White steel, also known as Shirogami, is a type of carbon steel from Japan manufactured by Hitachi Metals and known for its purity and high carbon content. This steel is easy to sharpen and can achieve a mirror-like edge finish. Its high hardness also results in exceptional edge retention, making it a popular choice among knife and sword makers.
However, due to its reactivity, special care must be taken when using and maintaining carbon knives, such as wiping the blade regularly and thoroughly washing and drying it after use. A patina can form on the blade over time, which can help reduce the chances of rusting. The steel composition includes Carbon (C) at 1.05–1.15%, Silicon (Si) at 0.1–0.2%, and Manganese (Mn) at 0.2–0.3%.
Creating Shirogami blades can be difficult due to the restricted temperature range required for hardening. The process of Yakiire, which involves edge hardening and quenching (the rapid cooling of the metal in water or oil), must be performed quickly to achieve the desired hardness and maintain the sharpness of the blade.
Shirogami White Steel Types
- Shirogami #1 has a carbon content of 1.25-1.35% and a hardness of 61-64. It is the hardest type of white steel but can be brittle.
- Shirogami #2 has a carbon content of 1.05-1.15% and a hardness of 60-63. It is the most common type of white steel used for knife manufacturing and offers good edge retention and ease of sharpening, but it is less hard than Shirogami #1.
- Shirogami #3 has a lower carbon content of 0.8-0.9%. It is commonly referred to as yellow steel and is durable and long-lasting, but it does not hold its edge as well as the other two types.
To ensure the best quality and structure of Shirogami steel, carefully control the temperature during forging. The recommended range for forging is between 800°C and 900°C, the red-yellow heat ranges are ideal for steel with high carbon content.
Higher temperatures can lead to oxidation and loss of the fine-grained structure, while lower temperatures can cause cracks in the steel. To maintain the highest quality, finishing the forging process at a slightly lower temperature of 750 °C and with a higher frequency of hammer blows is recommended.
Carbon loss is minimized by reducing the time spent heating and forging the steel. The best results can be achieved using a charcoal fire free of sulphur and phosphate. Additionally, Shirogami Steel can be forge-welded to wrought iron using Borax at 1100°C and yellow-white heat. It is crucial to ensure that the forge and anvil are clean before beginning the forge-welding process.
The Process of Treating Forgings Includes Three Stages:
- Annealing: A prolonged heating process at temperatures slightly below the hardening level to create a consistent, crystalline structure.
- Hardening: The steel is heated to the hardening temperature and then rapidly cooled in either lukewarm water or oil.
- Tempering: The steel is heated at a lower temperature to alleviate the mechanical stress accumulated during the forging process and improve the steel's structure.
Before heat treatment, the Shirogami steel must be entirely shaped and polished. It is crucial to guarantee a consistent heat distribution during the forging process or to utilize a designated hardening chamber.
Adjusting the temperature for the desired effect on the material includes heating it to a range of 740–770 °C for annealing, then increasing the temperature to 760–830 °C for hardening. The material is then cooled quickly by immersing it in lukewarm water or oil. Finally, it is tempered at a temperature range of 180–220 °C, resulting in a final hardness of approximately 62 on the Rockwell scale.
The grinding process should be approached cautiously when working with hardened and annealed steel. It is crucial to avoid excessive heat, as temperatures above 150 °C can negatively impact the steel's crystalline structure. To prevent this, it is best to use a water-cooled sharpening machine or manually sharpen the steel using Japanese waterstones.
Additionally, it is essential to note that "white paper steel" is not immune to corrosion. Apply an acid-free oil such as camellia oil or Ballistol after completing the grinding process to protect the blades from damage.
Why, Therefore, Aren’t All Knives Made of Shirogami?
Not all knives are made of Shirogami because, while it can be sharpened to a very sharp edge, it has the drawback of being more brittle and prone to rust.
A solution to this issue was to create Aogami, or "blue paper steel," by adding Tungsten and Chromium to increase durability and resistance to corrosion. However, this added durability also makes the blade more difficult to sharpen, which is why some chefs prefer using Shirogami kitchen knives.
What Distinguishes White Steel from Blue Steel?
The primary distinction between these two types of high-carbon steel is their amount of carbon and their hardness level.
Though it can be challenging to discern differences with the naked eye, the two feel distinct when in use.
- Typically, white steel blades are sharper, but blue steel blades have better edge retention, which means they maintain their sharpness for longer during kitchen cutting and food preparation tasks.
- Blue paper steel is known for being particularly sticky and resistant to wear due to its chromium and tungsten content, making it less prone to chipping than white paper steel.
- White steel tends to oxidize faster than blue steel but has a much sharper edge. Blue steel is more rigid than white steel and easier to sharpen as it maintains its edge for longer.
- During the manufacturing process and power grinding, white steel produces more bright sparks, while blue steel creates fewer and smaller sparks. Thus, you can distinguish between the two based on their spark.
If precision and sharpness are your top priorities, white steel is the optimal choice. On the other hand, if you value edge retention and infrequent sharpening, the blue paper steel blade is the best option.
Regarding durability, all Japanese knives are superior to standard Western-style blades. However, it should be noted that white and blue steel knives require specialized sharpening stones and can be more challenging to sharpen. Despite their higher cost, the skill and craftsmanship in creating these carbon steels and unique blades justify the expense.