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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. more...

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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.

Usage

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.

Read more at Wikipedia.org