How to Blue Steel

Co-authored by Christopher Thomas

Last Updated: December 4, 2024 Fact Checked

Bluing steel is the process of oxidizing the surface of the metal to protect it from rust. There are several different techniques, and each one has its own particular uses. The most commonly used are cold bluing and hot bluing, but rust bluing was the standard for many years. Before any bluing process, the metal must be prepared.

Method 1

Method 1 of 4:

Preparing the Metal

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If there is any finish left on the metal, it will interfere with the bluing process. This will result in an uneven and ineffective bluing. Grind and sand the piece until you have removed any and all finishes. You should see only bare metal.^{[1]
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If there are any pits in the metal, this is the time to deal with them. Use your sander or grinder to take the metal down around the pit. This will smooth the metal and give you a better finish.^{[2]
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Gloves will not only protect your hands, but they will also protect the metal from your hands. Your hands will leave oils on the metal that interfere with the bluing process. Wear nitrile or latex gloves to keep this oil from reaching the metal.^{[3]
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Clean the metal with an alcohol wipe. You should then follow by cleaning it with a wax and grease remover (an organic solvent). Finally, use a soft cloth to dry the metal.^{[4]
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Method 2

Method 2 of 4:

Cold Bluing

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You can buy bluing solutions premixed. Dip a cotton ball into the solution and apply it to the part evenly. If you are bluing a larger piece, use a rag or brush to apply the solution.^{[5]
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Once you have covered the piece with bluing solution, wipe it dry. Use a soft cloth, and make sure not to leave any solution in cracks or crevices. This will help ensure an even finish.^{[6]
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Use a fine grit sandpaper (for example 800 grit) to sand the finish. Sand slowly and carefully so that you do not go through the finish. This will smooth out your finish and make it shine.^{[7]
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Continue applying and polishing coats. When you have reached the desired color, you can polish one final time and then stop. Protect your finish by covering it in a protective oil (e.g. gun oil).^{[8]
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Method 3

Method 3 of 4:

Hot Bluing

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Submerge the piece in cold water and scrub with steel wool or a scuff pad. This will create small scratches for the bluing solution to adhere to. This should be done after any finish has been removed and the metal has been cleaned.
A bluing solution is made of potassium nitrite and sodium hydroxide dissolved into water. The solution is heated to approximately 275 °F (135 °C). You should leave the part immersed for 15-30 minutes.
When you remove the piece from the mixture, put it immediately into cold water. This will cause the metal to contract and lock in the finish. Do not scrub the metal again.
Boil plain water (with no salts). Dip the metal piece into the boiling water. This is the last step in the bluing process.
Removing the water from the metal is important. Put some water displacing oil onto a soft cloth and rub it into the metal. It will also leave a protective layer over the metal.

Method 4

Method 4 of 4:

Bluing Other Ways

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Rust bluing is the process of coating the metal in acid. Most commonly, nitric acid or hydrochloric acid is used. This allows the part to rust uniformly. Then, you scrub the rust off and repeat the process until the desired color is achieved. You can do this in a large container or vat that is tolerant to acid.
Fume bluing is similar to rust bluing, but it creates a more uniform rusting. The part is placed in an airtight chamber and either nitric acid or hydrochloric acid (gas) is pumped into the chamber. The gas causes a uniform bluing. Any airtight container will work, as long as it cannot be corroded by the acid.
Niter bluing is the hottest form of bluing. In this case, you take a mixture of potassium nitrate salt and sodium hydroxide salt and melt them (700 °F (371 °C). The part will achieve a nice bluing when dipped into the solution for.
There are many types of bluing professionals. If you are not up to trying it yourself, you can reach out to a bluer to learn their specialty. Ask what kinds of bluing they do and what pieces they are willing to treat (e.g. large or small).