Gun bluing is a method of treating the steel surfaces of a firearm so that it is protected against wear, corrosion and rust. The process is not as effective as plating the gun with an alternate metal like nickel in chrome plating, or Parkerizing. There are different types of gun bluing that have varying effects on the long-term protection of the gun.
What Bluing Does
The process of bluing is an oxidation process, like rust. It brings the black oxide of iron to the surface and stabilises it to prevent any further change from destructive rust. Red oxide rust, on the other hand, is destructive in that it does not occupy the same volume of the ferrous metal and can damage and pit the metal and weaken it.
In the 1900s, firearm makers would immerse the parts into boiling acid baths and leave them until the surface was uniformly rusted; then they would bathe the metal in water to stabilise the process and stop it. They would card and rasp the rust off the surface, leaving a uniform blue surface that was more durable and rust proof.
Modern hot bluing developed from rust bluing. It takes less time and is more efficient, so more guns can be produced in a shorter time. In the process of hot bluing, the parts are bathed in a solution of potassium nitrate, sodium hydroxide and water that is at boiling temperature. Stainless steel requires a different combination of chemicals, but the process is the same.
Cold bluing is a process where no heat is required; chemicals are applied directly to the metal in sections or spots that need treatment. A full bath can be used but is not required. This is most often used after the gun is sold by the owner to keep the gun from showing signs of holster wear or to cover small scratches.
For hot or cold bluing, either one will leave a uniform effect over a uniform surface. That means that any fingerprints, oil, dirt or grime you leave on the part before bluing will stand out. Preparation of the surface is the most important part of the process. Bluing is not a thick coating of the metal; it is a chemical change to the thin atomic layer of the surface, so a uniformly clean and particle-free surface is the only way to get a uniform cosmetic look on your blued gun.