So, why is Honyaki considered in a league of its own among countless other knives? The answer lies in their unique manufacturing process. Honyaki knives are fundamentally made from a single, hard piece of metal (hagane). (As a side note, some Western-style knives made from a single, hard stainless steel are often called "zenko" (full steel) and are distinguished from Honyaki.)

This simple yet highly advanced method is the source of their extraordinary performance. Let's unravel the specific reasons why Honyaki is treated as special, from a professional's perspective.

One of the most distinctive processes in Honyaki manufacturing is "Tsuchioki" (clay coating). This involves applying a special clay to the surface of the knife when it's heated to high temperatures and then rapidly cooled in water during the "quenching" process.

By varying the thickness of this clay in different areas, differential hardening occurs in the steel after quenching. Specifically, the edge becomes hard, while other parts remain slightly softer, providing a "cushioning" property that absorbs impact. It's this contrast in hardness that manifests as the beautiful "hamon" (temper line) on the knife's surface. The hamon is a major appeal of Honyaki and a testament to the artisan's skill.

During the quenching process, either "water" or "oil" is used to cool the heated steel. This choice significantly impacts the knife's final characteristics.

• Water Quenching: Results in an extremely hard finish, but also carries a higher risk of cracking. This is the method chosen by artisans pursuing ultimate sharpness.
• Oil Quenching: Less prone to cracking compared to water and often produces a cleaner hamon, but tends not to achieve the same level of hardness as water quenching.

Master craftsmen choose their quenching medium based on the desired sharpness and characteristics. Many of the finest Honyaki achieve their true potential through water quenching.

In the manufacturing of Kasumi knives, tiny iron powder-like substances are sometimes used as an "adhesive" when laminating the base metal and blade steel. While much of this iron powder is removed through forging, a minuscule amount can remain as an impurity, potentially affecting the final sharpness.

However, since Honyaki is made from a single piece of steel, there's no concern about such impurities being introduced. This allows for the pursuit of a purer, more ideal performance as a cutting tool.

Forging is a crucial process that influences a knife's sharpness. It involves hammering heated steel to refine its grain structure. During this, "low-temperature forging" is considered ideal, where the steel's temperature is kept as low as possible. Too high a temperature can coarsen the metal's grains, impairing the delicate sharpness.

Kasumi knives require heating the base metal and blade steel to high temperatures of around 1000 degrees Celsius for lamination. In contrast, Honyaki does not have this lamination step, allowing for forging at the optimal "low temperature" for the steel. This helps maintain a dense and uniform metal structure, resulting in exceptional sharpness.

From a technical standpoint, with Kasumi knives that combine different types of metals (soft iron with low carbon content and steel with high carbon content), there's a possibility of "carbon migration" where carbon from the steel moves into the soft iron. This could potentially prevent the steel from achieving its intended hardness.

Since Honyaki is made from a single, hard piece of steel, there's no concern of carbon migration. This ensures that the craftsman can achieve the precise hardness they intended.

Kasumi knives, made from different metals, can experience slight distortion over years of use or changes in environment. However, since Honyaki is composed of a single steel material, they are less prone to age-related distortion. This allows them to maintain their superior performance over a long period.