Introduction

The process of forging a Japanese sword (nihontō, 日本刀) begins long before the hammer strikes hot steel. One of the most critical preparatory steps is the assembly and preparation of the steel block (billet), which sets the foundation for the blade’s strength, flexibility, and cutting performance. This involves selecting the right steel, stacking, welding, and preparing the material for the forging process.

A well-prepared billet ensures that the sword will have a homogeneous grain structure, proper carbon distribution, and high durability. This essay explores the materials, techniques, and procedures used in assembling and preparing the steel billet before forging a traditional Japanese sword.

The Importance of Proper Billet Preparation

The quality of the billet determines the final strength, resilience, and sharpness of the blade. Proper preparation ensures:

  1. Purification of the Steel
    • Impurities and excess carbon are removed through the folding and forging process.
    • The steel’s composition is homogenized for even hardness and toughness.
  2. Layering for Structural Integrity
    • Traditional Japanese swords use a layered steel construction, providing a balance of hardness and flexibility.
    • A high-carbon steel (kawagane, 皮鉄) outer layer is used for the cutting edge, while a low-carbon core (shingane, 芯鉄) provides shock absorption.
  3. Reduction of Weak Points
    • The folding and layering process eliminates gas pockets and slag inclusions, reducing the risk of cracks or breaks in the final blade.

By meticulously preparing and assembling the billet, the swordsmith ensures that the blade will be highly functional, durable, and visually beautiful.

Step-by-Step Process of Preparing the Steel Block (Billet) for Forging

1. Selecting the Steel (Tamahagane, 玉鋼)

The best Japanese swords are forged from tamahagane (玉鋼, jewel steel), a traditional steel made in a tatara furnace.

  • Tamahagane is carefully sorted based on its carbon content:
    • High-carbon steel (kawagane, 皮鉄, “skin steel”) – Used for the cutting edge.
    • Low-carbon steel (shingane, 芯鉄, “core steel”) – Used for the inner core for toughness.
    • Medium-carbon steel – Sometimes used to transition between kawagane and shingane.

Each piece of steel is visually and chemically inspected to ensure ideal carbon distribution.

2. Cutting and Stacking the Steel

Once the appropriate pieces of tamahagane are selected, they are cut into smaller sections and stacked in preparation for welding and folding.

  • The steel is arranged in layers, with each piece carefully aligned to maximize structural integrity.
  • The stack is typically 5–10 cm high, depending on the size of the final blade.
  • If the swordsmith is using a laminated construction technique, different layers of high-carbon and low-carbon steel will be placed strategically.

Laminated Construction Methods:

  • Kobuse (甲伏せ) – Soft steel core with hard steel jacket.
  • Sanmai (三枚) – Three-layered steel with a hard cutting edge and softer sides.
  • Shihōzume (四方詰め) – A four-layer construction surrounding a tough core.

Once stacked, the billet is wired together to keep the layers aligned before welding.

3. First Forge Welding (Tsumi-wakashi, 積み沸かし)

The stacked steel is placed into the forge and heated to around 1,300°C (2,372°F). This is a critical stage where the layers are welded together into a single solid block.

  • The swordsmith adds straw or powdered charcoal to the fire, creating a reducing atmosphere that prevents oxidation.
  • When the steel reaches welding heat, it is carefully hammered to fuse the layers together.
  • This process eliminates air pockets and impurities, ensuring a strong, cohesive billet.

At this stage, the first impurities are expelled, and the block is now a solid mass of forge-welded steel.

4. Folding and Refining the Steel (Orikaeshi Tanren, 折り返し鍛錬)

Once the billet is welded into a single piece, it undergoes multiple folding cycles to refine its internal structure.

Folding Process:

  1. The billet is heated to forging temperature (~1,200°C).
  2. The smith hammers the billet into a long bar.
  3. The bar is cut in half, stacked, and welded together.
  4. This folding process is repeated 10–15 times, creating a layered structure with thousands of layers.

Each fold reduces impurities, homogenizes the steel, and refines the grain structure (jihada, 地肌), forming unique patterns in the final blade.

5. Shaping the Final Billet (Sunobe, 素延べ)

After sufficient folding, the billet is hammered into a rectangular shape, ready for the next phase of forging.

  • The swordsmith ensures that the carbon distribution is even.
  • The billet is adjusted for length and thickness, preparing it for forging into the final blade shape.
  • If a laminated core (shingane) is used, the outer kawagane is wrapped around it before final welding.

At this stage, the steel block is fully prepared for the actual forging process, where it will be stretched and shaped into a sword.

The Role of Experience in Billet Preparation

The quality of a sword starts with its billet. A skilled swordsmith must:

  • Carefully select and sort the tamahagane, ensuring the right balance of hard and soft steel.
  • Control the number of folds, as too many can make the steel brittle, while too few leave impurities.
  • Monitor carbon levels, preventing the blade from being too soft or too hard.

Each school of swordsmithing (e.g., Bizen, Soshu, Yamato, Mino) has its own traditions and methods for assembling and preparing the billet