Bluing (steel)

Bluing (steel)

Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron, which occupies the same volume as normal iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action.

In contrast, rust, the red oxide of iron (Fe2O3), does not occupy the same volume as iron, thereby causing the typical reddish rusting away of iron. Both "cold" and "hot" oxidizing processes are called bluing, but only the "hot" process provides any significant rust and corrosion resistance, and then only when also treated with an oiled coating.


Bluing is most commonly used by gun manufacturers, gunsmiths and gun owners to improve the cosmetic appearance and provide limited resistance against rust of the firearm. All blued parts still need to be properly oiled to prevent rust. Bluing, being a chemical conversion coating, is not as robust against wear and corrosion resistance as plated coatings, and is typically no thicker than 0.0001 inches (2.5 micrometres). For this reason, it is considered not to add any appreciable thickness to precisely-machined gun parts.

New guns are typically available in blued finish options offered as the least-expensive finish, and this finish is also the least effective at providing rust resistance, relative to other finishes such as Parkerizing or hard chrome plating.

Bluing is also used for providing coloring for steel parts of fine clocks and other fine metalwork.

Bluing is often a hobbyist endeavor, and there are many methods of bluing, and continuing debates about the relative efficacy of each method.

Historically, razor blades were often blued steel. A non-linear resistance property of the blued steel of razor blades, foreshadowing the same property that would later be discovered in semi-conductor diode junctions, along with the ready availability of blued steel razor blades, led to the use of razor blades as a detector in the crystal set AM radios which were often built by soldiers during World War II.

Hot vs. cold bluing

Bluing may be applied, for example, by immersing the steel parts of the gun to be blued in a solution of potassium nitrate, sodium hydroxide, and water heated to the boiling point. Similarly, stainless steel parts of the gun to be blued are immersed in a mixture of nitrates and chromates, similarly heated. Either of these two methods is called "hot bluing". There are many other methods of hot bluing. Hot bluing is among the most effective forms of bluing, providing the most permanent degree of rust-resistance and cosmetic protection of exposed gun metal.

"Rust bluing" was developed between hot and cold bluing processes. It was originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes. The process was to coat the gun parts in an acid solution, let the parts rust uniformly, then immerse the parts in boiling water to stabilize the rusting process by removing any remaining residue from the applied acid solution. Then the rust was karded (scrubbed) off, leaving a deep blue finish. This process was later abandoned by major firearm manufacturers as it often took parts days to finish completely, and was very labor intensive. It is still sometimes used by gunsmiths to obtain an authentic finish for a period gun of the time that rust bluing was in vogue, analogous to the use of "browning" on earlier representative firearm replicas.

There are also methods of "cold bluing", which do not require heated solutions. Commercial products are widely sold in small bottles for cold bluing firearms, and these products are primarily used by individual gun owners for implementing small touch-ups to a gun's finish, to prevent a small scratch from becoming a major source of rust on a gun over time. At least one of the cold bluing solutions contains selenium dioxide, to accomplish the bluing. Cold bluing is not particularly resistant to holster wear, nor does it provide a large degree of rust resistance. It does, however, often provide a very good cosmetic touch-up of a gun's finish when applied and additionally oiled on a regular basis.

Large scale industrial hot bluing is often performed using a bluing furnace. This is an alternative method for creating the black oxide coating. In place of using a hot bath (although at a lower temperature) chemically-induced method, it is possible through controlling the temperature to heat steel precisely such as to cause the formation of black oxide selectively over the red oxide. It, too, must be oiled to provide any significant rust resistance.


Bluing only works on steel or stainless steel parts for protecting against corrosion. Because it changes the Fe into Fe3O4, it does not work on non-ferrous material. Aluminum and polymer parts are largely unaffected by bluing; no protection against corrosion is provided by bluing processes on them, although uneven staining of the aluminum and polymer parts can be caused by attempts at bluing.

Holster wear will remove hot bluing over long periods of use; it will remove cold bluing over relatively short periods of use, from any wear areas that are "touched up" with cold bluing solutions.


Some prefer to call thin coatings of black oxide by the name "gun bluing", and to call heavier coatings by the name "black oxide", but they are both the same chemical conversion process for providing true gun bluing.

Browning is controlled red rust Fe2O3 and is also known as "pluming" or "plum brown". One can generally use the same solution to brown as to blue. The difference is immersion in boiling water for bluing. The rust then turns to black-blue Fe3O4. (Be careful when browning or bluing using any of the older methods; many of these older browning and bluing formulae are based on corrosive solutions (necessary to cause metal to rust), and often contain cyanide solutions that are especially toxic to humans.)

ee also



*Harvard reference | Surname=Budinski | Given=Kenneth G. | Title=Surface Engineering for Wear Resistance | Publisher=Prentice Hall | Place=Englewood Cliffs, New Jersey | Year=1988 | p. 48
*Harvard reference | Surname=Brimi | Given=Marjorie A. | Title=Electrofinishing | Publisher=American Elsevier Publishing Company, Inc. | Place=New York, New York | Year=1965 | pp. 62–63 .
*Citation | title = Coating, Oxide, Black, for Ferrous Metals (MIL-DTL-13924D) | publisher = Department of Defense | date = 1999-03-18 | url =
*Citation | title = Phosphate and black oxide coating of ferrous metals (MIL-HDBK-205A) | publisher = Department of Defense | date = 1985-07-15 | url =

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