Introduction

Tsuchioki (土置き) is the traditional clay coating process applied to a Japanese sword before yaki-ire (焼入れ, differential hardening). It plays a critical role in shaping the functional properties and aesthetic characteristics of a Japanese sword (nihontō, 日本刀). By carefully applying a specially prepared clay mixture, the swordsmith controls the cooling rate during quenching, ensuring that the cutting edge hardens to extreme sharpness while the spine remains tough and flexible.

Beyond its functional importance, tsuchioki is also an artistic practice, as the pattern in which the clay is applied determines the hamon (刃文, temper line)—a signature visual feature of Japanese swords. This essay explores the purpose, materials, techniques, and impact of tsuchioki on Japanese swords.

The Purpose and Importance of Tsuchioki

Tsuchioki is an essential step in the yaki-ire process, influencing both the mechanical properties and aesthetic appeal of the blade.

1. Controlling Hardness and Flexibility (Differential Hardening)

  • The cutting edge (hasaki, 刃先) is left exposed or lightly coated, allowing for rapid cooling during quenching. This creates martensite, an extremely hard steel structure, which gives the blade its legendary sharpness.
  • The spine (mune, 棟) and body (hiraji, 平地) are coated with thicker clay, slowing down the cooling rate, which results in a softer, pearlitic structure that enhances flexibility and impact resistance.

2. Preventing Warping and Cracking

  • Quenching a heated steel blade too quickly can cause severe warping or even cracking.
  • A proper clay coating insulates the steel, reducing thermal shock and controlling how the sword bends and curves during quenching.

3. Creating the Hamon (刃文, Temper Line)

  • The clay coating’s pattern directly determines the shape and visibility of the hamon.
  • Each swordsmith develops unique hamon designs, which act as a signature of their craft and school of forging.

4. Enhancing Blade Durability

  • The combination of hardness along the edge and softness along the spine ensures the blade can cut effectively without becoming brittle.

Thus, tsuchioki is both a functional and artistic technique, ensuring the perfect balance of performance and beauty in a Japanese sword.

Materials Used in Tsuchioki

The clay mixture used in tsuchioki is carefully formulated to adhere properly to the steel while providing effective thermal insulation during quenching.

Traditional Ingredients in Tsuchioki Clay

  1. Clay (土, Tsuchi)
    • Acts as the base material, providing adhesion and heat resistance.
    • Different swordsmiths use specific types of clay, often sourced locally.
  2. Charcoal Powder (炭粉, Sumiko)
    • Finely ground charcoal reduces heat conductivity, controlling how quickly the steel cools.
    • It also influences nie (沸, crystalline martensite formations) in the hamon.
  3. Silica or Stone Powder (石粉, Ishiko)
    • Improves adhesion and texture.
    • Helps stabilize the clay layer, preventing it from cracking or falling off before quenching.
  4. Iron Oxide (酸化鉄, Sanka-Tetsu, or 赤土, Red Earth)
    • Affects heat retention and hamon formation.
    • Often included in secret swordsmith formulas.
  5. Water (水, Mizu) or Natural Binders
    • Adjusts the clay’s viscosity to ensure proper thickness and even application.

Swordsmiths often adjust their clay formulas based on experience, and many keep their exact mixtures a closely guarded secret.

Application Process of Tsuchioki

1. Blade Preparation

Before applying the clay mixture, the sword must be properly cleaned and dried.

  • Any remaining oils or residues from forging are removed to ensure the clay adheres properly.
  • Some swordsmiths lightly heat the blade to improve adhesion.

2. Applying the Clay Coating

The clay is carefully applied in specific thicknesses to control the differential hardening process.

  • Thin Layer on the Edge (刃先, Hasaki)
    • A very thin layer (or no clay at all) is applied to the cutting edge.
    • This ensures rapid cooling during quenching, creating extremely hard martensite.
  • Thicker Layer on the Spine (棟, Mune) and Sides (平地, Hiraji)
    • A thicker layer of clay is applied to the spine and upper portions of the blade.
    • This insulates the steel, slowing down the cooling rate and preserving toughness and flexibility.
  • Creating the Hamon Pattern
    • Using a brush, spatula, or fingers, the swordsmith sculpts waves, peaks, or curves into the clay layer.
    • These patterns determine the final shape of the hamon, which can be:
      • Suguha (直刃, Straight Line) – A simple, straight hamon.
      • Midareba (乱れ刃, Irregular Waves) – A wavy, chaotic pattern.
      • Choji (丁子, Clove Blossom) – Round, flower-like shapes.
      • Togari (尖り, Pointed Hamon) – Sharp peaks resembling flames.

Once the clay is applied, the blade is left to dry completely before heating.