Hot oil black finishing, also known as hot blackening or hot black oxide, and cold black finishing lies in the temperature at which the processes are carried out.

Hot oil black finishing involves immersing the metal part in a heated bath of blackening solution, typically between 140 to 285 degrees Celsius (284 to 545 degrees Fahrenheit). The heat facilitates the chemical reaction between the metal surface and the blackening solution, resulting in the formation of a black oxide layer. This process provides corrosion resistance and enhances the appearance of the metal.

In contrast, cold black finishing, also known as cold blackening or cold black oxide, is carried out at room temperature or slightly higher. It involves applying a blackening solution directly onto the metal surface without the need for heating. The blackening solution typically contains chemicals such as selenium or copper salts, which react with the metal surface to form a black oxide layer.

The choice between hot oil black finishing and cold black finishing depends on various factors such as the type of metal being treated, the desired finish quality, and the specific requirements of the application. Hot oil black finishing generally offers better corrosion resistance and a more uniform black finish compared to cold black finishing. However, cold black finishing may be more suitable for certain metals or applications where high temperatures could potentially damage the metal or other components of the part.