The boshi (pronounced boh-shee) is where the temper line turns the corner. It is the portion of the hamon that runs up into the point and curves back along the spine, the hardened steel wrapping itself around the very tip of the blade. The literal sense is “cap” or “hood,” and that is exactly what it looks like: a hood of hardened steel pulled over the head of the sword.
The hardest part of the quench
Among forge people, the boshi is the truth-teller. The kissaki is the most fragile geometry on the whole sword, a fine triangular point bounded by the crisp yokote ridge line. Getting the clay to behave there during the quench, so the hardening curls cleanly over the tip without cracking or running off the end, is the most demanding moment of the entire process. A confident, well-shaped boshi tells you the smith controlled the fire all the way to the last centimetre.
How to read it
Tilt the point to the light and you should see the misty hardened band sweep up from the edge, round the kissaki, and turn back a measured distance along the back edge before fading. The named shapes, a rounded komaru, a pointed togari, a sweeping o-maru, are part of a blade’s character. The warning sign is a boshi that simply runs off the tip and vanishes: on many low-cost swords the temper never properly reached the point, which leaves the most-used striking zone soft and prone to rolling.
Why it matters to you
If you intend to actually cut, the boshi is your assurance that the business end of the blade is hardened where it counts. A real, returning boshi is the mark of an honest battle-ready katana, and you will see it at its finest on a properly finished master-grade katana. To understand how the quench produces it, see how a blade is forged, and continue through the rest of the katana anatomy guide.