This blade is forged from a reclaimed horse-drawn carriage spring made from shear steel that is more than a century old. Materials for the koshirae include copper bus bar for the habaki, driftwood Nootka Cypress with natural urushi lacquer finish for the tsuka and saya, reclaimed Congolese silver jewelery for the mekugi pin, and local Oceanspray wood for the ki-fuchi, koiguchi, and kurikata accents. See the finished work: Kuromon Aikuchi Tanto
The raw material for this blade spent more than the last century as a leaf spring for a horse-drawn carriage. It is rare to come across this type of steel and is a treasure to find. It is a type of steel called “shear steel”, predating the bessemer process and blast furnaces. Labour intensive to produce, it was made in small batches from wrought iron, carbonizing it in a charcoal furnace for several hours to create blister steel and then further flattening and forge welding layers together to distribute the carbon and form shear steel. The result is a simple carbon steel with a layered distribution of carbon and other inclusions. At the time, triple shear was the finest grade of steel available.
The best forge fuel for high carbon steel knives is softwood charcoal. Using scrap wood from local sources, we cook and chop small batches of bladesmithing charcoal in a homemade charcoal kiln.
The horse drawn carriage spring assembly as found on a homestead.
Disassembling, inspecting, cleaning, and sorting the pieces. The deep pitting on the surface from decades of weather will mean a lot of polishing time and less steel to work with so forging must be done very accurately.
Blade
The blade is formed in two distinct stages, sunobe and hizukuri. Sunobe is a tapered rectangular pre-form which determines the volume of steel that will be allocated to each part of the blade. It is then forged further to form the bevels and the approximate blade geometry.
The end is too thin and pitted to forge properly so it will be cut off for about three inches. The bolt hole in the centre determines extent of the tang end. Half of this spring will form the blade for this project, the other half will be forged into a sister blade of similar dimensions.
Forging the peaked spine known as “iori mune”. Forging as close to the finished shape as possible will save precious steel as well as time-consuming hand filing work.
At this point the blade is finished forging and will be cleaned up with files and scrapers. It will also need to be shortened slightly from the tang end due to a large area of pitting on one side. Elements of the form are based on the 700 year old Aizu Shintogo Tanto.
The iori mune is shaped and smoothed by drawfiling, a technique that turns a file into a fine plane for steel.
The original shear steel layers can be seen flowing with the curve of the tip where the steel was forged into shape. This is one reason why shortening cannot be done from the tip with a forged knife.
A sen dai staple vise holds the blade while the fire scale is removed with a sen scraper and hand files. The bevels are cleaned up and the final geometry begins to emerge.
An extra coarse diamond stone is used to refine the geometry of the bevels and true up all blade lines.
The blade shaped as far as possible before it is hardened. At this point the edge is still 2mm thick to protect it from cracking or warping during the stress of yaki-ire, the “clay tempering” process.
A layer of clay mixture in place and drying before the hardening process. Read more about yaki-ire here: Process: Yaki-Ire
Immediately after hardening, before tempering. It has acquired a small amount of curvature from the tension between the two different types of steel crystal formation now making up the blade.
With the clay removed and after yaki-modoshi, tempering. Tempering removes some of the internal stresses and achieves a proper balance between hardness and toughness for the blade’s intended purpose. It will be polished by hand on Japanese waterstones.
Habaki
Habaki is a non-ferrous collar for the blade that strengthens the base of the tang and holds the blade tightly in the scabbard. Often made of copper, a large and small piece are forged and filed to shape and then soldered together before polishing and patinating. The copper for this habaki comes from a reclaimed bus bar, used in a high tension power station.
The rough habaki jacket is hot and cold forged from a piece of copper bus bar.
Files and rasps are used to clean up and refine the shape.
A small compound wedge called machigane is forged and filed to bridge the gap along the edge of the tang.
The machigane will provide a solid solder joint between the two sides of the jacket and serve as a bearing surface for the hamachi and tang edge.
A piece of oxidized steel wire keeps everything from moving and a strip of silver solder is fluxed and placed on the machigane.
Slowly heating in the charcoal forge keeps oxygen from corrupting the joint. Just after the solder melts the piece is removed from the forge and cooled in air.
The excess machigane is cut off and the copper oxide cleaned off.
The habaki is made slightly undersized and then final fitting is done by cold hammering to stretch and harden the copper.
Files and stones are used to refine the shape and give texture to the surface. A natural patina will form on the copper while the rest of the fittings are being completed.
Tsuka
Tsuka is the wooden handle core of a Japanese sword. A block of wood is split and carved to fit the tang snugly and then the halves are glued back together with sokui, rice paste glue. Once dry, the outside is shaped with chisels and hand planes to create the final shape.
