Burnishing
What does burnishing mean?
Burnishing is the technical term for “refining” metal with a brown or black oxide layer. This gives the material a unique “antique” look. Derived from the French word “brunir”, translated as “to brown”, DIN 50938 distinguishes between single-, two- and three-bath burnishing.
Depending on the process, the workpieces are immersed in a boiling alkaline-oxidising solution at 133 and 144 degrees. The chemical process forms a largely resistant coating to bending and abrasion – the so-called burnished layer. Dark mixed oxide layers are produced by surface transformation of iron crystals and iron mixed crystals. The colour tone depends on the material quality, surface treatment, and burnishing solution composition. A protective layer one to two micrometres thick is created, and the workpieces remain largely dimensionally stable. The conductivity and magnetic properties of the base material are maintained.
The burnishing is, in other words, a controlled oxidation – so to speak, high-grade rust, which protects against further corrosion. However, the corrosion protection for outdoor use should be improved by applying suitable oils or waxes.
Unalloyed, low-alloy steels and cast iron are particularly suitable for burnishing. Stainless steel, on the other hand, can only be burnished with a special process. To ensure that the surface of the material undergoes uniform chemical changes during the burnishing process, the uncoated material surfaces must be clean and free of silicone oils or greases.