Bluing - often referred to as black oxide - is one of the oldest and most widely used surface treatments for steel firearm components. The process creates a deep, matte black finish and provides basic corrosion protection. For Picatinny rails and scope mounts, bluing is a classic choice, still used by several renowned manufacturers, including on a wide range of components in our Rusan assortment.
Relevant fun-fact: Although in Danish we call it "brunering" (literally "browning"), and the surface now appears completely black, the name originates from earlier times. Originally, chemicals were used that left the steel with a deep, dark brown hue. Today, the chemistry has been optimized to create the characteristic deep black color that perfectly matches traditional rifle barrels.
What is bluing, technically speaking?
Bluing is a chemical conversion process that transforms the outermost layer of the steel into magnetite (Fe3O4), which is a form of black iron oxide. Unlike paint or coatings (such as Cerakote), bluing does not add an extra layer on top of the metal. This means that the critical dimensions and tolerances of the material are preserved with 100% precision – which is crucial for the fit of a precision-machined Picatinny rail.
Since bluing is a process that only works on ferrous metals, the treatment is primarily applied to traditional steel Picatinny rails and classic steel mounts where one wishes to maintain the rifle's traditional appearance.
Advantages and limitations of bluing
The greatest advantage of a blued surface is its aesthetics and dimensional precision. The thickness of the oxide layer is microscopic - typically only 1-2 micrometers - which ensures that rings and mounts fit perfectly into the grooves without resistance.
The limitation lies in its corrosion resistance. As the surface itself is porous, it requires regular maintenance and a thin layer of gun oil to effectively repel moisture. Without oil, the steel base is vulnerable to the elements, including salt from sweat or acid from animal blood during hunting, which can quickly cause superficial rust.
Wear resistance in practice
Since the black oxide layer is very thin, a blued rail will, over time, show wear and patination in areas of mechanical contact – for example, on the edges of the Picatinny slots if you frequently mount and dismount your optical rings. This does not affect the rail's function but changes its visual appearance.
If you're looking for steel but want a completely weatherproof and extremely durable surface, you should instead choose our ultra-strong QPQ-treated steel rails. If, on the other hand, you desire minimal weight, our hard-anodized aluminum rails are the right choice.
Frequently asked questions about bluing (FAQ)
Is a blued Picatinny rail rust-proof?
No. Bluing provides excellent basic protection, but the surface must be kept lightly oiled to effectively repel moisture and dirt in the field.
Does bluing change the rail's dimensions and tolerances?
No. Since bluing is a chemical conversion of the surface metal itself (a conversion layer), the precise milled dimensions are maintained completely intact.