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 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.
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.
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.
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.
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.
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.
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.
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.
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.