Why The Black Rust Forming On Your Knife Is Actually A Good Thing

Have you ever picked up your favorite fancy knife — because we all have a favorite — and noticed that it's started to take on some strange coloration? Maybe you've tried rubbing it off, but the dark stains on both sides simply won't budge. As long as the marks are dark blue, purple, and black as opposed to red, the "stains" on your knives are actually called "patina," and it's the best possible outcome for your blade.

Patina is a thin layer that grows naturally on carbon and oxidized steel knives, including some of the best Japanese knife brands. While it may look like the knife is deteriorating as it changes color, patina actually protects the blade from damage, such as the formation of rust. Think of patina as a varnish or finishing coat for the knife, one that can occur all on its own over time or with a little encouragement from its owner. Patina is widely revered and celebrated by chefs who regularly work with knives for its unique and unpredictable patterns, giving the knife a visible history.

Patina forms on knives when the metal is exposed to acids and moisture, two common components of cooking. Not every kind of metal is susceptible to patina. Some are more susceptible to plain old rust, but you'll most commonly spot patina growing on carbon-based steels, thanks to their varying levels of carbides and chromium. Carbon steel is different than stainless steel in that the patina begins to form almost from the moment it's unsheathed because of the lack of chromium. It's still possible to see patina form on stainless steels and powder steels, though it's less common, and you might even spot the black and green tell of patina on materials like copper or brass.

The difference between patina and rust often hinges on slight nuances, and if you mishandle your knives in their early stages, you're more likely to grow the latter. Rust is equally unsafe for your carbon steel knives as it is for cast iron skillets, and manifests when the metal is exposed to water. While you want to avoid rust on your blades at all costs, you might look to accelerate the process of forming patina to protect your knives by rubbing certain materials on the blade, such as mustard, coffee, mayonnaise, onions, vinegar, or even egg yolks. You could buy a knife set with pre-formed patina, but experts argue that naturally formed patina is not only stronger, but better looking, too.