Black oxide is a blend of caustic, oxidizers, and additives that is used on ferrous metals. There are three types of metal blackening: hot, mid, and room temperature blackening. EPi was the first company to offer all three processes.
Hot black oxide (285 F°) is a conventional hot process which produces a true black oxide iron magnetite (Fe3O4) finish. It is a premium-grade salt mixture which will blacken a wider range of steel alloys than any other process on the market.
Mid-temperature black oxide (225–245 F°) is the latest development in blackening processes, producing a true black oxide iron magnetite (Fe3O4) finish. It produces no caustic boiling fumes and provides a safer working environment.
Room-temperature blackening (65–85° F) is the preferred metal-blackening process for safe and convenient in-house blackening. It is an excellent non-bleed black finish for powdered metal and cast iron. It produces super deep blackness and corrosion resistance equal to hot oxide blackeners.
It is important to note this is not a true black oxide process; however, it may be referred to as room-temperature black oxide. Room-temperature blackening processes are autocatalytic reactions of a black selenium-copper compound that deposits on ferrous parts through an immersion process.