Tsuchigoki (土置き) Technique—The Clay Coating Process Before Hardening That Determines the Hamon (刃文)

What is Tsuchigoki — The Most Critical Process Before Tempering

Tsuchigoki is a process performed immediately before tempering (yaki-ire) in Japanese sword manufacturing, in which heat-resistant clay called "yaki-tsuchi" is applied to specific areas of the blade. This process determines the final shape and pattern of the hamon on the blade and is one of the most important processes in forming the artistic characteristics of Japanese swords.

At first glance it appears to be a simple "mud coating," but in reality it is a task that condenses the profound knowledge and sensibility cultivated by the swordsmith over many years. The composition of the clay, its thickness, the application pattern, the drying state — all of these directly affect both the beauty of the hamon and the functionality of the blade.

The Composition of Yaki-tsuchi — Materials and Secret Traditions

The composition of yaki-tsuchi varies from swordsmith to swordsmith and has often been passed down as "secret traditions not to be divulged outside the school." The main components are as follows.

• Clay (nendo): Refractory clay with high heat resistance is the main component. It must withstand the sudden heat changes during tempering and function uniformly during the cooling process. Since characteristics vary by source and purity, swordsmiths typically select and use clay they are familiar with.
• Whetstone powder (toishiko) and charcoal powder (sumiko): Mixed to adjust clay shrinkage and control thermal conductivity. Whetstone powder consists of fine grinding particles, while charcoal powder is an organic component that carbonizes during tempering and produces unique effects.
• Charcoal ash (mokutonbai): Since thermal stability varies depending on the type of wood used as raw material, some swordsmiths pay careful attention to material selection.

These ingredients are mixed with water to create the yaki-tsuchi, but the mixture ratios, water content, and consistency vary from swordsmith to swordsmith, and even the same swordsmith makes fine adjustments depending on the season and humidity.

The Tsuchigoki Process — Designing the Hamon

In a sense, tsuchigoki is the work of "designing the hamon." The swordsmith positions the clay while holding in mind an image of the desired hamon pattern, with the newly polished blade (shirauchi) before him.

1. Base coat (shitanuri): A foundation layer of thin, uniform yaki-tsuchi applied over the entire blade. This serves as a base to ensure even heating of the blade.
2. Edge delineation coat (hatorinuri): Yaki-tsuchi is positioned along the outline of the hamon pattern. This work determines the basic shape of the hamon. The distinctive hamon patterns of each school — such as choji-midare, gunome-midare, and suguha — are reproduced through the tsuchigoki pattern.
3. Back coating (hadayosenuri): Thick application of clay to the back of the blade slows the cooling rate during tempering and creates a difference in microstructure between the edge and back. This difference affects the clarity of the hamon and the expression of the grain pattern in the back.
4. Drying and inspection: After the tsuchigoki is complete, the blade is properly dried. Uneven drying can cause cracking and warping during tempering, so this step cannot be overlooked.

The Science of Tsuchigoki and Hamon — Metallurgical Mechanisms

The mechanism by which tsuchigoki produces the hamon can be explained metallurgically as "selective control of martensitic transformation."

When steel heated to high temperature (around 800°C) is rapidly cooled, the crystal structure changes before the carbon dissolved in the iron precipitates, generating an extremely hard structure called "martensite." This forms the bright part of the hamon.

The thinly coated edge portion cools rapidly, allowing active martensitic transformation. In contrast, the thickly coated back portion cools more slowly, maintaining relatively softer microstructures such as pearlite and bainite. This difference in hardness is visualized as the "hamon."

The difference between nie and nioi also stems from tsuchigoki. The granular "nie" is related to the coarseness of martensite grains due to relatively rapid cooling, while the haze-like "nioi" arises from a continuous pattern of finer martensite grains. The balance between these two elements enriches the expression of the hamon and forms the foundation of its aesthetic value.

Tsuchigoki Styles by School — Tradition and Individuality

The five great schools of Japanese sword-making (Yamashiro, Yamato, Bizen, Soshu, and Mino) each have distinctive hamon styles, which reflect differences in tsuchigoki patterns.

• Bizen school: Complex hamon exemplified by choji-midare. Creating shapes resembling clove blossoms requires delicate control in the tsuchigoki process. The techniques refined by master smiths of the Bizen school, including those of the Osafune line, have been passed down to modern swordsmiths.
• Soshu school: Characterized by intense nie-type hamon with broad straight edges and large undulations. The hamon of Masamune and Noroshige is considered the pinnacle of the Soshu school, with a unique combination of tsuchigoki volume and rapid cooling.
• Yamato school: Features mostly pure, straight hamon patterns. The tsuchigoki outline is clear, with a robust, unadorned beauty that excludes unnecessary decoration.

Modern swordsmiths, too, build on the tsuchigoki techniques inherited from their masters while adding their own refinements. Tsuchigoki is a prime example of a technique that "appears simple on the surface but is profound in depth," requiring decades of practice to consistently produce hamon of professional quality. Even in the modern era of scientific analysis, its true essence lies in accumulated experience and sensibility, and its value as a traditional technique remains unshakeable.