroxide and 400 g/l of sodium nitrite. My solution starts to boil at about 140 °C. I have heard that black oxidation solution starts rising when it boils, but in my case only bubbles are formed on the surface of the solution.

The time I am giving to the parts is about 1 hour. I have tried doing it on quite a few types of steels but no success.

Osama Farooq [returning]
- rawalpindi Pakistan

March 2017

A. Hi again. All I know if what I read in the textbooks and forums ... but are you sure you're using sodium nitrite, not sodium nitrate? Rod Henrickson says he uses only sodium nitrate (2 pounds/ gallon = 240 g/l), and some textbooks suggest a mix of nitrite and nitrate, but I'm not aware of using sodium nitrite without sodium nitrate, as it sounds to me like you would not have sufficient oxidation power.

I don't understand what you mean when you say that you're supposed to see "black oxidation solution starts rising" but it's not happening -- but the solution must be boiling.

The process should only take a few minutes, not an hour.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

March 7, 2017

! I have got to the root of the problem actually I was giving way more time to the parts after pickling. It resulted in the oxidation of the surface of the parts hence they were not being properly black oxidised.
Sodium nitrite can be used as I am using it and it has also passed the oxalic acid spot test.

Osama Farooq
- rawalpindi, pakistan

March 2017

thumbs up sign Thanks for the feedback, Osama. Good to know that sodium nitrite works.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

October 30, 2017

A. Osama, delete the chromic acid it will destroy the black oxide finish immediately, If you must pickle the metal use the following in this order:
degrease with hot caustic solution
Pickle with acid (light sandblasting will produce the same effect and a lot safer) Pickle if you must, then rinse with Boiling water then into the bluing solution when the black oxide finish is a good deep color remove into a hot water rinse then into a hot soluble oil bath, be careful of soluble oil concentrations to avoid fire.
The old masters used a 300 grit finish and then blued the part, some applications call for use of a buffing wheel and abrasive wax; care and skill is needed with these devices they will hollow around screw holes.
Chromic acid will dissolve flesh and I think should be avoided at all costs.

Read Rod Henrickson's comments -- he is 100% correct in all that he has said. If You read and digest his comments there is little else to know in the short term.
You do not often get such detailed information offered freely!

Vaughn Gunthorpe
- Plainland Qld Australia

November 1, 2017

A. This is in reply to Vaughn's comment the other day:

If following the MIL-SPEC, a chromic acid dip will not destroy the black oxide finish. It calls for a final rinse that is 8 oz per 100 gallons of water. Even though the pH is quite low, the concentration is too weak to attack the finish. Minimum of 30 seconds at elevated temperatures will help corrosion resistance, not destroy it.

Jameson Grout
- Indian Orchard, Massachusetts

November 2017

thumbs up sign Thanks Jameson. According to MIL-DTL-13924 [link is to free spec at Defense Logistics Agency /] you are correct.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

June 18, 2017

Q. Sometimes we use glycerol to remove high iron content from the bath. Are there any alternatives?

Phahad hashim
- Dubai, UAE

July 25, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. My situation: I don't know how to prepare for blacking can you explain the blacking process and chemical ratio?

Manoj Kumar Vn
Shop employee - Bangalore, Karnataka, India

May 2020

A. Hi Manoj. We added your inquiry to a thread that answers your question. Feel free to add any followups or requests for clarification on anything you didn't follow. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

February 17, 2020

Q. Good day!
I read and followed the instructions I learned on this thread but I always have a problem with a certain gun barrel. They call it "military barrel". When I blue it, it turns red. I don't know what kind of steel they use so I don't know how to adjust my mixture. I use caustic soda and sodium nitrate. Temp is between 280 °F to 290 °F. I think I need to make it up until 300 °F to see if it will turn black.
Hope someone could help me or maybe give me advise. Thank you and more power.

Inocencio Abril Mayo III, Asst. Gunsmith
Gunshop - Al Araqi, Ad Dhahirah, Oman

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