Steel is blackened with heat, chemicals or a combination of both. Steel has two oxides: red and black. Red oxide is what we know as rust. It is corrosive and destructive. Black oxide provides a protective, abrasion-resistant coating that prevents rust formation. Depending on the method used, black oxide can be applied in an even finish. According to chemical engineer Fred Perlstein of the Frankford Arsenal, "The black oxide and manganese phosphate coatings (tested) were approximately equal in abrasion resistance and superior in this regard to the other black coatings tested."
Wipe the entire item with a soft cloth dipped in acetone to remove any dust, oils or other build-up. Blot dry with a clean paper towel. Use petroleum-based oils for tools and items that will not be used to prepare food.
If the part or item will be used for cooking or eating, substitute vegetable oil and heat the item on a stove or in an oven for one hour at temperatures below 288 degrees Celsius (vegetable oil has a flash point of 316 degrees Celsius). Otherwise, proceed as follows.
Wipe the entire item with a thin coating of petroleum-based penetrating oil.
Don a full face shield, protective clothing ("leathers") and heavy leather gloves. Work in a well-ventilated area, with shop doors and windows open. Heat the item with a torch until it begins to smoke and turn black.
Spray the item with penetrating oil until it is dripping wet. Heat the item again until it begins to smoke. The petroleum molecules will react with the molecules of heated steel to form a chemical bond, thus sealing the metal. Allow the item to cool to room temperature for at least 60 minutes.
Once the item cools, remove any burnt penetrating oil using acetone and a polyester scrub pad. Spray it again with penetrating oil and wipe away any excess with clean rags.
According to Bob Neidorff of Metal Web News, "There are many commercial products that claim to displace moisture and prevent rust. Some common ones are WD-40, Starrett M1, LPS-1, and LPS-3. All come in aerosol cans, which is convenient although expensive and bad for our environment. All leave a film of light oily stuff on the metal. Other products are Sprayon Corrosion Supressant, Rustlick 631, CRC SP-350, CRC SP-400, Boeshield T9, Birchwood Casey Sheath, and Rust-X. Some of these products are petroleum based, while others seem to be wax based. However, they all prevent rust by keeping air and water off of the metal surface."
Do not inhale lubricant fumes. Work in a well-ventilated area with doors and windows open or with an industrial ventilation system operating. Keep a large, fully-charged ABC fire extinguisher within reach at all times when using a torch. The blackening process described is used for steel machine parts and weaponry. It cannot be used with steel items that will be used for cooking, baking or eating. Petroleum-based lubricants are not food-safe.