A patina (/pəˈtēnə/) is generally defined as “a surface appearance of something grown beautiful especially with age or use.” The word was first used by Italians in the 17th century to refer to the green film that forms on the surface of copper. The original word was Latin for “a shallow dish,” so the Italian meaning probably developed after they had observed the film forming on copper dishes. By the mid-18th century, English speakers had also begun using the word to describe the discoloration of copper, and by the early 20th century, the English use of “patina” was being applied to the change in coloration and texture of a variety of materials over time and with regular use.
On metal, a patina is more specifically defined as a coating of chemical compounds that results from exposure to various atmospheric elements, like oxygen, water, carbon dioxide, and others. Essentially a form of mild corrosion, these chemical compounds may take the form of oxides, carbonates, sulfides, or sulfates. While patinas on metals generally develop naturally, they may also be accelerated or deliberately created through the application of chemicals. This artistic process is called patination and can be used to create a layer that is both attractive and protective.
For serious knife enthusiasts, understanding patinas is important—especially when it comes to blades. Stainless steel blades with a high volume of chromium produce chromium oxide, a thin layer that coats them and protects them from rust and patination. Tool steels, however, are not only vulnerable to rust, over time they will also naturally develop patinas in the form of black iron oxide.
Red iron oxide, or ferric oxide (Fe2O3), is what we commonly know as rust. It forms when iron reacts to oxygen in the presence of water. Ferric oxide is destructive to steel because the oxide molecule has a larger volume than the underlying iron atoms. Eventually, the oxide molecule “spalls” or flakes off, exposing fresh steel and compromising the physical structure of the material.
Black iron oxide (Fe3O4), however, is different. Also known as magnetite, it is a grey/black discoloration of the steel caused by its exposure to oxygen. This discoloration occurs naturally with tool steels because they do not contain the chromium content necessary to create a protective chromium oxide layer. Unlike red iron oxide, black iron oxide does not spall or flake and actually serves as a protective layer to help prevent rust. This layer is in fact what creates the patina on tool steel blades and, over time, gives them rich character and appearance. It is a natural aspect of non-stainless knives and is to be expected.
Tool steel knives will naturally patina over time. While some knife enthusiasts prefer to resist this by polishing them back to their new-in-the-box state, most prefer to let nature take its course and accept the character and protective function of a patina. Others take the opposite approach, purposely accelerating the process by patinating their blades by soaking them in hot vinegar, coating them with mustard, cutting acidic foods, or similar methods. Like bluing a firearm, this process can be used to purposely enhance the character of a blade and create a protective patina faster than it would occur naturally.
Any way you look at it, if you invest in tool steel blades, patinas “come with the territory.” Whether you choose to embrace them, accelerate them, or polish them away, understanding them is the first step in the process.
Leave a Comment: