Traditional Japanese swordsmithing forges are purpose-built with certain design elements specific to the tasks involved in tanren and hizukuri. This article will discuss several of the features that are common to this type of construction and follow the process of crafting a swordsmith’s forge at a museum on Vancouver Island.
Most of history was forged with very simple equipment made from found and natural materials. A basic charcoal forge can be made with clay, brick, or even mud and stone.
Sword Forge Overview
The swordsmith forge consists of a fuigo box bellows separated from the forge by a low wall, a firepot consisting of two parallel walls to contain the fire, and a charcoal reservoir where fresh charcoal is stored for use.
The bottom half of the firepot area is filled with fine charcoal to provide an insulated bed for the fire and extends under the charcoal reservoir to provide an area to push longer blades into while heating. Often there is a hood and chimney above the forge but the oldest examples are left open and smoke is vented through windows and roof openings.
The smith is positioned so that everything is within reach and the fuigo, forge, and anvil are located on radiating lines from a central location. In order to lower the anvil relative to assistants and apprentices acting as sakite (strikers), the smith works from a sitting position at or near ground level. The most basic examples involve half-kneeling on a mat flat on the ground but many permanent installations include a shallow pit for the feet to rest in.
Although several options exist, from the most basic found mud and stone forges through traditional clay and charcoal refractory to a modern castable refractory material, for this project hard firebrick will be the material of choice. Firebrick can handle the high heat and temperature extremes of forge work, is relatively easy to build with, and can be reconfigured to make adjustments as necessary. Regular fireplace bricks can be used but will not last as long as firebrick, regular clay brick will not handle the heat at all unless it is coated inside with some type of fireclay mixture.
The outside of the forge and the charcoal reservoir will not be exposed to high temperatures and can be made from any type of clay or brick, in this case a mixture of salvaged house and fireplace brick. The pipe from the fuigo to the forge is steel and the tuyere on the end of it is made from a refractory mixture of charcoal powder and clay.
For the support wall, brick, concrete blocks, or sheet metal can be used, for this project a traditional style frame of salvaged wood and driftwood will be inlaid with bamboo lath and filled with an earthen mixture of natural clay, sand, and straw. The hood is a repurposed piece of agricultural or mill equipment hung from the ceiling at the outside corners.
The fuigo wall separates and protects the wooden fuigo from heat and sparks and also holds up one side of the hood. It should be constructed of a non-flammable and somewhat heat resistant material. The wood frame provides structure and the bamboo lath provides anchor points for the earthen mixture to hang on to.
The frame wall is made from driftwood and salvaged lumber, and the bamboo lath is held in place in slots or holes in the wood frame. A minimum of one vertical and one horizontal row of bamboo are tied together using twine. If available, the traditional style often calls for larger split bamboo and twice as many horizontal pieces, about 1 sun apart rather than two or three.
The back section of the forge is divided into upper and lower parts at the level of the floor. The upper part will be the charcoal fuel reservoir where fresh charcoal can be raked into the fire, or burning charcoal can be temporarily pushed when replacing the billet into the fire. The lower part will be filled with charcoal fines and provides a space to push the blade into when heating without pushing away the fire with it. The lower part is the width of the forge and the upper part is one brick wider on each side.
Clay, sand, and chopped straw can be combined to form several different earthen mixtures, from a thin spreadable plaster to a thick cob style material. There should be enough clay to stick the mixture together, sand and straw can vary quite a bit in their proportion. This traditional craft playlist provides a look at several different tsuchikabe and wara juraku techniques.
The steel tuyere pipe connects the fuigo to the tuyere in the bottom center of the left side of the firepot. It generally slopes slightly but the main thing is that the actual tuyere should point downwards into the firepot at about 30 degrees (this varies according to the school). The location of the tuyere is midway between the front of the forge and the charcoal reservoir, and midway between the top and bottom of the forge walls, at the level of the floor.
The tuyere pipe is around 1.75 to 2 sun diameter and cut from sturdy steel pipe. Often a kink or bend is added to the pipe so that the infrared from the charcoal does not shine directly on the inside of the fuigo.
The firepot contains the burning charcoal and must endure high temperature. The width should be appropriate to the majority of work performed, too wide and fuel is wasted, too narrow and larger high temperature work becomes difficult. The width is usually between 6.5 sun and 7.5 sun but knifemakers can use 4-5 sun or insert spacer bricks for smaller work. The standard length is 3 shaku and the peak height is about 2 shaku.
