What is a patina? How do you “force a patina”?
A patina is a thin layer of oxidized steel that develops over the blade with age and use. Though a mild form of rust, it actually helps prevent against deeper corrosion. Patinas occur due to a mix of chemical compounds interacting with the steel—acids in particular will speed up this process.
To “force a patina” is to manually induce this process in a short period of time. Many knife makers force a patina with vinegar in order to add a thin protective (and aesthetically pleasing) layer to their knives. I’ve written more on what a patina is and how to force a patina yourself in this article.