SOTW #16 - Differential Hardening Sunnobi Tanto - yakiire with water, charcoal, clay

Yakiire - Differential hardening a classical style sunnobi tanto blade forged from century-old reclaimed shear steel. Sunnobi tanto are larger than ordinary tanto (nagasa above 1shaku) and may have sori similar to ko-wakizashi.

see the whole process of forging this tanto here: http://islandblacksmith.ca/2018/03/utsushi-study-of-a-sunnobi-tanto/

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.

more about the process: http://islandblacksmith.ca/process/
more about yaki-ire: http://islandblacksmith.ca/process/yaki-ire-clay-tempering/