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.