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 the rust was karded (scrubbed) off, leaving a deep blue finish. The 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 sued 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. Rust bluing is also used on shotgun barrels that are soldered to the rib between the barrels, as hot bluing solutions would dissolve the solder during the bluing process.
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 provide 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 a cold bluing solution.
Bluing is most commonly used by gun manufacturers, gunsmiths and gun owners to improve the cosmetic appearance of, and provide a measure of corrosion resistance to, their firearms. Bluing also helps to maintain the metal finish by resisting tangential scratching, and also helps to reduce glare to the eyes of the shooter when looking down the barrel of the gun. 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 2.5 micrometers (0.0001 inches). For this reason, it is considered not to add any appreciable thickness to precisely-machined gun parts.
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. Many 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.
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