Bower-Barff process
A method of coating iron or steel with magnetic iron oxide in order to minimize atmospheric corrosion. The articles to be treated are put into a closed retort and a current of superheated steam passed through for a period, followed by a current of producer gas (carbon monoxide), to reduce any higher oxides that may have been formed.
Bower-Barff: steam-and-gas oxidation armor for iron
The Bower-Barff process creates a protective layer of magnetic iron oxide (Fe3O4) directly on the surface of iron and steel parts by exposing them to superheated steam inside a sealed retort, followed by producer gas containing carbon monoxide. The result is a thin, adherent, black oxide coating that resists atmospheric corrosion without the expense or weight of plating, paint, or enamel.
The process works in two stages. First, superheated steam oxidizes the iron surface; this creates a mixed layer of oxides. Then producer gas (a mixture of carbon monoxide and nitrogen from incomplete combustion of coke or coal) passes through the retort at high temperature. The carbon monoxide reduces the higher oxides back to magnetite, the hard, stable black oxide that gives the coating its corrosion resistance and magnetic properties. The sequence must be carefully controlled: too much steam creates brittle scale; incorrect timing leaves the surface in the wrong oxidation state.
Bower-Barff was widely adopted in the late 19th and early 20th centuries for firearms, tools, and precision engineering where rust bloom on bare steel was unacceptable but weight and cost ruled out plating. The coating is typically 0.0005 to 0.002 inches thick and provides good protection in dry or moderately humid conditions. It performs poorly in marine or wet industrial environments; for those applications, hot-dip galvanizing or modern epoxy systems are more reliable.
Variants and limitations
The core process is straightforward, but retort design, steam temperature (usually 200 to 300 degrees Celsius), gas flow rate, and cycle time all affect results. Some shops combined the process with light oil coating or wax to extend protection further. The method requires careful supervision and proper ventilation; producer gas is toxic, and steam under pressure poses burn and explosion hazards. Because the coating is purely oxidation-based with no plating layer, it cannot repair mechanical damage; any scratch or gouge exposes bare metal that will rust.
The process has largely been displaced by electroplating, galvanizing, and synthetic coatings in modern shops, but remains in use for high-precision tool work and for restoration of antique firearms and machinery where authentic appearance is valued. Understanding the Bower-Barff oxide is essential when restoring or maintaining older equipment; aggressive wire brushing or grinding will remove the protective layer and must be followed by re-blacking if the part is to remain corrosion-resistant.