Tsuchitori Clay Mixing Techniques: Ingredients, Mixing Ratios, Drying Process, and Their Effect on Hamon Formation

What is Tsuchitori—The Preparatory Process that Determines Hardening

When discussing Japanese sword hardening (yaki-ire), attention often focuses on "water temperature" and "the moment of hardening." However, what fundamentally determines the shape of the hamon is the composition and application method of the "clay (tsuchi)" applied to the blade before hardening. The process of making this clay is called "tsuchitori" (clay preparation).

Tsuchitori may appear simple at first glance, but it is an extremely demanding process requiring considerable technical skill and experience. The clay's materials, particle size, moisture content, mixing ratios, and drying state after application directly influence the hamon's width, activity, and nie formation. To be precise, tsuchitori is the process of creating the blueprint for the hamon, with the sword smith's intended hamon design condensed into the clay's composition.

Basic Raw Materials for Clay—Three Types of Components

Traditional Japanese sword clay is generally composed of three primary types of materials.

Whetstone powder (toishi-ko): Powder from whetstones used by sharpeners (primarily medium to fine-grain whetstone powder). The primary material that provides adhesion and insulating properties to the blade. The particle size varies depending on the type of whetstone and affects the heat distribution during hardening. Using larger particle whetstone powder increases the insulating effect of the clay, tending to produce a hamon that spreads less easily and forms a stable shape.

Charcoal powder (mokutan-ko): Pulverized charcoal. It primarily functions as a buffer (cushioning material) for the rapid heating and cooling during hardening, and plays a role in adjusting the clay's shrinkage. The particle coarseness and water absorption vary depending on the charcoal type (pine, bamboo, oak, etc.), and sword smiths have their preferred charcoal types.

Clay (nendo): Natural clay-based soil. It functions as a "binder" in tsuchitori, adhering the whetstone powder and charcoal powder to the blade. The shrinkage rate varies depending on the clay's origin and quality, so even when mixed at the same ratios, the result may differ. Some sword smiths insist on clay from particular regions, which is passed down through generations as "the sword smith's secret clay formula."

Composition Ratios—Traditions and Individual Secrets

The clay composition ratio is considered a secret formula unique to each sword smith, and in many cases is not disclosed. However, the following general trends are known.

A general compositional framework is approximately "whetstone powder : charcoal powder : clay = 5-7 : 1-2 : 2-3 (by weight)," but this is merely a rough guideline, and each sword smith's composition adjusted from years of experience varies widely.

Sword smiths who employ "thick clay (atsu-tsuchi)" application during hardening and those who use "thin clay (usu-tsuchi)" applied uniformly have fundamentally different objectives for the hamon's width and activity. When aiming for the choji-midare (clove-leaf pattern) of the Bizen tradition, precise placement of specific clay only on the blade edge is required, while when pursuing the o-nie (large crystalline pattern) of the Soshu tradition, there is a tendency to combine thin clay application overall with rapid hardening.

Mixing and Drying—The Process that Tests Craftsmen's Intuition

When mixing the clay, the sword smith first measures appropriate quantities of each material, then gradually adds water while mixing. Control of the water amount is particularly important—the phrase "slightly firmer than an earlobe" is often used in craftsmen's terminology. Too much moisture causes the clay to drip after application to the blade and becomes prone to cracking during drying. Too little weakens adhesion to the blade and increases the risk of flaking during hardening.

The prepared clay is carefully applied to the blade using a brush or fingers. The basic principle is to apply it thinner near the edge (or with a unique pattern on the edge itself) and thicker near the spine (mune), but exactly how thick each section should be is entirely at the sword smith's discretion. Whether the hamon forms a gunome (eye pattern) or choji-midare is determined by this differentiated clay application.

Drying after application is also carefully managed. Rapid drying causes internal stress in the clay and leads to cracking, so natural drying (several hours to overnight) is the principle. Direct sunlight and strong hot air are strictly forbidden, and in modern workshops where temperature and humidity control of drying rooms is possible, appropriate environmental control is sometimes performed.

The Science of Clay—Insulation and the Physics of Hardening

Explaining scientifically the impact of clay on hamon formation comes down to "controlling rapid cooling through the clay's insulating effect." During hardening, the blade edge where clay is thin (or not applied) rapidly cools upon contact with water, forming a hard metallic structure called martensite. This is the part that appears visually as the hamon.

Conversely, the spine and back (ji) area where clay is applied thickly cool more gradually, with hardening suppressed and a resilient structure remaining. The difference in these two cooling rates is the fundamental mechanism that creates the hamon boundary. Differences in the clay's particle size, density, and thickness create subtle variations in cooling speed, which appear as nie, nioi, and the activity of the hamon.

From the perspective of modern materials science, clay composition is a "localized cooling rate design," and the sword smith's empirical knowledge is actually based on sophisticated thermodynamic intuition. The complex phase transformations occurring at the moment of hardening have been controlled by sword smiths using centuries of transmitted empirical knowledge. The profound essence of tsuchitori lies at the intersection of Japanese sword science and craftsmen's knowledge.