The term burnishing means a process for the darkening of mainly ferrous materials, such as cast iron and wrought iron, unalloyed or alloyed steels.
The most frequently used variant is dip burnishing. For this process, the iron and steel pieces are dipped in highly concentrated, boiling, alkaline, oxidizing salt solutions, generating very dark iron oxide layers on their surfaces. The shade of black being determined by the material quality, the surface treatment and the type and condition of the burnishing bath. This process is mainly used to give workpieces an attractive appearance and to increase their corrosion resistance, while maintaining their dimensional stability.