When steel is heated to red Hot condition, there is layer of Black coating formed. This coating is taped away and then heated. Why and how is this black coating formed.

Black oxide or darkening is a transformation covering for ferrous materials, stainless steel, copper and copper based compounds, zinc, powdered metals, and silver weld. It is utilized to include mellow consumption resistance, for appearance and to limit light reflection. To accomplish maximal erosion resistance the dark oxide must be impregnated with oil or wax. One of its points of interest over different coatings is its negligible development.