Forge Design and Equipment for Modern Swordsmiths: The Layout and Tools of a Traditional Japanese Sword-Making Workshop

What Kind of Space is a Modern Forge?

"Kajiba" (forge) is a specialized workshop where a sword craftsman forges tamahagane and creates the blade of a Japanese sword. Modern sword craftsmen are permitted to create swords only in certified manufacturing locations, based on the Cultural Affairs Agency's "Law Regarding the Manufacture and Possession of Japanese Swords as Art Objects" (the sword manufacture certification system). For this reason, the kajiba is not merely a "workshop," but rather requires a special spatial design where legal requirements, traditional forging methods, and the craftsman's physical movements intersect.

Basic Structure and Building Scale of the Kajiba

Traditional forges are typically single-story wooden structures. Modern forges sometimes use steel-frame or partially reinforced concrete buildings, but among sword craftsmen, the traditional belief that "spaces handling fire are best built from wood" remains strong. The reasoning is that wooden construction allows heat absorption and dissipation to occur naturally, resulting in gradual temperature changes in the work environment. Additionally, in the event of a fire, complete burning would limit the spread of flames—a paradoxical advantage.

The ideal floor area is a minimum of 30–40 square meters, ideally 50–80 square meters. Master workshops with multiple apprentices working simultaneously may exceed 100 square meters. Ceiling height is traditionally kept low (approximately 2.5–3 meters) to minimize spark scatter, though with the development of modern ventilation systems, slightly higher ceilings are now sometimes seen.

Position and Types of the Furnace

The heart of the kajiba is the "ro" (furnace), the heating kiln used to heat tamahagane. The furnaces used by modern sword craftsmen are classified into three main types:

Jiro (ground furnace): A traditional form in which the ground is dug out, charcoal is placed within, and air is blown in using bellows. Because the furnace sits below ground level, the sword craftsman naturally assumes a forward-leaning posture while working. Many traditionally-oriented workshops still use jiro furnaces today.

Dairo (elevated furnace): A furnace set on a raised platform above ground level. The working posture is more comfortable than with a jiro, reducing strain on the lower back. This type is increasingly adopted in workshops that prioritize apprentice training.

Electric furnaces and gas furnaces (auxiliary use): Some sword craftsmen use electric or gas furnaces as supplements for low-temperature processes such as tempering, but the main processes of forging and quenching must always use charcoal-fire furnaces. The Cultural Affairs Agency's certification requirements also specify charcoal-fire furnaces as the standard.

Furnaces are typically positioned in the center or back of the building. The ideal layout places the entrance, furnace, anvil, and water bucket in a straight line for efficient workflow.

Installation of Bellows and Treadle Bellows

The device that pumps air into the furnace is called "fuigo" (bellows). Before the early modern period, "tatara" (treadle bellows—a box-shaped air pump operated by foot) was the standard, but many modern workshops have replaced it with electric blowers. However, some craftsmen remain devoted to the traditional tatara, using it in apprentice training.

Electric blowers present noise and vibration issues. To consider nearby residences, modern designs increasingly place the blower outside the building in a soundproof box, connected to the furnace via duct.

Placement and Fixation of the Anvil

The "kanatsuko" (anvil) where the sword craftsman wields the hammer is positioned beside the furnace, within immediate reach by turning. The anvil is made of cast iron or forged steel, and its surface must be smooth. Small anvils weigh around 50 kg, while large ones may exceed 200 kg.

Traditional fixation methods include direct embedding in the floor or securing to a hardwood (such as zelkova) base. When directly fixed to the floor, the hammer's impact travels through the floor to the entire building structure. In modern designs, it is common to insert vibration-damping materials (rubber, sand bed) between the anvil and floor.

Water Bucket and Cooling Tank

The water bucket used for quenching (water hardening) is positioned adjacent to the furnace and anvil. Water temperature management is a critical factor in quenching quality, and many sword craftsmen have their own management methods, such as "maintaining a consistent water temperature even in deep winter" or "in summer, using well water or ice water to lower the temperature."

Water buckets have traditionally been made of wood (cypress or zelkova), and many modern craftsmen still prefer wooden buckets. Metal buckets are avoided because the potential trace ion leaching into the water could negatively affect quenching, according to empirical experience.

Ventilation, Smoke Exhaust, and Modern Regulatory Compliance

Smoke, carbon monoxide, and dust generated by the charcoal furnace have significant impacts on worker health and the surrounding environment. Modern kajiba must implement the following ventilation measures:

• Installation of smoke exhaust outlets: A large smoke hood is installed directly above the furnace, with a duct directing exhaust directly outside.
• Installation of ventilation fans: Exhaust fans are installed in high locations (near the roof) to provide forced ventilation.
• Constant CO detectors: CO detectors are recommended to be installed near the furnace to prevent carbon monoxide poisoning.

Additionally, to consider noise impact on neighboring residences, many workshops have established self-imposed regulations limiting hammer-working to daytime hours, with no work conducted at night.

Understanding the Kajiba Deepens Sword Appreciation

Understanding the sword craftsman's kajiba provides essential context for appreciating Japanese swords as works of art. The traces of fire-forging marked on the blade, the hamon's shape determined by the water temperature during quenching, the thickness and curve of the blade formed by the rhythm of hammer on anvil—all of these are the product of a specific workspace, the kajiba. When holding a sword, imagining the craftsman's workspace behind it represents the first step toward true appreciation.