So, you bought a carbon steel knife…
High carbon steel is a metal with a 1,300 year history of making high-quality blades. Knives made of carbon steel tend to keep an edge much longer than their stainless-steel counterparts; however, that extended edge life comes with a trade-off: carbon steel is susceptible to rust.
The blade will change color – becoming grey or even black as it is used. This is called patina and it’s a good thing. Patinas develop on carbon steel when exposed to the air for extended periods of time or to acids for short periods of time. It’s a type of oxidation that helps defend against rust.
Black or grey = good, red or orange = bad.
As you use the knife, use a kitchen towel to keep it clean and dry. Onions, tomatoes and other acidic food will make the patina darker at a more rapid pace. My personal carbon knives have a strong patina that I acknowledge as a reward for using an exceptional knife.
To help the patina develop, keep the blade oiled. Spray a little non-stick spray or wipe a little cooking oil on the blade once a week for the first few months to let the patina develop without worrying about rust. If stored for a while between uses, mineral oil is a better option since it does not degrade like a food oil. If rust does appear, you can wipe most of it off with a kitchen sponge or a paper towel, but deeper rust might require steel wool. If it gets out of control, you may bring the knife back to me for repolishing. Once the patina is set in, the knife will be fully protected and much easier to maintain.
Do not stick this knife in the dishwasher. Wash it with soap and water and dry it thoroughly after each use. A little maintenance and care will ensure that this knife will last a lifetime.