When the Japanese talk about good sharpness, they mainly refer to a sharp blade that does not influence the cell structure of food during cutting. To attain such a smooth edge, the blade must be made of steel composed from particles that are as small and homogeneous as possible. This structure of steel makes it possible to sharpen a blade to a fine and smooth edge that does not tear the cells but rather smoothly cuts through them. It is also very important that the steel has a hardness of 60 HRC, thus enabling smaller sharpening angle. All these features are most commonly found in carbon steels that have a very high content of carbides and, consequently, a low content of chromium. A knife made from such steel, however, behaves slightly differently compared to more common, stainless steel knives. With use and time, these blades acquire a patina. The knife is actually the most sensitive when it is still new and had not yet developed a patina which will later protect it and make it more resistant.
What is a patina and how do you recognize it?
Traditional Japanese steels can, due to their low chromium (Cr) content, discolor and take a patina through use as they oxidize.
A patina is a natural protective layer on carbon steels that protects the blade from further oxidation and gives it a look of rough finish. It is produced by the oxidation of steel surface and this process is even quicker if a knife comes into contact with various acids. You can recognize a patina by its characteristic colours: first you see golden yellow that changes to deep blue, which then transforms to purple and, in the end, to grey that darkens over time. If you notice red dots of rust, you should remove them.
What is the difference between a patina and rust?
A patina is a thin layer that forms on the surface of oxidized steel and protects it from further oxidation.
The patina should not be confused with rust as the former protects the blade, while the latter causes corrosion and deterioration of material.
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. Rust does not protect the blade but rather speeds up further corrosion on the spot where it developed. If you notice rust on the blade, you should remove it before using the knife.
Which steels acquire a patina and which are the most sensitive?
There are three types of steel: carbon, stainless and powdered steel.
- As its name suggests, carbon steel has high carbon content and contains little or no chromium. The most popular Japanese carbon steels are white steel (Shirogami / White Paper Steel), blue steel (Aogami / Blue Paper Steel), SUJ2, SKS93 etc. Among the above, Shirogami is the most sensitive because it does not contain chromium and thus quickly develops a patina. Knives made of Shirogami steel require a little extra care and are therefore intended for experienced users. SUJ2 steel is less sensitive than white steel, however, it still acquires a patina relatively quickly. The least sensitive carbon steel is Aogami that can contain up to 1.5% of chromium. View our selection of Carbon steel knives >>
- Stainless steels contain more than 12% of chromium that reacts with oxygen, forming a passive layer that protects the steel from oxidation and corrosion. View our selection of Stainless steel knives >>
- Powder steels are a special type of steels since some alloys contain enough chromium to be classified as stainless, while others contain less chromium and are prone to tarnishing (HAP-40, 70-4.70% Cr). View our selection of Powder steel knives >>
How can you speed up the formation of a patina?
Since patina protects the blade from corrosion, you can also speed up the process of its formation. There are several ways you can to do that: you can use instant coffee, vinegar, mustard, acids and similar substances. For all DIY enthusiasts, coffee and mustard are the best option.
Speeding up the formation of a patina with coffee:
Brew coffee, wait for it to cool down and soak the blade in it. Take care not to damage the tip and protect it by placing a piece of cloth or sponge at the bottom of the container. Also take care not to soak the handle because wood can absorb coffee and get stained. Soak the blade in coffee until you like the look of patina.
Speeding up the formation of a patina with mustard:
By using mustard, you can make patina in a desired pattern. Mix the mustard with some vinegar and apply the mixture to the blade with a cotton bud. If you apply a thin layer of mixture, more patina will form, whereas if you apply a thicker layer, less patina will develop on the blade.
Maintenance of knives with a developed patina
The knife is the most sensitive when it is still new and a patina had not developed yet, but later patina will protect it and make it more resistant. These types of knives require a touch more care and we recommend you follow the steps below:
- Wipe it regularly with a clean and dry cloth (you can also do this during use).
- After use, make sure you rinse it with lukewarm water and wipe it dry.
- Gently coat it in Ballistol or Camellia oil or plain refined sunflower oil.
- You can store the knife by wrapping it in paper which will absorb moisture and protect the blade.
How do I remove rust?
If rust appears on the blade despite the protective layer of patina, you can remove it with a rough kitchen sponge, special sponge for cleaning knives (it looks like an eraser) or a fine water sandpaper (500-800 grit). You should polish the blade slowly and carefully to not scratch the knife’s surface. You should also pay special attention to the Damascus pattern and Kuro-uchi finish because it could fade or even disappear due to rough cleaning. Once you remove the Kuro-uchi finish, you cannot bring it back to life.