Black oxide is a finish applied to iron and steel. There is also a ' black oxide' process applied to the copper inner layers of printed circuit boards, but that is another topic.

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).

While most metal finishing processes use toxic chemicals, the black oxide process is especially hazardous, and amateurs are most definitely discouraged from attempting hot blackening! One of the things that makes black oxiding so dangerous is that the black oxide bath operates at about 290 degrees F. Note that this is well above the boiling point of water, and the difference is the biggest problem. Water will evaporate from the black oxide tank rapidly, but when replacement water (which turns to steam at 212 degrees) is introduced to a 290 degree tank, it will have a strong tendency to explosively flash into steam--'erupting' and spraying everything and everyone with this terribly hot and terribly caustic solution. People have been killed. It is vital that the water make-up system be properly engineered and knowledgeably operated.

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).

There are dozens of on-line letters here related to black oxide and cold blackening. Here is a partial list of some of the early ones.