Surface Coatings

The reasons for treating the surface of a firearm is to get rid of reflections from bright surfaces which could dazzle the firer and/or reveal the firing position to the enemy or game, to improve the appearance of the firearm, and more importantly to provide a degree of protection against corrosion.

Early firearms had no finish at all and rusted rapidly when exposed to black powder residues and atmospheric moisture. A process known as "browning" (artificial rusting) was the first attempt at a rust-resistant finish, and records on the process exist as early as 1637. The problem with browning was stopping the rusting action. The early browning finishes were known as "russetting."

"Bluing" started to replace browning by the 1800s although it originated much earlier as records exist dated 1719.

Both browning and bluing are essentially controlled artificial rusting processes using special oxidizing mixtures. The process consists of a number of stages:

1. Degreasing (by using any suitable solvent).

2. Application of browning of bluing solution.

3. Rusting (at room temperature or in a steam oven).

4. Drying.

5. Scratching (removing loose rust with a wire brush or fine steel wool).

6. Repeating stages 2 to 5 from two to six times (usually three is sufficient) until the desired color is obtained.

7. Fixing (oiling, lacquering, or waxing). Depending on the nature of the chemicals involved a further step may be necessary to neutralize or remove traces of the chemicals used before fixing.

There are hundreds of formulae for browning/bluing solutions, to the extent that it is not possible to give a typical example. However, the majority of chemicals involved are given below101,102:

Organic

Inorganic

Acetic acid

Ammonia, ammonium carbonate, chloride, persulfate, sulfide

Acetone

Antimony trichloride

Benzoin

Arsenic

Butyl alcohol

Bismuth chloride, nitrate, oxychloride

Carbon

Boron, i.e., boric acid

Chloroform

Chromium, i.e., chromic acid, oxide

Diethyl ether

Copper chloride, oxalate, sulfate

Ethyl alcohol

Iodine

Ethyl nitrate

Iron; ferric acetate, bisulfate, chloride, permanganate, sulfate

Formic acid

Lead acetate, oxide

Gallic acid

Manganese oxide, dioxide, peroxide, nitrate

Oxalic acid

Mercury, i.e., mercuric nitrate, chloride, mercurous nitrate

Picric acid

Mineral acids, i.e., hydrochloric, nitric, sulfuric

Tannic acid

Potassium bisulfate, chlorate, cyanide, dichromate,

ferricyanide, ferritartrate, iodide, nitrate, oxalate,

permanganate

Tartaric acid

Selenium, i.e., selenious acid

Fixing Stage

Vaseline

Silver nitrate

Amber varnish

Sodium chloride, dichromate, hydroxide, hyposulfite, nitrate

Linseed oil

Sulfur

Shellac

Tin, i.e., Stannic chloride, oxalate, stannous chloride

Copal

Zinc; chloride, nitrate, sulfate

Mineral oil

Quartz sand, glass powder, water

The process of browning and bluing can be very time-consuming and labor intensive. Today, nearly all bluing (blackening) is done by the hot salt, black oxide process because of its speed and cheapness of materials. Hot salt blackening can vary in color from a blue black to a deep black depending on the concentration of the chemical solutions, the temperature, and the alloy content of the steel. The vast majority of firearms currently manufactured are finished by the use of a blackening solution containing sodium hydroxide, potassium nitrate, and sodium nitrate in the typical ratio 65:25:10, respectively.

One of the best finishes for firearm steel is "phosphatizing" (Parkerizing) but few manufacturers offer this finish other than if required for military or police markets. The process deposits a crystalline layer of phosphates on the metal surface by immersion in a bath of iron, zinc, or manganese dioxide and phosphoric acid. Of these, a manganese phosphate finish is preferred for military use.

After phosphatizing some firearms are then oil-coated to provide extra protection against corrosion. Other protective coatings applied over the phosphate coating include electrostatic spray painting, epoxy, zinc chromate, and Teflon. When the coating includes molybdenum disulfide or fluorocar-bons such as Teflon, there is the added advantage of reduced friction between moving parts.

Some firearms are plated with anodized aluminum, nickel, or chromium which gives durability and good looks, and some are made from stainless steel which is much less prone to rust than conventional steel. Electroless nickel coating is an alloy coating of 88% to 96% nickel and 4% to 12% phosphorus, which is produced by chemical (not electrical) reduction of nickel on to the metal surface.

Good care and maintenance of a firearm are the best protection against corrosion. There is a wide range of commercial gun cleaning and maintenance products available.

Bore cleaning products include an electrochemical cleaning device and numerous chemical cleaning mixtures, containing both organic and inorganic compounds, the compositions of which are commercial secrets.

Lubricating products range from light mineral oil to dry lubricants incorporating molybdenum disulfide, fluorocarbons (PTFE), and other synthetic lubricants.

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