Fuigo and Forge Fuel: How Bellows Technology and Charcoal Selection Define Sword Quality

The Fire of Japanese Sword Smithing—Temperature Control Determines Quality

At the heart of sword forging lies "fire technology." Tamahagane is heated to the appropriate temperature, shaped by striking, and then quenched and hardened through rapid cooling. In all of these processes, how temperature and the atmosphere of oxidation and reduction are controlled fundamentally determines the quality of the blade. It is the bellows (fuigo) and charcoal (sumi) that take charge of controlling this fire.

The furnaces used by Japanese sword smiths are called "kajiiro" and come in two types: the "jiro" dug into the ground and the "dairo" installed on a platform. In modern times, the dairo is the mainstream, whereas extensive jiro furnaces were used in tatara iron production.

The temperature required for each part of the blade is not uniform. In the stage of folding and forging, it is necessary to heat the entire steel uniformly, but in the stages of sunobe and hizukuri, localized heating is required with awareness of the temperature difference between the edge and the core steel (jigane). The skillful use of the "fire aspect (kasou)" within the forge—which position to place charcoal and where to insert the steel—is one of the techniques passed down from master to apprentice through oral transmission.

Structure and Historical Evolution of Bellows (Fuigo)

The bellows (fuigo) is a blower that sends air into the furnace and plays a central role in managing the heat of Japanese sword forging. Historically, it has undergone the following evolution:

• Hako-fuigo (box bellows): A wooden box-type blower widely used until the Edo period. Air is sent by pushing and pulling a leather-covered piston inside. They were often used by two people in pairs, requiring the coordinated action of "right pull, left push." Air volume was adjusted by the speed and depth of the push-pull motion, and the skill of a trained assistant (tanteki) directly affected the furnace temperature.
• Toyo-fuigo (trough bellows): A trough-shaped blower made by splitting bamboo or wood in half, a form transmitted from China. Simpler in structure and more portable than box bellows, but inferior in wind volume stability.
• Tenbin-fuigo (balance bellows): An improved type that became popular in the late Edo period, using the principle of a balance beam to alternately move pistons on both sides to achieve continuous air flow. One person could now operate it, reducing dependence on assistants.
• Modern blower (electric fan): From the Showa period onwards, many sword smiths adopted electric blowers. Wind volume became stable and constant temperature maintenance became easier, but among sword smiths who prioritize tradition, some still insist on manual bellows. The culture of "reading fire through hand sensation" continues to persist in some quarters.

The technique of handling bellows was once considered one of the basic movements to be mastered in the early stages of apprenticeship. To stabilize the heat, continuous air flow at a regular rhythm was necessary, and it is said that within this seemingly monotonous work, the eye for observing changes in furnace temperature was cultivated. Even today, while electric blowers serve as the main tool, some sword smiths switch to manual bellows only when delicate temperature adjustments are required, such as in finishing or just before hardening, and many workshops make use of both types.

Types of Charcoal and Selection Criteria—How Charcoal Differences Affect Steel Properties

Charcoal (mokutan) is traditionally used as fuel for forging furnaces. Coal and coke became popular in Western sword forging from the Meiji period onwards, but charcoal is the foundation of traditional Japanese sword manufacturing. The type of charcoal directly affects furnace temperature, burning time, and ash properties.

• Binchotan: The highest quality charcoal made by firing Ubamegashi at high temperature, with strong heat and sustained long-term burning. High carbon content and low ash. Well-suited for the finishing stage of forging and precision heating of the blade tip, but high price means it is often used only for important processes.
• Shiro-sumi (white charcoal): Charcoal made by firing hardwoods other than Ubamegashi at high temperature, with quality second to binchotan. Provides firm, uniform burning.
• Kuro-sumi (black charcoal): Soft charcoal fired at relatively low temperature, easy to ignite and inexpensive. Used for initial heating or bulk heating, but unsuitable for prolonged forging due to rapid burning and high ash content.
• Matsu-sumi (pine charcoal): Charcoal made from pine wood with high oil content and good ignition properties. Used in traditional forging furnaces in some regions, with some saying it has the effect of "carburization," supplementing carbon on the steel surface.

Charcoal is not thrown into the furnace as is, but rather is broken into appropriately sized pieces and used selectively according to the process. If left in large chunks, the flame becomes uneven; if crushed too finely, burning becomes too fast and temperature becomes unstable. Also, charcoal loses ignition properties and burning uniformity when it absorbs moisture, so storage in a dry state is also considered part of heat management. It can be said that not only the quality of the charcoal but also how it is handled reflects the skill of the sword smith.

Fire and Steel Carbon Management—Oxidation, Reduction, and Carburization

When charcoal is burned, a mixture of carbon monoxide (CO) and carbon dioxide (CO₂) is generated. Strong blowing with the bellows increases oxygen, creating an oxidizing atmosphere, causing carbon on the steel surface to burn and decarburization (dattan) to occur. Conversely, reducing the blowing to create incomplete combustion increases carbon monoxide, creating a reducing atmosphere, which can result in carbon supply to the steel (carburization).

It is unclear whether Edo-period sword smiths scientifically understood this principle, but it can be inferred from the oral traditions of various schools that they empirically read the atmosphere within the furnace from the "color of the fire," "color of the steel," and "rate of charcoal consumption," and controlled it through the pace of bellows operation. The traditional saying "when yellowish-white sparks fly, carbon is flying" was wisdom gained from empirically sensing decarburization.

Carbon management through charcoal is closely linked to the hardening process. By placing a thin layer of clay on the blade edge and a thick layer on the core steel section in a process called "tsuchiyoki" before hardening, the blade edge undergoes rapid cooling to become hard, while the core steel cools gradually and retains flexibility. This hardness/softness difference forms the foundation for creating hamon, but as a prerequisite, the overall carbon content of the steel must be properly managed during the heating stages with bellows and charcoal. Heat management and tsuchiyoki are understood as a series of techniques for achieving both sharpness and resilience.

Modern Forge Management—Fusion of Tradition and Science

Most modern sword smiths manage furnace temperature by combining electric blowers with infrared temperature sensors. On the other hand, the judging criteria of the Japanese Art Sword Preservation Society (NBTHK) require "traditional methods," with the traditionality of materials and processes serving as a prerequisite for certification, so there are limits to complete mechanization.

As of 2026, binchotan production is declining in its main production regions (Wakayama, Mie, Kochi), and securing stable supplies of high-quality charcoal has become a challenge for sword smiths. Some sword smiths have begun learning charcoal-making themselves to produce specialized charcoal in-house.

Changes in temperature and humidity also affect the burning state of charcoal, and the sense of fine-tuning air volume and charcoal amount according to season and weather is said to be something that cannot be completely replaced by measurement instruments alone. This kind of difficult-to-quantify experiential knowledge continues to be largely transmitted through oral tradition between master and apprentice and through hands-on practice. The challenge of choosing bellows and charcoal, dating back to ancient times, remains at the heart of sword forging technique in the modern era. It is now an age when fine swords can be carefully examined and purchased online—please feel the dedication of each sword smith to their craft and materials through their works.