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. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. A distinction can be made between traditional bluing and some other more modern black oxide coatings, although bluing is a subset of black oxide coatings.
In comparison, rust, the red oxide of iron (Fe2O3), undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off causing the typical reddish rusting away of iron. "Cold", "Hot", "Rust Blue" and "Fume Blue" are oxidizing processes simply referred to as bluing.
"Cold" bluing is generally a selenium dioxide based compound that colours steel black, or more often a very dark grey. It is a difficult product to apply evenly, offers minimal protection and is generally best used for small fast repair jobs and touch-ups.
The "Hot" process is an alkali salt solution, referred to as "Traditional Caustic Black", that is typically done at an elevated temperature, 135 to 155 °C (275 to 311 °F). This method was adopted by larger firearm companies for large scale, more economical bluing. It does provide good rust resistance which is improved with the use of oil.
"Rust Bluing" and "Fume Bluing" provide the best rust and corrosion resistance as the process continually converts any metal that is capable of rusting into magnetite (Fe3O4). Treating with an oiled coating enhances the protection offered by the bluing. This process is also the only process safely used to re-blue vintage shotguns. Many double barrelled shotguns are soft soldered (Lead) / silver brazed together and many of the parts are attached by that method also. The higher temperatures of the other processes as well as their caustic nature will weaken the soldered joints and make the gun hazardous to use.
Bluing can also be done in a furnace, for example for a sword or other item traditionally made by a blacksmith or specialist such as a weaponsmith. Blacksmith products to this day may occasionally be found made from blued steel by traditional craftsmen in cultures and segments of society who use that technology either by necessity or choice.
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. It was also used by machinists, who protected and beautified tools made for their own use. Bluing also helps to maintain the metal finish by resisting superficial 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 micrometres (0.0001 inches). 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 or nitriding processes like Tenifer.
Bluing is also used for providing coloring for steel parts of fine clocks and other fine metalwork. This is often achieved without chemicals by simply heating the steel until a blue oxide film appears. The blue appearance of the oxide film is also used as an indication of temperature when tempering carbon steel after hardening, indicating a state of temper suitable for springs.
Bluing is also used in seasoning cast-iron cookwear, to render it relatively rust-proof and non-stick. In this case cooking oil, rather than gun oil, acts to displace water and prevent rust.
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 semiconductor 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.
Bluing may be applied, for example, by immersing the steel parts to be blued in a solution of potassium nitrate, sodium hydroxide, and water heated to the boiling point, 275 °F to 310 °F (135 °C to 154 °C) depending on the recipe. Similarly, stainless steel parts may be immersed in a mixture of nitrates and chromates, similarly heated. Either of these two methods are called hot bluing. There are many other methods of hot bluing. Hot bluing is the current standard in gun bluing, as both it and rust bluing provide 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 (often nitric acid and hydrochloric acid diluted in water). Then the rust was carded (scrubbed) off, using a carding brush or wheel. A carding brush is a wire brush with very soft, thin (usually about .002 thick) wires. This process is repeated until the desired depth of color is achieved or the metal simply will not color any further. This is one of the reasons rust and fume bluing tend to be more rust resistant than any other method. The parts are then oiled and allowed to stand overnight. This process leaves a deep blue/black finish.
Fume bluing is another process similar to rust bluing. Instead of applying the acid solution directly to the metal parts, the parts are placed in a sealed cabinet with a moisture source, a container of nitric acid and a container of hydrochloric acid. The cabinet is then sealed. The mixed fumes of the acids will produce a uniform rust on the surface of the parts (inside and out) in about 12 hours. The parts are then boiled in distilled water, blown dry, then carded, as with rust bluing.
These processes were later abandoned by major firearm manufacturers as it often took parts days to finish completely, and was very labor-intensive. They are 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. 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.
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.
There are also methods of cold bluing, which do not require heat. 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 an adequate cosmetic touch-up of a gun's finish when applied and additionally oiled on a regular basis, however, rust bluing small areas will often match and blend better and wear better than any cold bluing process.
Parts to be niter blued are steel which has been polished and cleaned, then immersed in a bath of molten salts; typically potassium nitrate and sodium nitrate (sometimes with 9.4 grams (0.33 oz) of manganese dioxide per lb of total nitrate). The mixture is heated to 310 to 321 °C (590 to 610 °F) and the parts are suspended in this solution with wire. The parts must be observed constantly for colour change. The cross section and size of parts will affect the outcome of the finish and time it takes to achieve. This method must not be used on critically heat-treated parts such as receivers, slides or springs. It is generally employed on smaller parts such as pins, screws, sights, etc. The colours will range through straw, gold, brown, purple, blue, teal, then black. Examples of this finish can be seen commonly on older pocket watches whose hands exhibit what is called "peacock blue", a rich iridescent blue.
This is the predecessor of all metal colouring typically employed in the firearms industry. Contemporary heat-treatable steels did not exist or were in their infancy. Soft, low-carbon steel was used, but strong materials were needed for the receivers of firearms. Initially case hardening was used but didn't offer any aesthetics. Colour case hardening occurs when soft steels were packed in a reasonably airtight crucible in a mixture of charred leather, bone charcoal and wood charcoal. This crucible was heated to 730 °C (1,350 °F) for up to 6 hours (the longer the heat was applied the thicker the case hardening). At the end of this heating process the crucible is removed from the oven and positioned over a bath of water with air forced through a perforated coil in the bottom of the bath. The bottom of the crucible is opened allowing the contents to drop into the rapidly bubbling water. The differential cooling causes patterns of colours to appear as well as hardening the part.
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 formulas are based on corrosive solutions (necessary to cause metal to rust), and often contain cyanide or mercury salts solutions that are especially toxic to humans.
Bluing only works on steel, cast iron, or stainless steel parts for protecting against corrosion. Because it changes iron into Fe3O4, it does not work on non-ferrous material. Aluminium and polymer parts cannot be blued, and no corrosion protection is provided. However, the chemicals from the blueing process can accomplish uneven staining on aluminium and polymer parts. Hot bluing should never be attempted on aluminium, as it will react violently with the bath of caustic salts, potentially causing severe chemical burns.
Friction, as from holster wear, will quickly remove cold bluing, and will also remove hot bluing, rust, or fume bluing over long periods of use. It is usually inadvisable to use cold bluing as a touch-up where friction is present. If cold-bluing is the only practical option, the area should be kept oiled to extend the life of the coating as much as possible.
In colloquial use, thin coatings of black oxide are often termed gun bluing, while heavier coatings are termed black oxide. Both refer to the same chemical process for providing true gun bluing.