The Sword-Making Process of the Koto Period: From Forging to Finishing

The Koto Period—The Golden Age of the Heian, Kamakura, and Nanboku-chō

Koto (古刀) refers to Japanese swords made before the Keichō era (before 1596). The tachi made particularly from the late Heian through the Kamakura and Nanboku-chō periods are still highly valued today as representing the pinnacle of technical achievement in Japanese sword history.

The five great traditions of sword-making established by smiths of this era—Yamashiro-den, Yamato-den, Bizen-den, Sōshū-den, and Mino-den—each developed their own distinctive forging methods and hardening techniques, profoundly influencing generations of craftsmen to come. This article traces the representative tachi-making process of the koto period step by step.

Selection of Tamahagane and Material Preparation

The first step in tachi production begins with the selection of tamahagane, steel refined from iron sand. Tamahagane obtained through tatara iron-smelting varies greatly in properties depending on its carbon content. Drawing on experience and intuition, smiths distinguish between high-carbon steel (for the outer layer) and low-carbon steel (for the core), selecting materials suited to their specific purposes.

After sorting, the tamahagane is broken into small pieces, stacked on a flat iron plate (bōshin), and covered with Japanese paper and straw ash. The steel is then heated in this state through a process called "tsumijakashi"—bonding the steel into a unified mass—marking the first forging stage.

Folding and Refining—Uniform Steel and Impurity Removal

The steel mass obtained through tsumiakashi is refined through "orikikaeshi-kitae"—repeatedly folding and hammering the hot steel to extend it. Through this process, impurities (slag) are expelled, carbon distribution becomes uniform, and the steel's structure becomes denser.

The representative forging patterns of the koto period include the following:

• Itame-kitae (板目鍛え): Wood-grain-like patterns created by repeated folding, common in Yamato-den and Yamashiro-den
• Masame-kitae (柾目鍛え): Linear patterns created by folding in one direction, distinctive of Yamato-den koto
• Mokume-kitae (杢目鍛え): Complex patterns mixing itame and masame, common in Bizen-den and Sōshū-den

The number of folds is typically around 10 to 15, but too many folds cause excessive carbon loss and weaken the blade, so balance is critical.

Tsukurikomi—Combining Outer and Core Steel

One of the most important technical features of koto tachi is "tsukurikomi." By combining hard high-carbon steel (outer layer) with softer low-carbon steel (core), a structure is achieved that balances hardness and toughness in the blade.

Representative types of tsukurikomi:

• Kōbuse (甲伏せ): The most basic structure, where core steel is wrapped with outer steel, used in Yamato and Yamashiro traditions
• Hon-sanmai (本三枚): A three-layer structure with separate steel applied to the back of the blade
• Shihō-zume (四方詰め): The most refined structure, with all four sides wrapped in outer steel, common in superior Sōshū-den works

The tsukurikomi process demands the smith's greatest skill and judgment, since the combination of materials critically affects the finished blade's performance.

Sunobe and Blade Shaping

After tsukurikomi is complete, the blade undergoes "sunobe"—heating and hammering to extend the form. The overall shape of the blade—length, curve, width, and thickness—is determined during this process.

Koto tachi are characteristically marked by "koshi-zori"—a pronounced curve from the hilt to the middle of the blade—though the exact curve varies subtly across different periods and schools. The smith shapes the blade by carefully observing the color of the flames and the steel's condition, refining the form with each precise hammer blow.

At this stage, the shinogi (ridge), mune (back), jihada (surface patterns), and hamachi (edge section) are also formed, establishing the blade's basic shape.

Tsuchioki and Quenching—Creating the Hamon

The step that modern experts and enthusiasts most closely examine in koto production is the "yaki-ire" or quenching process. The "hamon"—the hardening pattern formed on the blade's surface—most distinctly reveals each school's distinctive character.

Before quenching, the smith applies a layer of refractory clay (yaki-ba-tsuchi) over the entire blade in a process called "tsuchioki." By applying thin clay to the edge and thick clay to the spine, the smith intentionally creates a temperature differential during quenching.

Characteristics of hamon by school:

• Yamashiro-den: Fine straight hamon (hososuguha), small interlocking patterns (kogunome), and other refined, elegant patterns
• Yamato-den: Straight hamon (suguha) as the standard. Fine nioe patterns, often accompanied by deep "mune-yaki" (back tempering) extending toward the spine
• Bizen-den: Clove-blossom patterns (chōji-midare), with abundant variation as its defining characteristic
• Sōshū-den: Large irregular patterns (ōmidare) and undulating hamon (notare), with abundant activity including sand-stream and gold-line effects

After tsuchioki, the blade is heated to approximately 800–900°C, then quickly plunged into water. This rapid quenching hardens only the edge while the spine retains its flexibility and toughness. A single mistake in this crucial moment causes the blade to crack, and there is no possibility of repair.

Polishing and Finishing—Honing with Whetstones

After quenching is complete, the blade is sent to a polisher (togishi) for finishing. Even in the koto period, the division of labor between smith and polisher was standard practice.

Polishing proceeds through multiple stages—from rough grinding to final finishing—using different whetstones to achieve a mirror-like brilliance. Koto-era polishing techniques remain highly regarded today, and the whetstones used by period polishers—such as nakanagura-to and uchigumori-to—are still prized as the finest polishing stones.

Koshirae and Mei—Completion as a Tachi

After polishing, the signature (mei) is cut into the blade. Koto-era signatures were cut with careful attention to the edge direction (verified by compass), which is why modern experts place such emphasis on signature position and style in authentication.

Finally, the blade receives its "koshirae"—a hilt, guard (tsuba), habaki, menuki, and other fittings, along with a scabbard—completing it as a tachi. Koto-era tachi fittings are called "tachi-koshirae," worn suspended from the waist blade-down in the "haitō" style.

Koto tachi-making—with its spirit and techniques passed down to contemporary smiths—remains revered worldwide as a concentrated expression of Japanese sword's essential beauty and functional elegance.