Your carbon-steel knife's patina not only looks great, but is protective too.
While researching carbon-steel knives (see related content), we learned that they require more care than stainless-steel knives because they can oxidize in two ways. Hematite—rust—is an aggressive form of oxidation that eats into the metal, creating a flaky, orange surface. Magnetite is a mild form of oxidation that affects only the outer surface of the metal; its presence actually prevents further corrosion. Magnetite turns carbon-steel knives a charcoal gray, giving them what we call a patina.
Both types of oxidation occur when carbon steel comes in contact with oxygen and moisture (reactions that are sped up in the presence of salt). If water is left in contact with the blade for an extended period of time (for instance, if a wet knife is allowed to dry naturally), rusty spots will quickly form. Magnetite will form when exposure to oxygen and water is limited.
We found that if a knife developed a light charcoal-gray patina naturally over time, it was less likely to rust if left wet. To put this protection in place quickly, some manufacturers suggest forcing a patina to develop on the blade. We tried one method: soaking the blade in vinegar (a low pH environment favors the production of magnetite) and then washing and wiping it dry. The approach gave the knife a matte, grippy finish that created undesirable drag in food, and more important, the blade ended up rusting more easily. The upshot? The best way to develop a protective patina and avoid rust is to use your carbon-steel knife regularly, to wipe it dry continually during use, and to rinse and dry it thoroughly as soon as you’ve finished a cutting task.