High Carbon Steel in Japanese Knives
What makes a chef’s knife stand out from the rest? Fancy handles, damascus patterns and lightly burnt forearms aside, the single distinguishing feature of a professional’s blade is just how gosh-darned sharp it is. On my first day as a kitchen hand, 16-year-old me asked the sous-chef how sharp his knives got. He muttered to himself, shrugged, picked up the closest one and lazily shaved a six-inch long patch of his forearm of all hair in one smooth motion. That moment a lot me about the concept of “sharp”, and the importance of being polite to this slightly mad bloke at all times. Read on for more info on why some knives can be “sharp”, while some struggle.
The key ingredient that turns a lump of iron into steel capable of taking and holding an edge is carbon. Traditionally, blacksmiths in Japan and around the world employed many and varied techniques to impart their desired amount of carbon into their steel during the manufacturing process. The higher the carbon content of a blade, the easier it is to temper the steel and the harder it will be. It is an important point that carbon steels are prone to rust and any high carbon blade requires regular maintenance to ensure a long and useful life.
In modern times, blacksmiths generally purchase steel from larger manufacturers rather than forging it themselves. Two very common steels in Japanese knives are considered High Carbon Steels — Shirogami (White Paper Steel) and Aogami (Blue Paper Steel). Japanese knives made from Swedish steel are also considered to be made from a high carbon steel (HCS).
The benefit of high carbon steel is simple — it gets really, really sharp. The exceptionally keen edge it can be honed to makes HCS a hit with chefs around the world. One key difference between Japanese carbon steels and Western ones, besides the steel itself, is the heat-treatment applied during forging. While Western knives are generally treated to a Rockwell hardness of about 56–57, Japanese knives often achieve a Rockwell hardness of 61 or higher. This means that they will retain that edge for longer, at the expense of being less ductile, and therefore less “tough”.
Shirogami is perhaps the “purest” example of a high carbon steel, in the sense that it has very few unintended impurities and no alloys. It is generally segregated into three grades, the hardest being №1 White Paper Steel, and the softest being №3. For those looking for a forgiving introduction to the world of high carbon steel knives and a simpler sharpening experience, it is hard to go past Shirogami.
Aogami steel is manufactured by adding alloys of tungsten and chromium to Shirogami. The resultant Blue Paper Steel is harder than the simpler Shirogami and will take a sharper edge for longer. Chefs and home cooks looking for the ultimate in edge refinement need look no further than Aogami steel knives, always bearing in mind that they should only be used for preparing appropriate ingredients (bones, fruit stones and pumpkins are not generally adviseable).
The trade-off of using HCS is that as the steel becomes harder it also becomes more brittle. Generally a high carbon steel cutting edge will be sandwiched between two cladding leaves of a softer steel. This impartsw the benefit of a hard cutting edge while affording the user a degree of ductility and “give” in the blade.
Even so, the high carbon steel cutting edges of Japanese knives are generally more prone to chipping than their western cousins, especially if accidentally brought into contact with unexpected bones or fruit stones. Using a levering action when cutting a dense food such as pumpkin can risk damaging a thin, hard Japanese blade.
The second compromise of HCS kitchen knives is a susceptibility to patination, though with proper care and consideration this will have no effect on the cutting edge. The steel’s reaction to air and water mean that it will discolour with time and use, though immediate hand washing and a light coating of camellia oil after use will minimise this and help the edge retain its sharpness.
Unfortunately, this trait also means that if a carbon steel knife is not properly cared for or is left wet it will begin to rust. If your HCS knife does rust, immediately remove the rust (carefully) with a kitchen scourer, or soak in vinegar for a few hours if stubborn. Re-hone the edge before coating the blade with camellia oil and storing it as per your normal routine.
Although they require an extra few moments of attention after use, in my opinion high carbon steel knives are well worth the effort. Growing out of the Japanese smithing and culinary traditions, Japanese HCS knives are a fantastic tool for any kitchen, whether in the home or in the restaurant.