The forge floor beneath a full depth charcoal bed should not normally be exposed to high heat but should certainly be fireproof. Earth, bricks, steel plate, or even reclaimed concrete blocks may work. Using brasque, refractory mixture, kiln shelf, or firebricks would be a better option in a shallower or smaller forge design.
The back of the forge is divided by a steel shelf which holds fresh charcoal ready to be raked into the fire, and holds burning charcoal temporarily when replacing a large billet into the fire during tanren. because the charcoal reservoir is wider than the forge, some sort of angle cut bricks or steel “ears” help channel the charcoal into the firepot.
The tuyere (羽口 haguchi) must be made from refractory material to withstand the intense heat in this area of the forge. A common recipe uses sand, clay, and castable refractory with a very small amount of water. For this project traditional brasque fireclay made from charcoal powder and clay will be used. There are several recipes but the most basic is 6 parts charcoal powder to 2 parts clay with just enough water to make it sticky.
The tuyere should convert the ~2 sun pipe diameter into a 1 sun opening, increasing the velocity of the air just before entering the fire. The walls should be close to 1 sun thick and the tuyere protrudes into the forge between 0.5 sun and a little less than 1 sun. The area around the tuyere is packed with similar material to seal the opening in the sidewall of the forge but should be designed for easy replacement when the tuyere wears out.
The lower half of the entire forge is filled with charcoal fines, not quite powder but small enough that it isn’t useful for fuel. The charcoal bed provides an insulated and adjustable floor for the forge area, and creates a space under the charcoal shelf to push a long blade into while heating without pushing the fire along with it.
The charcoal bed should begin at the level of the front of the forge and end at the bottom of the charcoal shelf but is scooped out in between to about 2-4 sun below the tuyere opening. For different operations the depth may be easily adjusted. The charcoal should be packed down and often will need to be dampened slightly to prevent it burning up.
…the part 5 video is making the 475lb. anvil from scrap metal.
Aku is charcoal made by controlled burning and then smothering of straw (specifically rice straw if available). The resulting fine flaky mixture is used as a flux and as protection from carbon loss during tanren and forge welding operations. A tray is usually placed between the forge and anvil during tanren so the billet can be rolled in aku on the way to the anvil then rolled again and coated with clay slurry on the way back to the fire.
The raw material for this tray comes from some salvaged farm equipment, the shape and size reflect the original dimensions of the sheet metal part. Dents are hammered out, the sides are bent up, the corners folded over and riveted.
Clay slurry (泥汁 doro/dorojiru) is a thin mixture of natural clay and water used as a flux during certain stages of tanren and forge welding. The mixture is spooned over the billet after it has been rolled in aku and quickly dries to form a coating on the way back to the forge.
The container should hold water and be resistant to rust, in this case an earthen jar large enough to hold a stable supply of slurry. Depending on the school and the smith’s preferences it is usually placed either in front of the fuigo wall or to the right of the anvil and water bucket. It is buried almost to floor level and the ladle and lid are made from salvaged farm equipment.
An important tool for proper fire management is the charcoal rake, also called hikaki or hikakibou (火かき / 火かき棒). Large amounts of burning charcoal are pushed back from the firepot to place the billet back in the fire, and fresh charcoal is added as needed during all stages of forging. A long handled rake with a blade size appropriate to the width of the forge allows the smith to reach right to the back of the charcoal shelf and keep a well managed fire at all times.
Salvaged farm machinery parts, a large socket wrench, and a piece of fallen tree branch are the raw materials for this seat. The wooden handle offers some protection from heat build up as the rake sits next to the fire. If there was hot forging involved a couple of elegant refinements would be to upset the rod right at the socket and taper towards the blade end.
The earliest forges in most cultures were placed into the ground and required the smith to sit flat on the ground. Swordsmiths continue to work at ground level because it allows the anvil to be much lower in relation to the strikers standing with heavy (6.5kg/14lb) sledge hammers.
The wooden platform for this project is designed to operate in two positions, either in conjunction with the foot pit for chair style sitting or spanning across it for flat ground style sitting. Salvaged lumber, including two table leaves, are the raw materials for this seat.
Special thanks to Pierre Nadeau for generously providing the fuigo for this build, along with much helpful advice on embracing wabisabi and making the most of measurements and materials. Thanks also to Dave Kasprick for enthusiasm, encouragement, and sharing his shop space, to Larry and Dee for the firebrick, Lorne for the anvil components, and to the Parksville Museum for providing the location for the forge.