Tsuchioki: The Clay Coating Technique That Creates the Hamon

What is Tsuchoki — Final Preparation Before Quenching

Among the processes of Japanese sword-making, tsuchoki (clay coating) is the most important yet most invisible technique. Just before quenching, a sword-maker applies specially formulated clay to the blade. This seemingly unremarkable process determines the hardness distribution across the entire blade, the beauty of the hamon, and the overall strength of the sword.

The principle of tsuchoki lies in "controlling the cooling rate for each part of the blade" during quenching. Different parts of the blade — the edge, the body surface, and the spine (mune) — must cool at different rates, and the role of tsuchoki is to control this through the thickness and composition of the clay.

Clay Preparation — Secret Recipes

The clay used for tsuchoki is by no means simple pottery clay. Each sword-making school has prepared clay according to secret formulations.

The basic component is locally-sourced yellow clay (yellow earth (odo)). This clay has fine particle size and appropriate viscosity, with excellent balance between adhesion to the blade and ease of removal during quenching.

The following materials are added as additives:

• Charcoal powder: Absorbs radiant heat during quenching and adjusts the cooling rate of the body surface.
• Straw ash (warabai): Reduces the viscosity of clay and facilitates removal after quenching. At the same time, it has heat-retaining properties that moderate the cooling rate.
• Salt: Reacts with the quenching water and adjusts the formation of a steam film.
• Grain powder (kokuko): Rice bran, soybean powder, etc. Burns away during quenching, creating complex cooling conditions.
• Animal bone ash: Rare, but used in formulations of particularly high-end sword-makers. Adjusts chemical reactions during quenching.

These materials are mixed carefully while adding water and left to "rest" (nekas) for several days to several weeks. Through aging, the clay particles recrystallize, resulting in a more uniform texture. The reason that tsuchoki of famous swords is considered a secret formula is that the proportions of this mixture were the greatest knowhow of the sword-maker.

Application Technique — The Culmination of Craftsmanship

The application of tsuchoki clay is a more sophisticated technique than it appears.

Application to the edge: Applied in the thinnest layer (0.5–1.5 mm). By applying thinly, the edge cools most rapidly during quenching, achieving maximum hardness. Even a 1 mm deviation in thickness changes the edge hardness by tens of units.

Application to the body surface: Applied at a medium thickness (2–4 mm). By applying thicker than the edge, the cooling rate is slowed, ensuring toughness (resistance to cracking).

Application to the spine: Applied in the thickest layer (3–6 mm). To avoid excessive hardening of the spine, the thickest clay layer is applied here. This is designed to allow the spine to function as the "axis of flexibility" of the blade.

These thickness variations are applied multiple times while being adjusted using a burin (tagane) or bamboo spatula. For a single blade, the sword-maker spends 30 minutes to over an hour, shaping the clay with millimeter-level precision.

Tsuchoki's Contribution to Creating the Hamon

The reason that tsuchoki is more than just a "cooling control tool" and defines the boundary of "a work of art" is that the hamon (wave-like pattern seen on the edge) is determined here.

The "surface shape" of the clay applied to the edge — whether completely smooth, slightly wavy, or with small protrusions — subtly changes the cooling during quenching. Where areas that cool quickly are adjacent to areas that cool slowly, "cooling irregularities" occur in the blade, which are visible as hamon through the play of light and shadow.

Renowned sword-makers intentionally adjusted the surface shape of the tsuchoki to deliberately create "a distinctive hamon that showed their school affiliation." For example:

• The "suguha" of the Bizen tradition: The tsuchoki on the edge is made completely smooth, producing a straight and orderly hamon.
• The "choji-ba" of the Yamashiro tradition: Fine irregularities are made on the surface of the tsuchoki to create a distinctive "T-shaped" hamon.

Thus, tsuchoki is the crystallization of the sword-maker's skill, achieving both "scientific control of the quenching process" and "the expression of artistic intent."

Appreciating the Traces of Tsuchoki — The Perspective of Evaluation in Certificates of Authentication

In the hamon appearing on the blade surface after polishing, structures called "nioi" and "nie" are observed. Nioi is a fine structure that appears misty along the edge boundary, while nie refers to particle-like structures within that shimmer like stars. Both are records of the temperature difference during quenching created by tsuchoki, which is inscribed in the blade as changes in the steel's structure.

In sword authentication, the appearance of nioi and nie, as well as the irregularities or regularity of the hamon, serve as important clues for estimating the sword-maker and school that produced it. Since the quality of the tsuchoki directly determines the completeness of the hamon, the state of the hamon is treated as an important evaluation item when issuing certificates of authentication.

Note that the clay used for tsuchoki flakes off from the blade during the quenching process, and no clay remains on existing swords. However, the record of the cooling difference brought by that clay continues to be permanently inscribed in the blade as the structure of the steel. This is the reason that even in swords that have existed for many years, we can read the technical level of tsuchoki at the time of production even today.

Tsuchoki in Military Swords — Balancing Efficiency and Reliability

Tsuchoki technique was also important in Showa-era military swords, but it was used for different purposes than in art swords. The tsuchoki of military swords was designed to prioritize above all the balance between "efficient mass production" and "ensuring consistent quality." Unlike art swords, there was no room for complex and precise surface shapes, so clay application was simplified as much as possible. In its place, records show that the thickness of the tsuchoki was strictly controlled to meet measurable "hardness standards."

On the other hand, in high-end military sword workshops, precise tsuchoki work comparable to art swords was sometimes performed, and such swords are still highly valued today. This is why the quality of the tsuchoki is "the most direct indicator of the sword's individuality and quality."

Frequently Asked Questions — About Tsuchoki

Q. Does the hamon change if the blade is re-polished? Polishing is the process of bringing to the surface the structure that was already inscribed within the steel through tsuchoki and quenching; it is not a process that creates a new hamon itself. Therefore, the basic shape of the hamon does not change through normal re-polishing.

Q. Can anyone master the technique of tsuchoki? Tsuchoki is based on the refined sense of proportion and application technique that each school and sword-maker has cultivated over many years, and it is said to be difficult to reproduce the same hamon just by following the steps. For this reason, it is said that many sword-makers have personally handled the tsuchoki process rather than entrusting it to apprentices.

Q. How can we distinguish the quality of tsuchoki in existing swords? Points to check when appreciating or authenticating include the regularity of the hamon outline, the uniformity of the appearance of nioi and nie, and the clarity of the boundary with the body surface. For professional judgment, it is best to refer to a certificate of authentication from an authentication organization.