Hizukuri: Forging the Blade
The raw material for this blade comes from a reclaimed shear steel leaf spring taken from a horse-drawn carriage and estimated to be a century and a half old.
The steel is forged into a rectangular bar called sunobe. The sunobe determines the volume of steel allocated to the blade and tang and largely the final dimensions. The tip is cut at a 45 degree angle and filed smooth to prevent cold shuts. Note that the edge is facing up in this photo.
The tip is then forged to the opposite position, running the layers of the steel along the edge. Note that the edge is still facing up in this photo.
The peak of the spine is forged to shape next, followed by the bevels. A thin layer of water on the anvil creates steam explosions that clean the forge scale off the steel, preventing it from building up and marring the surface.
Any excess is hot cut from the end of the tang. At this point all of the shaping has been done with the hammer alone.
Ara-Shiage: Shaping the Blade
Using a file, the profile of the blade is cleaned up and the machi are made true.
The bevels are cleaned and shaped using a hardened steel scraper called a sen.
The rough scraped surface left by sen.
Filing and drawfiling are used to true and smooth the surface left by the sen.
The rough filed surface left by drawfiling. At this point the edge is left 1-2mm thick in order to decrease the chances of warping and cracking during hardening. A coarse stone may also be used to ensure the surfaces are a straight and true as possible before yaki-ire.
Yaki-Ire: Hardening the Blade
Applying the clay layers in preparation for hardening the steel (yaki-ire). The thicker layer insulates the steel, causing it too cool slower and form pearlite/ferrite. The thin layer on the edge increases the surface area, causing it to cool faster, forming martensite.
A blade is born. The steel has hardened successfully without cracking, a file will not cut the edge at all but slides off without biting.
The clay is removed, the edge, straightness, and shape of the blade are inspected. Then the steel is cleaned and tempered to remove some of the internal stress.
An inspection polish with a coarse waterstone reveals a distinct hamon marking the division between the tougher blade and harder edge. The blade is shaped on coarse stones until it reaches final geometry and then all other mounting work is done before the final polish.
