Introduction to Japanese Kitchen Knives Part 3: Heat Treatment and Layering

As we discussed in Part 1, knives are made via forged steel or stamped steel. After either process the steel needs to be changed to make it usable for cooking, done via a process called heat treatment. Think of it like baking a cake; cake mixture needs to be baked to create a structure, otherwise it's just liquid in a bowl that won't work as a cake. Knives need heat treatment, or they won't work properly as knives.

Each forger or factory has its own approach to heat treatment, and it's worth paying attention to - as the heat treatment stage is the most important part of any knife. A VG-10 stainless steel knife that is properly heat treated will vastly outperform one that has had cost cutting measures implemented, or isn't heat treated with the right techniques. To use another cake example, compare a cake made from scratch by a master baker by hand, against a packet cake from the supermarket baked at home, against a microwaved cake in a cup using the same mixture. The end results would all be vastly different. Even if the packet cake had better ingredients, the baker-made cake would very likely turn out better because of the care and expertise taken when baking it. Same analogy works with knives too. No knives in the microwave though, please!

This is one of the key differences between a cheaper knife and a higher end one - they're probably heat treated differently. Good heat treatment equals good quality. And good heat treatment is the most important step of any knife.

How a knife is heat treated depends on many factors, such as steel type, thickness, size, whether it's forged or stamped, and intended layering. There are many layering methods, but the two key ones are mono-layered steel and multi-layered steel.

Temperatures, timing and even ambient temperature influence the heat treatment process, on top of variables like technique, time spent and attention to detail. A knife made by an artisan at the right temperature for the right time will outperform a poorly heat treated, mass-produced knife, even if it has a higher quality steel.

Heat treatment has three major steps: Normalizing, Quenching, and Tempering.

Normalizing involves evenly heating the steel to above 1400°C (2552°F) then slowly cooling it. This balances out the steel, removing potential weak points on the knife that appeared during forging. Some knifesmiths will hammer out further distortions by hand to improve the end result.

The steel is then reheated to over 1000°C (1832°F), giving it a bright glow. Quenching involves dipping that steel into oil or water to rapidly cool it. In Japan, smiths often quench with water, sometimes using a special clay coating over the blade too. High end stamped knives will also go through sub-zero quenching.

Quenching makes a knife hard, and is crucial to get right. The best knives in the world have perfect quenching over anything else.

After quenching and another cooling, the steel becomes fragile. Tempering fixes this, removing stress from the steel to increase its durability.

The steel is baked or tempered in an oven with precise temperatures and timing. These determine how the knife will come out, removing brittleness and adding hardness, durability, and edge retention.

Stainless steel reacts differently to carbon steel during this process, so different approaches are required to get similar results.

After this, the knife is ready for cleanup, final sharpening, and polishing!

The above steps are necessary for a good knife. You can cut corners or skip steps in production to reduce cost or mass produce, but the knife won't perform or last as well.

Remember, a knife is more than just its steel. It's how it's made.