Steel is made up of iron, carbon, and small percentages of other elements such as silicon, manganese, phosphorus and sulfur. Carbon is the key ingredient, as iron cannot be tempered, or hardened, without it. The greater the amount of carbon, the harder the steel will be. "Carbon steel" is made of iron combined with 0.1-2.7% carbon. Below 0.1% carbon, the metal is no longer called carbon steel, but "soft iron."
Stain-resistant steel is produced by introducing at least 12% chromium to the basic mixture of iron and carbon. A chromium-oxide film forms on the metal surface that prevents the iron from coming into contact with oxygen and water. However, stain-resistant steel knives must still be washed after each use as the salt and acids in food can erode the chromium-oxide film and reduce the knife’s rust resistance.
Japanese knife makers have always chosen their materials carefully. Traditional sword craftsmen used a form of steel called tamahagane which is only produced in western Japan in a high heat smelter, or tatara. Tamahagane is made of iron dust and pure charcoal. Traditional craftsmen use tamahagane to produce swords that are both sharp and strong, but it is extremely expensive and difficult to forge. It is the ultimate material for sword making. Today’s chef knives are forged with similar methods used by sword craftsmen for generations using shiro-ko ("white steel") and ao-ko ("blue steel").
Shiro-ko and Ao-ko Carbon Steels
Shiro-ko steel is a highly refined carbon steel that has no added ingredients (though it may contain varying levels of the impurities (phosphorus and sulfur). Adding chromium and tungsten to shiro-ko steel creates ao-ko steel. With these additional ingredients an ao-ko steel blade becomes more durable, easier to temper, and capable of maintaining a longer-lasting edge than a shiro-ko blade.
Kirenaga or "Duration of Sharpness"
Kirenaga means "duration of sharpness." In a busy Japanese restaurant, the chef will often have to make hundreds of precise slices of fish. Therefore, it is essential that he chooses a knife that will stay sharp throughout the entire day. This chart shows that between an ao-ko and a shiro-ko blade, both perfectly sharpened, the ao-ko blade will have a longer edge life.
Stain-Resistant Steels for Traditional Knives
As technology advances, new types of steel are being developed that harmonize the performance benefits of carbon steel with the practical benefits of stain-resistant steel. High-carbon, stain-resistant steels such as ginsan-ko, INOX , VG-10, and 8A are becoming increasingly popular with professionals.