Introduction

Forging a Japanese sword (nihontō, 日本刀) is one of the most complex and revered metalworking processes in the world. It is a blend of science, craftsmanship, and tradition, requiring not only technical skill but also artistic intuition and spiritual dedication. The forging process is far more than simply shaping steel—it involves layering, folding, differential hardening, and precise hammering to create a blade that is exceptionally sharp, resilient, and uniquely beautiful.

This essay explores the intricate steps involved in forging a Japanese sword, including material selection, the forging and folding process, shaping, differential hardening, and final refinement.

The Purpose and Importance of Traditional Sword Forging

The traditional forging process improves the strength, durability, and cutting performance of the sword. It achieves:

  1. Purification of the Steel
    • Repeated folding removes impurities and excess carbon, refining the steel’s microstructure.
  2. Differentiated Hardness for Strength and Flexibility
    • The process creates a hard cutting edge while maintaining a tough, flexible spine, preventing breakage.
  3. Unique Beauty
    • The folding process forms the jihada (地肌, grain pattern), which is unique to each sword and enhances its aesthetic appeal.
  4. Spiritual and Artistic Mastery
    • Swordsmiths see their work as a spiritual practice, ensuring that each blade carries a sense of honor, discipline, and tradition.

Step-by-Step Process of Forging a Japanese Sword

1. Selecting and Preparing the Steel (Tamahagane, 玉鋼)

The best Japanese swords are forged from tamahagane (玉鋼, jewel steel), a high-purity steel produced in a tatara (踏鞴) furnace.

  • Tamahagane is selected based on carbon content:
    • High-carbon steel (kawagane, 皮鉄) is used for the cutting edge.
    • Low-carbon steel (shingane, 芯鉄) is used for the core, adding toughness.
  • The swordsmith carefully inspects and sorts the steel before heating it.

2. Folding and Layering the Steel (Orikaeshi Tanren, 折り返し鍛錬)

Once the steel is prepared, it undergoes a repetitive heating, hammering, and folding process to remove impurities and create a strong, layered structure.

Step-by-Step Folding Process:

  1. The steel is heated in a forge until it reaches approximately 1,200°C (2,192°F).
  2. It is hammered flat, removing slag and inconsistencies.
  3. The blade is folded in half and welded together, effectively doubling the number of layers.
  4. This process is repeated 10–15 times, creating thousands of layers.

Each fold purifies the steel further and enhances its durability.

  • The folding pattern creates visible steel grain (jihada, 地肌), which becomes a distinctive feature of the sword.
  • Depending on the number of folds and technique, the jihada can have different patterns, such as:
    • Itame-hada (板目肌) – Wood grain-like texture.
    • Mokume-hada (杢目肌) – Burl wood-like swirls.
    • Masame-hada (柾目肌) – Straight, parallel lines.

At the end of this stage, the steel contains the perfect balance of carbon and purity, making it ideal for forging into a blade.

3. Constructing the Blade (Kitae, 鍛え)

Japanese swords are made using either a single-steel method or a laminated (composite) construction, where different steels are combined.

Laminated Blade Construction Techniques:

  1. Kobuse (甲伏せ) – A soft core steel is wrapped in a harder outer layer.
  2. Sanmai (三枚) – A three-layered structure with a hard cutting edge and softer core sides.
  3. Shihōzume (四方詰め) – A complex four-part structure, maximizing strength and resilience.

The forging process ensures that the blade has the best combination of hardness, flexibility, and toughness.

4. Shaping the Blade (Sunobe, 素延べ)

At this stage, the swordsmith begins to shape the blade into its final form.

  • The heated steel is hammered into the desired length and curvature.
  • The basic geometry is established, including:
    • The shinogi (鎬, ridge line).
    • The mune (棟, spine).
    • The kissaki (切先, tip).
  • Any excess material is removed, and the sword’s curvature is adjusted to match its intended design.

Once the shape is refined, the blade undergoes tsuchioki (土置き, clay coating) in preparation for the differential hardening process.