One of the main ideas behind putting a finish on firearm steel is to protect it from corrosion, right? Ironically enough, one of the most common firearms finishes, bluing, results from a specific form of steel oxidation – that is, rust. Instead of the red-brown color associated with rust (Fe2O3), bluing forms the blue/black oxide Fe3O4. As the bluing process is only thin layer of controlled rust it offers minimal protection from environmental oxidation, but it is easy and affordable to apply to ferrous (that is, iron-based) metals. Which, also means that this process does not work with aluminum alloys or other non-ferrous metals.
The process for bluing steel evolved from an older rust finish process called “browning.” Browning a gun involves coating the steel in a corrosive solution (often composed of alcohol, nitric acid, water, ferric chloride, and copper sulphate), then removing the acid with a brush of many thin wires (called “carding”). To get a richer or darker shade, the process is repeated before washing and oiling the steel (to protect it).
There are two processes for bluing steel: cold bluing and hot bluing. There are a couple of different ways to cold blue steel, as well. The easiest is to purchase one of a number of different solutions or pastes sold commercially, for example, Birchwood Casey’s PermaBlue®, or Brownell’s Oxopho-Blue. These products are often used for small [cosmetic] “touchups” to coat scratches or wear on an original factory-blued finished steel. Another, older cold bluing process involves the same steps as browning, except submerging the parts in boiling water before the carding step. There are also cold bluing processes that involve heating the metal before applying a bluing solution.
Hot bluing can be achieved in many different ways. There are, however, two chief processes to achieve a hot blued steel finish. One way is to boil a potassium nitrate. sodium hydroxide, and water solution and submerge the steel. The steel oxidizes in that hot solution, and then is removed and rinsed. There is another method of hot bluing that uses the hot fumes of caustic chemicals (namely nitric and hydrocloric acid) in an airtight compartment with the steel and a water source instead of a chemical bath. Both these methods are highly toxic, however, and not recommended for home DIY bluers, but are often used in factory bluing processes. Although more difficult (and dangerous) to achieve, hot blued finishes are more wear resistant and longer lasting than those accomplished via cold bluing processes.
I find it interesting that one of the most common forms of protective finishes for firearms is, in fact, a form of rust. Before I started researching, however, I didn’t realize how old the process was, or how many different ways a similar-looking (i.e., blued) finish can be accomplished. Have you ever cold blued a gun yourself? What products or methods do you prefer?