A very old piece of Nootka Cypress driftwood that has been drying and seasoning under cover for several months will serve as raw material for the handle and scabbard. The outside layers are split away to reveal clean wood inside even after decades at sea and on the beach.
The split block is planed smooth inside and sawn into two parts, one for the tsuka and one for the saya. The tsuka blocks are carved to match the shape of the tang and then glued back together using sokui, rice paste glue. The leather wrapping and wedges hold it tight overnight while it dries.
Once the glue is dried, the block is trued up with a hand plane and the tang location marked on the outside.
The mouth is carved close to its final shape and then the corners are taken down with a chisel and plane.
The final shape of the profile is decided and the sides and corners continually refined and smoothed down.
The final profile roughed in with hand planes and refined with a chisel.
Ki-Fuchi
Fuchi is a collar or ferrule around the wood core of the handle where the blade emerges. Ki means wood. Fuchi are usually made from iron or copper, but in the case of kaiken or aikuchi, they are often made of horn or even wood. In this case a slice of a local island ironwood will strengthen and form an accent for the handle/blade juncture.
Oceanspray, locally known as ironwood, is a brush-like hardwood that grows near water. It is so tough that a quarter inch slice like this cannot be broken by hand, even though it is across the grain. A thin slice will provide a visual accent and a hard surface for the habaki to rest against.
A shoulder is formed by removing some of the tsuka to the thickness of the Oceanspray. This will stabilize the ki-fuchi and prevent contact between the steel and the harder wood.
The Oceanspray is chiseled and filed to fit tightly over the shoulder and a slight dish is carved in the top to ensure a tight closure with the saya later.
The profile is carved and filed to match the tsuka, given a rounded lip, and sanded smooth.
Saya
Saya means scabbard. A similar process to the tsuka, the halves of the saya are carved inside to fit the blade closely and grip the habaki tightly when sheathed. The halves are glued together using sokui, rice paste glue, and then the outside is shaped with chisels and hand planes.
The omote half of the split block is carved out first, a collection chamber for oil and dust is carved at the tip. The chisel for this work is called saya-nomi (鞘鑿).
The edge sits fully inside this half so that it does not rest against the glued joint. this stage must be done very carefully, testing continually for a snug and accurately aligned fit.
Once the fit is very accurate and snug around the habaki, the halves can be glued together. The offset nature of the carving is visible in this photo, ura to the left, omote to the right.
Sokui, rice paste glue, is used to join the halves and they are tightly wrapped and wedged to increase pressure and alignment while drying overnight.
When fully dry, the wrapping is removed, the outline of the fuchi is traced at the mouth of the saya, and the general profile of the saya sketched on the block.
The mouth is carved down first to set the shape and size guideline.
The corners are removed, and then their corners are removed in turn, carefully removing the excess wood.
A small hand plane is also used to round the shape and bring it further down to size.
After the tip is rounded and smoothed, the saya is polished with dried tokusa, horsetail plant, which has very fine natural silica in its cells.
Koiguchi
Koiguchi can be translated as the koi (carp) mouth and does bear a striking resemblance. It is the area where the saya meets the tsuka. Usually made from horn, in this case another slice of Oceanspray (island ironwood) will meet and mirror the ki-fuchi. This is a delicate operation as the mouth of the saya must be carved so thinly and closely to accommodate the large shoulders of the habaki while maintaining a slim and balanced exterior profile.
A fine saw, knife, and paring chisel are used to create a shoulder for the koiguchi reinforcement.
Another slice of Oceanspray wood is roughed out and then cleaned with coarse files until it just fits onto the shoulder at the koiguchi.
The outline of the saya is traced on the back and the slice is chiseled and carved to shape.
A slight dish is carved to allow tight closure, and the surfaces are rounded and smoothed.
The koiguchi is slightly smaller and thinner than the ki-fuchi. This creates visual balance and keeps the focus on the fuchi.
Kurikata
Kurikata translates as “chestnut shape” and serves as a rest for the sash and an attachment point for a retaining cord when worn. Usually made from horn or non-ferrous metal, in this case tough local Oceanspray wood will be used to compliment the other accents of the piece and keep the list of materials short.
Another, thicker, slice of Oceanspray ironwood is cut for the kurikata. Because of the extreme toughness of this wood, it can be used against the grain and still retain more strength than most wood.
The shape of the wood chosen follows the curve of the kurikata almost perfectly, it is carved, scraped, and filed smooth, then polished with dried tokusa plant.
The finished kurikata fits snugly into a channel cut into the saya. It will be attached permanently with sokui and nori-urushi, an adhesive made from rice paste, water, and natural urushi lacquer.
