An Inside Look at Kobuse Construction

Though the majority of tanto are muku (one-piece construction), after the beginning of the edo period larger swords (katana, wakizashi) are often intentionally constructed of multiple steel components containing differing carbon content. The reason partly stems from the increase in brittleness of steel made from mass-produced tamahagane which began around that time and also from the fashion of making wider “hade” hamon. After the advent of the large centralized tatara system at the beginning of the edo period an increase in broken blades led to the development of several construction systems to add shock absorbing toughness to large blades. The most common style that has continued to this day is called kobuse (甲伏せ, lit. “shell placing over”), a higher carbon jacket (kawagane) wraps around a lower carbon core (shingane) and is forge welded to form the edge and sides of the blade.

Once the two steels have been made or chosen, the first step is to form the jacket and core separately, in approximate thickness proportions of thirds. Then they are fitted together as closely as possible, and finally forge welded into a solid billet. Many japanese swordsmiths today use borax or a boric acid flux to ensure a clean weld, but it is also possible to perform this stage with traditional aku (charred rice straw) and tojiru (clay slurry) as exterior protection from oxidization (as was done in this case).

The stages that follow forge welding are the same as for a muku blade but additional care must be taken to hammer evenly along the length and on each side to keep to shingane as close to the center as possible, preventing it from being exposed by future polishing.

This was a study to observe and record the behaviour and proportion of the two steels at crucial stages as the billet is forged out into a finished blade. Accordingly, a 1cm slice of the steel was taken after each stage of tsukurikomi (forge welding), sunobe (blade preform), and hizukuri (bevelling and finishing). Additionally yaki-ire was performed on the hizukuri slice. Each slice was later polished on natural waterstones to reveal the boundary between the kawagane and shingane, as well as the hamon boundary in the finished blade slice. The remaining piece of the billet was forged into a small complete blade in order to observe the shingane proportions along the spine in relation to the tip and tang.

Process Photos

Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
The raw material for the kawagane jacket was a bar of layered material that appeared to be a mid-carbon shear-steel, it had been used to repair a section of horse drawn wagon axle.
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
The raw material for the shingane core was a piece of wrought iron salvaged from the sea.
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
Folding the formed and cleaned kawagane jacket into a u-shape. (more photos on the process at a later date)
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
The completed billet after forge welding the kawagane around the shingane. (more photos on the process at a later date)
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
Checking the spine of the billet with coarse waterstones to evaluate the weld and centering before proceeding to forge further. Note how the shingane tapers toward the tip to leave kawagane only, helping keep the weld cleaner.

Observations

Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
Another look at the spine after tsukurikomi. In kobuse technique the shingane is exposed along the mune (spine) in the finished blade as well. The tang will be formed closest to the handle of the shingane.
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
A cross-section of the billet after tsukurikomi (forge welding) and before wakashinobe (lengthening). Note the proportions are still approximately thirds at this point (also the welding flaw/inclusion right at the bottom of the shingane, which resolved a short way back from the front of the billet).
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
A cross-section of the sunobe (blade preform) after forging to dimensions for a full-sized tanto. Note that the softer shingane is now compressed more than the kawagane near the edge of the blade.
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
A cross-section of the blade after hizukuri (beveling) and yaki-ire (quenching). Note that the shingane is still relatively centered but is thinner proportionally and has been located away from the edge area by the beveling work (also the dark area at the edge is the hamon).
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
A comparison of size between the three. Note that the billet shows some of the hada (layering) of both the kawagane and the shingane wrought iron.
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
Another view which shows the hamon and blade form more clearly and truly.
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
Contrast to illustrate the shingane. Careful observers will note the relationship between the dimensions of the sunobe and the finished blade.
Island Blacksmith: Kobuse sword blade construction study made from reclaimed steel using traditional techniques
The smaller blade forged from the remainder of the billet right after yaki-ire. (more photos of the polish at a later date)

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