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Parkerizing is a technique used to add a phosphate coating to steel to provide it with protection against wear and corrosion. This coating is typically composed of a zinc phosphate, though it can also use iron or manganese instead. It cannot be applied to nonferrous metals such as copper and aluminum, and even steel that has a high content of nickel can be unsuitable. The technique is most commonly used on guns, and has been a popular alternative to bluing since the middle of the 20th century. Most parkerized metal takes on a matte gray finish, though some techniques can result in a darker black color.
The technique of parkerizing is based on iron phosphating processes that were first carried out in the late 19th century. Similar manganese phosphating was also experimented with in the early 20th century, which led to the development of parkerizing around 1915. A new process with the same name was developed around 1938, which used zinc instead of manganese. Since zinc was easier to come by and less expensive than manganese, this became the more popular method at the time.
Phosphating is a process that involves dipping a metal object into a bath of acid and salts. In the case of parkerizing, the metal is steel and the phosphate salts can be zinc, manganese or iron. A chemical reaction between the steel and acid results in the phosphate salts being pulled out of the liquid and into the metal of the workpiece. This results in the formation of zinc, iron or manganese phosphates on the metal surface.
Since phosphates are chemically deposited into the surface of a metal workpiece during the parkerizing process, they become an integral part of the steel. This is also the case for bluing, which is what sets these techniques apart from other types of corrosion protection. Methods that involve painting a substance onto metal can create a similar color and patina, but the metal is not changed chemically.
In order for parkerized metal to exhibit corrosion resistance, it typically needs to be coated in oil. This is due to the fact that the phosphate coating is porous and not resistant to corrosion on its own. When coated with oil on a regular basis, the pores are filled up and the phosphate coating can offer excellent protection against corrosion. Parkerized metal is typically more wear-resistant than untreated steel whether it is coated in oil or not.
@KoiwiGal - While I agree with you that a gun with a blue finish is more attractive than one without it, the technique isn't really all that good for protecting the metal. And, it will eventually wear off, because it's a less effective coating than parkerization.
Parkerization can, however, eventually provide a kind of greenish-grey patina, which some people like almost as much.
And, if you prefer, there are still people who use bluing methods, or who will sell you a gun which only looks as though it has been treated with bluing.
Really, when you come down to it, there are people who are enthusiastic enough about gun-smithing to come up with almost any combination of traits that you might want.
I've always wondered why guns seem to have a slightly different color and texture than other kinds of metals. I guess this is why.
I actually think they often get drawn in popular culture with that older technique, "bluing" in mind though. Guns in pictures always seem to have a blue sheen, and from what I can tell, the name of that technique came about because the guns would end up with a blue-black kind of color.
I wonder why they stopped using that, in that case? I personally think the blue-black color is more attractive than the grey-black that comes from the parkerizing process.