Mekugi
Mekugi is a peg or pin, usually made of bamboo and sometimes horn, that holds the tang in the handle and locks all the components of a tanto together securely. It can be removed for dis-assembly, allowing the blade to be cleaned and polished or even mounted in a new koshirae. For this work, the mekugi will be a unique type made of silver and generally reserved for presentation pieces of the highest order.
A scrap of rolled square silver bar and an ear cuff made from Congolese silver will serve as raw materials for this metal mekugi.
The three pieces of the silver mekugi along with the remaining scrap of the original sources. A strip of leather will be fed through the eye of the pin, locking the back washer in place and preventing accidental removal of the mekugi.
The design of the silver mekugi is based on a large hobnail traditionally used on the doors and gates of a castle. The hobnail symbolizes protection.
The assembled aikuchi tanto koshirae ready for lacquering.
Urushi
Urushi is traditional lacquer made solely from the sap of a certain tree. It reacts with humidity and heat to cure into a hard, smooth surface. Bowls made with urushi lacquer have been known to last for more than two centuries of regular use. The wood is first sealed with a wiped on layer of urushi, and then brush coated with mutiple thin layers that are cured for one to three days each before being polished with charcoal and water and then re-coated when dry. Curing takes place in an enclosed box misted with water and set in a warm area. The final stages involve polishing with tokusa, horsetail grass, coating with multiple layers that are wiped off and cured for a day each, and then burnishing final layers with a drying oil such as flax or tung oil.
Day 1: Nori-urushi, an adhesive made of rice paste, water, and urushi, is used to secure all parts together permanently at the joints. Then the first layer of natural urushi lacquer is applied and wiped off while wet to seal the wood. It is allowed to cure for two days.
Day 7: A mixture of natural urushi and very fine clay in the form of diatomaceous earth is used to make a paste to fill any gaps remaining in the woodwork joints and seams as well as the grain of the wood. It is cured for two or three days until fully dry and then the uneven areas are smoothed and polished very lightly with fine charcoal and water. Care is used not to break through to the raw wood again.
Day 10: After sitting for a day to allow the polishing water to fully evaporate, the second layer, in black, is applied quite thinly with a brush and allowed to rest for two days in the furo until cured. The wood can still be seen faintly through the urushi.
Day 11: It is water polished lightly with charcoal to remove any bumps, high spots, or irregularities and then allowed to dry out thoroughly. The charcoal can be shaped and conforms to the surfaces while working.
Day 15: The third layer is applied slightly thicker and brushed perpendicular to the length to give some very subtle texture imitating a rattan wrap. It is allowed to cure for two to three days in the furo.
Day 16: The third layer is polished in the same way as the second and then a fourth layer of black urushi is applied and allowed to cure for three days in the furo.
Day 19: The fourth layer is polished and smoothed with charcoal and water. The rough polishing stages take considerable time and focus in order to remove only as much material as necessary, especially near edges and corners.
Day 20: The fifth layer of urushi is applied and cured for three days. The colour is quite deep and the wood core cannot be seen through the black urushi at this point.
Day 23: It is water polished again with charcoal to remove any new irregularities. Once the surface is established, finer charcoal is used to remove scratches. It is then given a fine matte polish with tokusa, horsetail grass, which contains fine silica in the plant cells. Some of the remaining brush and layer texture can be seen in the surface even though it is quite flat.
Day 24: A thin layer of black urushi is applied over the surface with a spatula and a brush and then wiped off with a cotton cloth after a few minutes. It is cured in the furo over a day or two as usual. This technique is known as fukiurushi and is used to build up very thin successive layers on a wood or lacquer surface.
Day 28: The shine begins to deepen after a second and third layer of fukiurushi in black are each followed by a day or two of curing. At certain angles an interesting surface texture is still visible from the layering.
Day 30: A course of oiling, curing, and burnishing is begun with very minute amounts of pure flax oil, a drying oil. The surface is sealed and built up as each layer hardens, taking about four days to cure each application of about a drop. The furo is still used for curing, the warmth speeding up the process slightly and the box protecting from dust, the difference being that no humidity is used for the oil stages.
Day 42: After the last drop of oil has cured, the exterior surface is has a smooth matte finish. The Oceanspray (island ironwood) accenting the koiguchi has been finished with natural colour fukiurushi and oil to allow the warmth of the wood grain to show through.
Aikuchi Tanto
The finished blade with its intensely revealed shear steel hada texture and suguha hamon with turnback, its copper habaki, and the matching tsunagi blade and ki-habaki made from Nootka Cypress. The wooden blade and habaki were crafted to allow display of the blade alongside the koshirae.
In correctly angled light the boshi is visible as it turns back slightly towards the spine in ko-maru style.
The completed tanto blade and koshirae ready for assembly. Century-old shear steel, reclaimed copper, silver, Nootka Cypress driftwood, Oceanspray ironwood, hand-tanned buckskin, and urushi lacquer.
