Hot baths of sodium hydroxide (NaOH), nitrates such as sodium nitrate (), and/or nitrites such as sodium nitrite (NaNO) at  are used to convert the surface of the material into magnetite (FeO). Water must be periodically added to the bath, with proper controls to prevent a steam explosion. 
Hot blackening involves dipping the part into various tanks. The workpiece is usually dipped by automated part carriers for transportation between tanks. These tanks contain, in order, alkaline detergent, water, sodium hydroxide at  (the blackening compound), and finally the sealant, which is usually oil.
The NaOH (caustic soda) and elevated temperature cause FeO (black oxide) to form on the surface of the metal instead of FeO (red oxide; rust). While it is physically denser than red oxide, the fresh black oxide is porous, so oil is then applied as post treatment to the heated part, which seals it by "sinking" into it. The combination prevents corrosion of the workpiece. There are many advantages of blackening, including:
* Blackening can be done in large batches, which is ideal for small parts.
* There is no significant dimensional impact. The blacking process creates a layer about 1μm thick.
* It is far cheaper than similar corrosion protection systems, such as paint and electroplating.
The oldest and most widely used specification for hot black oxide is MIL-DTL-13924, which covers four classes of processes for different substrates. Alternate specifications include AMS 2485, ASTM D769, and ISO 11408.
Iron(III) chloride (FeCl) may also be used for steel blackening by dipping a piece of steel into a hot bath of 50% FeCl solution and then into a hot boiling water. The process is usually repeated several times.