Process of Polishing a Tanto Blade

After the blade is hardened and tempered, the final geometry is created and the surface smoothed and polished with various abrasive stones. Polishing is broken into two distinct stages, the rough polish occurring before the fittings and scabbard are made, and the final polish once the rest of the knife is complete.

A combination of Japanese waterstones, both synthetic and natural, and diamond stones are used to remove smaller and smaller amounts of steel and give the knife its final shape and surface. Each stone is progressively finer and is used with a different orientation so that the scratches from the previous stones may be clearly seen. Once they are erased, the next stone can be used, each time refining the geometry and surfaces towards the final goal. Once the rough polish is finished, work on the blade is halted until after the habaki, seppa, tsuka, and saya are created and fitted. This prevents accidental scratches while working on other parts of the knife.


Rough Polish (Kaji-Togi)

At this stage, the large volume of steel left around the edge for the process of yaki-ire must be removed, and the blade and tang are given their final geometry. The blade is much harder than is was during the rough shaping stage and steel cannot be removed with any metal tools. A combination of natural and artificial waterstones as well as a human powered carborundum wheel are necessary for this process.

Care is taken to refine each part of the blade geometry and bring the planes into proper alignment and proportion beginning with coarse stones and quicker removal and ending with very fine stones and subtle adjustments. Tagane-ha (chisel edge) is a common technique for establishing the centre line of the edge. Both sides of the edge are honed away on a 45 degree angle and then the excess material in between the edge and the spine is removed, similar to the method of using a sen to set the pre-quench geometry after forging.

Island Blacksmith: Hand forged reclaimed tanto made with traditional techniques
After yaki-ire and yaki-modoshi, a test with a Sun Tiger (朝日虎印) 120 grit Japanese waterstone reveals the hamon, the edge is still about 2mm thick.
Island Blacksmith: Hand forged reclaimed tanto made with traditional techniques
Sun Tiger used to make a hand cranked grinding wheel as well. This little human-powered stone helps to remove the bulk of the super-hard steel left at the edge after yaki-ire. After this work the edge is now less than a mm thick.
Island Blacksmith: Hand forged reclaimed tanto made with traditional techniques
Working again with the Sun Tiger 120 grit waterstone and a DMT 120mesh/120micron (extra-extra-coarse) diamond stone to remove the dips from the wheel and bring the blade geometry very close to finished.
Island Blacksmith: Hand forged reclaimed tanto made with traditional techniques
The edge is now less than a fourth of a mm thick and the tang is filed to adjust its final taper along the top and 2mm width along the bottom.
Island Blacksmith: Hand forged reclaimed tanto made with traditional techniques
This is the official “coarse” stage of the polish, working perpendicular to the blade with a 220mesh/60micron (extra-coarse) DMT diamond stone until all the diagonal lines of the rough shaping are erased. This is also the first stone to touch the mune (spine) since the drawfiling that was done before yaki-ire. This stone brings the edge right to zero width.
Island Blacksmith: Hand forged reclaimed tanto made with traditional techniques
Moving to a 325mesh/45micron (coarse) DMT diamond stone and working diagonally until the perpendicular scratches are erased. The polished area should extend along the tang to include the area that will sit under the habaki. Either this stone or the 600mesh/25micron (fine) DMT stone will be the last for kaji-togi.
Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials
A size comparison for scale. This is a full sized tanto kata (based on a blade by Shintogo Kunimitsu) next to the kotanto blade.

Before the rest of the polish, the habaki, seppa, and other fittings are created. See the interim steps here:
Making Habaki (Blade Collar) | Making Tsuka (Handle) |
Making Saya (Scabbard)


Final Polish (Togi)

Once the fittings and scabbard are complete, the blade is given its final polish. Depending on the condition of the blade, the last used stone is repeated again to ensure there are no new scratches from the workshop and then finer and finer stones are used to complete the finish. The last steps are carried out with Japanese waterstones which cause the hamon and other steel activity to show up against the body of the blade. The blade is carefully cleaned and oiled frequently for the first few days after polishing.

Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials
After a repeat of the 600mesh/25micron (fine) DMT stone at a 45 degree angle, a 1200mesh/9micron (extra-fine) DMT stone is used straight in line with the blade. All stones after this will be used in the same direction.
Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials
A Japanese waterstone forms a slurry like fine clay with suspended particles in it, water (with a little baking soda to combat rust) is used to control its viscosity and how much stays on the stone during use.
Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials
As soon as the Japanese stone starts to work the hamon becomes visible. A more technical approach could show this hamon in more detail, but goal of this polish is just to give a simple and honest look into the heart of the steel.
Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials
A different angle of light. Some very interesting activity surfaces during this stone. Near the tip on the omote side there are double-lobed shapes fanning out under the place where the clay lifted slightly during the quench. One of them extends back almost to the centre, above the hamon, and has a similar area mirroring it on the ura.
Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials
This is a finer synthetic Japanese water stone. The slurry is grey from the steel mixing in with the clay as it comes off the blade.
Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials
A natural stone gives a nice final finish due to its slight variation of hardness and grit size. The blade will be carefully rinsed and cleaned and then oiled. My favorite part of this hamon is the way it gets pebbly, like fine sand washing in a river, right as it turns back at the tip.

The Sunahama Kotanto

Island Blacksmith: Hand forged kotanto made from reclaimed and natural materials

View the finished work here: Sunahama Kotanto