Different Types Of Black Oxide

Black oxide is a finish applied to iron and steel. There are two general types of black oxide for iron and steel: hot black oxide (or hot blackening) and room temperature blackening (or cold blacking)

Hot blackening: 

Hot black oxide can be done from generic mixtures of caustic soda, sodium nitrite / nitrate, wetting agents and stabilizers or from proprietary mixes.

The result of the process is an attractive but very thin and marginally corrosion-resistant, dark black iron oxide finish. This black finish is familiar to consumers on gears and sprockets, some brands of spark plugs, and socket wrenches and other tools. It is also used on firearm components, such as rifle barrels.

Usually, the parts are cleaned, black oxided, and then waxed or oiled (with intervening rinses).

Cold blackening: 

In order to reduce the hazards of hot blackening, and to save energy, proprietary cold blackening solutions have been introduced. These operate at about room temperature, and on a different chemical basis, so they are substantially less hazardous. However, room temperature blackening is not a true black oxide process; rather it is the application of a copper selenium compound. This compound is not always an acceptable substitute for black oxide as it does not look as nice, and can tend to be very smutty (easily rub off onto hands and clothes).