White / Blue / Silver

I went down the Japanese knife rabbit hole over Christmas. Getting into something new can feel quite overwhelming, especially for me. I tend to skip the Dunning-Kruger effect and immediately notice that I know nothing about the topic and that it is infinitely complex. Even in this post, there are some details I skip because they are too difficult for my liking ¹.

This is me learning something new — welcome to the ride.

Steel

The base material of every Japanese kitchen knife is carbon steel. It is an alloy of iron and 0.05–2.1% carbon. The small carbon atoms fill the voids between the larger iron atoms without replacing them. The result is called an interstitial alloy.

Alloys have several properties that measure their performance:

• Strength is the ability of a material to withstand loads without failure. This includes yield strength, the maximum stress before permanent plastic deformation, and tensile strength, the maximum stress before breaking when stretched.
• Hardness is a measure of the resistance to localized deformation, such as indentation or scratching, induced mechanically by either pressing or abrasion. It is determined by performing the Rockwell hardness test, during which a 120° diamond cone is pressed into the metal. The penetration depth defines the Rockwell hardness C (0.002 mm = 1 HRC) ².
• Wear resistance refers to a material's ability to resist surface degradation or loss under mechanical actions such as rubbing, sliding, rolling or impact.
• Hardenability is the depth to which a steel is hardened after putting it through a heat treatment process.
• Ductility is the ability to stretch/deform without breaking. It is the opposite of brittleness.
• Toughness is a combination of strength and ductility. It measures how much resistance a steel has to fracturing. In knives, this includes chipping.
• Corrosion resistance is the ability to resist oxidation in air or water.

These properties can be altered by the addition of other elements. Their atoms are much larger than the carbon ones and don't fit inside the voids. Instead, they replace them. The result is called a substitutional alloy.

• Carbon (C) is present in all knife steels and is the most important hardening element. It increases tensile strength and edge retention and improves resistance to wear and abrasion. Added in isolation, it decreases toughness.
• Silicon (Si) increases strength. It also deoxidizes the molten material by reacting with oxygen, which is considered a contaminant. Oxygen promotes pitting, an extremely localized form of corrosion leading to small holes.
• Phosphorus (P) is considered an impurity and reduces toughness. It is present in most steels because it comes as part of the raw materials. In small amounts it improves strength, machinability and hardness.
• Sulfur (S) is also considered an impurity and comes from iron ore or coke in blast furnaces. It increases machinability but decreases toughness.
• Manganese (Mn) improves the grain structure and contributes to hardenability, strength and wear resistance. It deoxidizes and desulfurizes the steel during manufacturing. In larger quantities, it increases hardness and brittleness.
• Vanadium (V) refines the grain structure of the steel and contributes to wear resistance and hardenability by forming the hardest carbides. Carbides are hard little crystals, which form when carbon atoms bond with metal atoms inside the steel.
• Tungsten (W) is the strongest carbide former after vanadium and improves wear resistance.
• Chromium (Cr) increases wear resistance, hardness, tensile strength and corrosion resistance. Steel with at least 13% chromium is deemed stainless. The chromium atoms form a microscopic layer of chromium oxide on the surface when exposed to oxygen. Despite its name stainless steel can rust. It just stains less, not never.

Paper Steel

Paper steels are produced by Proterial Ltd., formerly known as Hitachi Metals, and make up most of the steel used for Japanese knives. The most relevant ones are known as white, blue and silver steel. Their names are based on the wrapping paper color ³:

• Shirogami (白紙 from shiro "white" and gami "paper")
• Aogami (青紙 from ao "blue" and gami "paper")
• Gingami (銀紙 from gin "silver" and gami "paper")
• Ginsan (銀三 from gin "silver" and san "three") stands for Gingami #3

Each steel comes in multiple variants with different compositions:

The composition influences the steel's properties:

The differences in hardness are caused by the carbon content. More carbon results in a steel with higher hardness, which in knives translates to longer edge retention and the ability to take a sharp edge. The increased toughness of blue steels is caused by the addition of rare materials like tungsten and vanadium, which drives up the price. High toughness prevents chipping but also makes the knife harder to sharpen.

So choosing a knife steel comes down to two questions:

1. Do you want stainless or high carbon steel? If you want stainless steel with minimal maintenance, choose either Ginsan or a powdered steel like SG2. If you want high carbon steel with character and a lovely patina, white and blue steel are the way to go.
2. Do you want short frequent or long infrequent sharpening sessions? If you want short frequent sharpening sessions, pick the pure white steel. If you want long infrequent sessions, go with the fancy blue steel.

This is the theory. In practice, you probably could not tell the difference between two steels.