Sujihiki

Carbon steel
Carbon steel is not stainless and therefore requires a little more care than stainless steel. Always clean and dry the knife thoroughly immediately after use. This is especially important after cutting acidic foods such as citrus fruits, tomatoes and onions, as the acid can quickly react with the steel.

Do this:

Clean the knife immediately after use.
Wash by hand using the soft side of a dish sponge and a little dishwashing liquid.
Dry the blade thoroughly after cleaning.
For long-term storage, the blade can be protected with a thin layer of tsubaki oil.

Avoid:

Dishwashers.
Leaving the knife in water or allowing it to remain damp.
Leaving food residue, especially from acidic ingredients, on the blade.

Patina, a completely natural oxidation of carbon steel

As carbon steel is used, the blade will gradually change colour and develop a patina. Depending on the foods you cut, it can take on shades of grey, blue, brown and sometimes even almost rainbow-like tones.

Patina is a natural reaction between the steel and food and is not the same as rust. In fact, an established patina provides some protection for the steel and makes the blade less reactive over time.

Rust, on the other hand, should be removed. It is often orange or reddish-brown and may feel rough on the surface of the blade. A patina is normally smooth and is a natural part of owning and using a carbon steel knife.