Utsushi Study of a Sunnobi Tanto – Crossed Heart Forge

Sunnobi tanto (寸延び短刀) are larger than ordinary tanto, with nagasa a sun or two above 1 shaku (sun nobi, “a sun longer”, from nobiru, to stretch or lengthen). Though there is some area of crossover with hira-zukuri ko-wakizashi and they may have sori similar to ko-wakizashi, the simplified difference would be that they are still designed with tanto geometry rather than wakizashi proportions and form.

An utsushi (写) is a careful and exacting replication of an existing work, undertaken for the purpose of study and further understanding of the process, techniques, and historical mindset of the original craftsman.

This project is the result of a very rare opportunity to document a traditional pattern in various stages of its forging. It is very closely based on the lovely form of an original forged by Sumihira~san, a master and mentor to Pierre of soulsmithing. Three similar blades were forged as part of a demonstration and I was privileged to be able to study and take careful measurements of the sunobe, the forged blade, the filed blade, and the hardened blade after yaki-ire. This level of detailed hands-on measurement is almost unheard of as most utsushi must be made from a single kata and a couple of measurements of the finished work alone.

Process Highlights

scroll down or jump to the sections below:

Kata
Material
Wakashinobe
Sunobe
Hizukuri
Yaki-ire
Specifications

Studying & Making the Kata

Three stages of the same size and style of sunnobi tanto forged by Sumihira~san, a rare opportunity for studying the approach of another swordsmith. Center is after forging (hizukuri), top is after rough filing the profile (arashiage), bottom is immediately after quenching (yaki-ire).

The kata (pattern) prepared from measurements and tracings of the original, in this case the tracing is of the blade before yaki-ire (something not usually available) with a slight reverse sori, in order to use as an exact forging pattern.

Raw Material

The tanto was forged from the remaining half of the billet used for the Hatsu Yari, a piece of an antique carriage axle with some subtle visible hada layering. The century-old steel is on the lower end of carbon content, similar to many older koto swords but has a very fine and even structure.

The raw material for this blade spent the last century as a repaired section of axle for a horse-drawn carriage.

This section of steel was inserted by a blacksmith as a repair to an axle, the forge weld can be seen at lower left. It has some subtle layering as well as slightly higher carbon content than standard axle steel of this era. The other half of this same piece was used for making a yari.

Chopping, screening, and sorting the pine charcoal in preparation for forging.

Lengthening the Billet

Wakashinobe is the process of forging a billet out into a rectangular bar in preparation for making sunobe. This piece of steel makes a cameo being forged out to length in the first scene of the short film by Trevor Komori, appropriately titled Study the Old the Know the New.

Forging the Sunobe

Sunobe is the rectangular preform which sets the size and proportions for each part of the tanto. In this stage the thickness, taper, blade length, blade depth, and tang size are largely decided. A proper sunobe lays the foundation for a proper blade and errors at this stage make it difficult to control the finished proportions.

Forging in the charcoal forge with fuigo box bellows providing the air.

Completed forging of the sunobe (pre-form). Another rare opportunity for study was a cast aluminum copy of the sunobe from the original blade (bottom).

The finished sunobe compared to the tanto kata, note that sunobe are rectangular in cross section, contain the distal taper, and that the spine is the lower (curved) part in this photo.

A comparison of the nagasa (length) and haba (depth) of the sunobe with the tanto kata, note for the purpose of illustration the sunobe has been flipped over.

Forging the Bevels

Hizukuri is the stage of forging the rectangular sunobe into a beveled blade. The thickness of the spine stays largely unchanged from the sunobe but drawing out the edge material causes the blade to curve and it must be corrected regularly and compensated for in advance of the edge becoming too thin to hammer on. A wooden block helps protect the peaked spine while adjusting the curve. The forging begins at the base of the blade and works toward the tip and then back again to refine the shape further.

Hizukuri is the process of forging bevels into a sunobe. Note the slight reverse sori in the spine to compensate for the effects of yaki-ire.

After cleaning up the profile with a file to add the machi (notches) and hot stamping the kamon on the tang.

The bevels are fairly clean and smooth as-forged, due to careful hammering and the use of water during the latter stages of hizukuri.

The bevels are fairly flat at this point (minimal ha-niku) and the spine is mitsu-mune (three-faceted).

Hot stamped katabami-ken-kamon.

Using a sen-dai, sen scraper, and files for hira-ji (smoothing the bevels) and preparing the surface for tsuchioki, application of clay.

Hardening the Blade

Because it is an utsushi and the sori needed to be as close as possible to the original, this blade underwent yaki-ire twice before it achieved the intended curvature. The amount of sori depends not only on the blade geometry but also on the steel makeup, the width and style of the hamon, the heating technique, and the quenching temperature.

After application of the thicker clay layer to delay the cooling of the body and a watery thin slip layer to protect the edge area and speed up its cooling rate.

The results are excellent but because the goal is to precisely reproduce the original pattern, slightly more sori is required for this tanto.

After normalizing and readjusting the spine, a wider area of edge is exposed with a new clay application to produce slightly more sori on the second attempt.

A general overview of the process being shown, read more about the process of yaki-ire.

1. Using approximately a 1:1:1 mixture of natural clay, polishing stone powder and ground charcoal to mask the back of a hand forged blade about 1-1.5mm thick to slow down the cooling rate.
2. Brushing on a thin slip layer with extra charcoal added along the exposed edge to speed up the cooling rate and protect from carbon loss.
3. Carefully heating in a charcoal forge supplied by air from a fuigo box bellows until the edge reaches critical temperature.
4. Plunging edge-first into cold rainwater to cool the blade quickly and harden the edge while leaving the rest tough and resilient.
5. Testing for successful hardening with a file and then removing the clay with a mild steel scraper.
6. Slightly reheating the blade over the flames to temper the edge.
7. Test polishing on a coarse Japanese waterstone to check the hamon.

After cleaning off the clay on the adjusted second yaki-ire the sori is a match to the original pattern.

A view of the sori introduced by yaki-ire, shown next to the reverse sori of another tanto in preparation for yaki-ire, intended to end up with a straight spine.

Narrowing the edge and shaping the bevels further with a coarse synthetic water stone. Coarse stones may be used dry and the powder is saved for use in the next yaki-ire clay mixture.

Using a coarse diamond stone for some clean up and checking planes and tapers after shaping with the coarse synthetic water stone. Read more about the stages of polishing.

Specifications

長さ/刃長 Nagasa: 11 sun 6 bu (351mm)
元幅 Motohaba: 1 sun 1 bu 5 rin (35mm)
重ね/元重 Motokasane: 2 bu 3 rin (7mm)
反り Sori: 1 bu (3mm)
中心/茎 Nakago: 3 sun 6 bu (110mm)
形 Katachi: hira-zukuri, mitsu-mune
中心/茎 Nakago: futsu, no mekugi-ana
銘 Mei: hot stamped kamon

Material: Reclaimed carriage axle steel

This tanto is a good challenge and a lovely form that I look forward to forging again. It is currently in the early shaping stages of kajitogi as it is given the final geometry corrections. Once it has its final geometry and the polish is taken to about 400 grit, the next step would be to make a habaki and then other fittings. The tang is signed but the mekugi-ana has not been drilled yet. The blade is just over 13.75″ long and the overall length to the tip of the tang is just over 18″. The spine at the munemachi is about 7mm thick.

Final Photos by Jordan Wende

Island Blacksmith: Hand forged knives reclaimed from antique steel.

This piece is in a private collection in California.