Hot caustic bluing

Bluing may be applied by immersing steel parts in a solution of potassium nitrate, sodium hydroxide, and water heated to the boiling point, 275–310 °F (135–154 °C) depending on the recipe.^[6] Similarly, stainless steel parts may be immersed in a mixture of nitrates and chromates, similarly heated. Either of these two methods is called 'hot bluing'. Hot bluing is the current standard^[7] in gun bluing, as both it and rust bluing provide the most permanent degree of rust-resistance and cosmetic protection of exposed gun metal, and hot bluing takes less time than rust bluing.

Rust bluing

Acid solution applied to bare metal

After boiling rusted parts

After eight rust, carding and oiling sessions

Rust bluing was developed between hot and cold bluing processes, and 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 convert the red oxide Fe
₂O
₃ to black oxide Fe
₃O
₄, which forms a more protective, stable coating than the red oxide; the boiling water also removes any remaining residue from the applied acid solution (often nitric acid and hydrochloric acid diluted in water). The loose oxide was then carded (scrubbed) off, using a carding brush – a wire brush with soft, thin (usually about 0.002 in (0.051 mm) thick) wires – or wheel.

This process was repeated until the desired depth of color was achieved or the metal simply did not color further. This is one of the reasons rust and fume bluing are generally more rust-resistant than other methods. The parts are then oiled and allowed to stand overnight. This process leaves a deep blue-black finish.^[8]

Modern home hobbyist versions of this process typically use a hydrogen peroxide and salt solution, sometimes preceded with a vinegar soak, for the rusting step to avoid the need for more dangerous acids.^[9]

Fume bluing

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 mixed fumes of the acids 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 melt 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.

Cold bluing

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. Cold bluing is not particularly resistant to holster wear, nor does it provide a large degree of rust resistance. Often it does 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 often match, blend, and wear better than any cold bluing process.

At least one of the cold bluing solutions contains selenium dioxide. These work by depositing a coating of copper selenide on the surface.

Nitre bluing

Niter and colour case

In the nitre bluing process, polished and cleaned steel parts are 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 pound 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 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 range through straw, gold, brown, purple, blue, teal, then black. Examples of this finish are common on older pocket watches whose hands exhibit what is called 'peacock blue', a rich iridescent blue.

Color case hardening

Color case hardening is the predecessor of all metal coloring 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 did not 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 colors to appear as well as hardening the part.

Different colors can be achieved through variations of this method including quenching in oil instead of water.

Browning

'Browning' is controlled red rust Fe
₂O
₃, 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 Fe
₃O
₄. 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.