Due to their low chromium (Cr) content, traditional Japanese carbon steel blades can develop a protective layer called patina.
➡️ A patina is a natural protective layer on carbon steel that forms on the surface of oxidized steel and protects it from further oxidation. You can recognize a patina by its characteristic colors: first you see golden yellow that changes to deep blue, which then transforms to purple and, in the end, to gray that darkens over time. Due to its protective properties (and cool look!), you definitely want it on your blade.
➡️ Rust is a thicker layer of usually brown iron oxide with a reddish tinge, formed by the reaction of iron and oxygen in the presence of water or air moisture. It causes corrosion and deterioration of the blade. You need to remove it from your blade.
Since patina protects the blade from corrosion, you can also speed up its formation by covering the blade in instant coffee, vinegar, mustard, acids, ... To learn how to do it, check out our “What is Patina? Is it Good or Bad for Your Knife?” article on our blog.
