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

Fire in Japanese Sword Making——Temperature Control Determines Quality

The essence of sword forging is "the art of fire." Tamahagane is heated to the proper temperature, shaped by striking, and then rapidly cooled and hardened. In every step of this process, how well one controls the temperature of the fire and the oxidizing and reducing atmosphere fundamentally determines the quality of the blade. The bellows (fuigo) and charcoal (sumi) are responsible for this control of fire.

The furnaces used by Japanese sword smiths are called "kaji-ro" (forges). There are two types: the "ji-ro" (pit furnace) dug into the ground and the "dai-ro" (elevated furnace) installed on a stand. In modern times, the elevated furnace is the standard, but in tatara iron production, vast pit furnaces were used.

The Structure and Historical Evolution of the Bellows

The bellows are the air pump that sends air into the furnace and play a central role in fire management for Japanese sword forging. Historically, they have undergone the following evolution.

Hako-fuigo (Box Bellows): A wooden box-type blower widely used through the Edo period. Air is pumped by pulling and pushing a piston with a leather interior. Often operated by two people working in tandem, requiring coordinated movements of "right pull, left push." Air volume was controlled by the speed and depth of the pushing and pulling, and the skill of the experienced assistant (tanteki) directly affected the furnace temperature.

Toyo-fuigo (Trough Bellows): A trough-type blower made from bamboo or wood split in half, a form transmitted from China. It had a simpler structure and better portability than box bellows, but lacked stability in air volume.

Tenbin-fuigo (Seesaw Bellows): An improved bellows that became popular in the late Edo period, using the principle of a seesaw to move pistons on both sides alternately, achieving continuous airflow. It became possible for one person to operate it alone, reducing dependence on assistants.

Modern Blowers (Electric Air Pumps): Since the Showa period, many sword smiths have adopted electric blowers. Wind volume is more stable compared to manual bellows, making it easier to maintain consistent temperature. However, some sword smiths who emphasize tradition continue to use manual bellows, and the culture of "reading the fire through hand sensation" remains in some areas.

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

Traditionally, charcoal (mokutan) is used as fuel in the forging furnace; coal and coke became common in Western-style sword forging after the Meiji period, but charcoal remains fundamental to traditional Japanese blade making. The type of charcoal directly affects furnace temperature, burning time, and ash characteristics.

Binchotan: The highest quality charcoal, made by firing Ubame oak at high temperature. It has strong heat output and long-lasting combustion. High in carbon content with minimal ash. Well-suited for the finishing stages of forging and precise heating of the blade edge. Due to its high cost, few sword smiths use it for all stages; most reserve it for critical stages only.

Shiro-zumi (White Charcoal): Charcoal made by firing hardwoods other than Ubame oak at high temperature, second in quality to binchotan. Produces hard, even combustion.

Kuro-zumi (Black Charcoal): Soft charcoal fired at relatively low temperature; easy to ignite and inexpensive. Often used for initial heating (the stage of beginning to heat the steel) and bulk heating. Because it burns quickly and produces much ash, it is unsuitable for extended forging work.

Matsu-zumi (Pine Charcoal): Charcoal made from pine wood with high oil content and excellent ignition properties. Used in traditional forges in some regions. Because of its high oil content, some believe it has a "case-hardening effect" that supplies carbon to the steel surface.

Charcoal Fire and Steel Carbon Management——Oxidation, Reduction, and Case Hardening

When charcoal burns, it generates a mixture of carbon monoxide (CO) and carbon dioxide (CO₂). Strong bellowing with the fuigo increases oxygen and creates an oxidizing atmosphere, causing carbon on the steel surface to burn away in a process called decarburization (dattan). Conversely, by reducing the bellowing and allowing incomplete combustion, carbon monoxide increases and creates a reducing atmosphere (kangenふんいき), which may cause carbon to be supplied to the steel (case hardening).

Whether Edo-period sword smiths understood this principle scientifically is unclear, but from the oral traditions of various schools, it is evident they empirically read the furnace atmosphere from "the color of the fire," "the color of the steel," and "the rate of charcoal consumption," controlling it through the speed of bellows operation. The transmission that "when yellow-white sparks fly, carbon is being lost" was wisdom gained through empirical observation of decarburization.

Modern Forge Management——The Fusion of Tradition and Science

Most modern sword smiths manage furnace temperature by combining electric blowers with thermometers (such as infrared sensors). On the other hand, the Japanese Art Sword Preservation Society (NBTHK) requires "traditional methods" in its certification, with the traditionality of materials and processes being a prerequisite for certification, so complete mechanization has limitations.

Regarding charcoal, the producing regions for binchotan (Wakayama, Mie, and Kochi) are declining, making stable procurement of high-quality charcoal a challenge for modern sword smiths. Some sword smiths learn charcoal-making themselves and produce their own specialized charcoal. The ancient question of selecting bellows and charcoal remains at the heart of sword-forging technique in the modern era.