Parkerizing is a form of treatment for metal, typically zinc or iron alloys, in order to produce a strong and very durable protective passive film on the surface of the metal. The steel thus treated is said to be parkerized and this process was invented by John Parker in 1870.

The rusting process can require days or even months while the parkerizing process takes only minutes. The black color of the iron oxide is similar to that which would be applied if the part were blued or case hardened, though with a slight sheen. Parkerizing can also be colored by dyeing the surface with aniline.

Parkerizing is used extensively on firearms in certain applications to prevent rust. The usual form of application is hot dip in molten zinc, but it can also be applied in the manner similar to bluing.

The parkerizing solution is made by mixing 1 part of Black Oxide Pigment with 2 parts Cold water and then boiling it for about 5 minutes. Some people add salt or other chemicals to the boiling solution. Then, after letting it cool, you can pour it into a spray bottle or paint-on with a brush.

The solution should be used within a few hours because the water will evaporate and give room for bacterial growth which can make your metal rust prematurely.

The surface to be parkerized should be thoroughly degreased to remove all traces of oil and should be dry before the solution is applied. This can be achieved by blowing with an air gun, or wiping with a rag wetted in meths/petrol or turpentine.

While the hot dip method is ideal for large scale production, it requires significant preparation time due to the need to heat the vat to Zinc’s melting point of 980 degrees Celsius.

Parkerizing was first done as a paint formulation produced by mixing together equal parts of a phosphoric acid and a manganese solution, which would be applied with a brush over the steel, usually after degreasing the surface., or as a powder mixture to be dissolved in water. Parkerizing is a similar process to bluing, which instead uses a nitric acid and sulfuric acid solution which turns the iron in the steel into magnetite.

Rust preventatives were not commonly used on firearms until it became apparent that firearms in storage were prone to rusting when exposed to moisture or humid environments. Firearms collectors and users began to research rust treatments, and many settled on the parkerizing process as a simple solution. Parkerizing is often confused with bluing as both processes produce similar results – a metal surface that has been colored by oxidization of the metal.

Parkerizing protects against oxidation in much the same way as bluing does.

The main difference between the two is that bluing requires a metal substrate to form on, whereas parkerizing can be done without a metal substrate.

In either case, the result is a black oxide with some sheen. Bluing produces a deeper blue-black color and has been known to have additional coloring agents added in the solution to enhance the color. However this is not typically done with parkerizing.

Parkerizing is best done by a professional and not at home.