This blade was named Sunagawa (砂川, sand river) because the texture of the blade and the flowing edge of the hamon are reminiscent of the bank of a calm river. It was hand forged in a charcoal swordsmith style forge powered by fuigo box bellows and water quenched with clay, an outdoor knife that has the foundation of the Japanese sword. The final piece has a hamidashi mounting is in the rustic kura (蔵, storehouse) style and includes antique fittings from swords carried long ago.
This article highlights the forging of a small utility blade from a reclaimed pre-1960s cultivator tine. The photos document the dimensions and shape of the piece at various stages from starting bar to finished and hardened blade. A few notes are provided to outline the process and steps.
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Steel: Raw materials
This blade was forged and yaki-ire performed at the museum forge. It began as a pre-1960s (integral) cultivator tine used to work a farmer’s land a generation or more ago. Parts of the same tine were used for a mountain kotanto and a sunnobi tanto.
Sunobe: Forging the preform
Sunobe is a pre-form that allocates steel for the tang and blade, sets the taper for the spine and tang, and determines the final thickness and width as well as approximate length. The more care that is given to the sunobe the more accurate and clean the final blade will be. In a swordsmithing license test the judges may stop the student at sunobe if they already see that the final blade will not be correct.
The step prior to sunobe is called wakashinobe (lengthening the bar/billet), which may take more or less time depending how close the starting bar is to the proportions of sunobe.
Hizukuri: Forging the bevels
If the sunobe is formed correctly and accurately, hizukuri should produce a blade of correct proportions. When steel is forged it does not compress, it simply moves outward from the hammer blows. This means that thinning along one edge of the sunobe will cause a curve to form in the blade. It can be counteracted in advance, afterward, or during each heat by the type and direction of the blows. At some point the edge will become too thin to correct so it is good practice to keep it relatively under control at all times.
During hizukuri a thin layer of water is used on the anvil to cause steam explosions to clean the forge scale off the hot steel. The oxide scale does not soften like the steel and will be driven into the surface if it builds up too thickly. Water forging helps prevent build up and produces a better finished surface.
Arashiage: Rough shaping
There is a proper order for arashiage (rough filing), moving around the profile and working from one step to another. First the spine, then the mune machi, then the nakago mune, then the edge, then the ha machi, then the nakago no ha, then the tip of the tang. In this case the bevels will remain as forged and the shape is already very accurately forged so very little filing is required.
Tsuchioke: Applying the clay
The clay is a mask to delay the cooling of the body of the blade by about a half-second longer than the edge. This will cause a different crystal structure to form in each part of the blade, producing a very hard edge (martensite) and a very tough resilient spine (ferrite/pearlite). The mixture is made from natural clay, charcoal powder, and polishing stone powder and the main purpose is that it needs to remain in place without shrinking while drying and without cracking while heating.
It is applied about 1.5mm thick and starts from a point on the tang where the heat will not reach, covering the spine and everything outside of the intended hamon area. Most modern low-alloy carbon steels will harden a small distance under the clay, some more than others, so this must be taken into consideration. It can be dried overnight or over the coals in the forge before yaki-ire is attempted.
Yaki-ire: Hardening the blade
Yaki-ire (quenching/hardening) is the most stressful step for the steel, if any one of several variables is off the blade could crack rendering the project into scrap. Careful heating to the lowest temperature possible to achieve hardness in the whole hamon area is the goal. Accurate and even temperature along the hamon area, not overheating the spine, and correct water temperature are some of the details to watch.
After the hamon area reaches critical/non-magnetic temperature it is plunged into water to cool. The remaining clay is removed and the hamon placement and hardening checked before some slight yaki-modoshi (tempering) over the flames and possibly more later if needed.
Hamon: A look at the results
A good hamon should be well proportioned, have a look of control and evenness, with a delicate turnback at the tip, and running off just beyond the machi. In the case of older classical tanto the style was often suguha, relatively straight and narrow hamon that have a reserved, elegant feel to them. This particular steel takes a hamon very close to the clay placement, allowing a high degree of control over the shape and form.
長さ/刃長 Nagasa (blade length): 111mm
重ね/元重 Motokasane (spine thickness): 5mm
元幅 Motohaba (blade width): 29mm
反り Sori (spine curve): uchizori (reverse) with slight drop point
中心/茎 Nakago (tang length): 79mm
形 Katachi (geometry): hira-zukuri, mitsu-mune, with slight ubuha
刃文 Hamon (edge pattern): suguha
帽子/鋩子 Boshi (tip pattern): ko-maru
中心/茎 Nakago (tang): futsu, kuri-jiri, one mekugi-ana
銘 Mei (signature): mumei (unsigned)