The Katana: From Backup Dagger to Clay-Quenched Steel and Strict Sword Laws

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From the 12th to 14th centuries, mounted samurai fought mainly with the yumi bow and the yari spear, and carried the long, deeply curved tachi, hung from the belt with the edge facing down for slashing at foot soldiers. In the large wars of the Sengoku period, massed ashigaru fought on foot in tight formations where the tachi was too slow to draw. Their short backup dagger, the sasuga, grew longer until it became the katana, thrust through the obi with the edge facing up so that drawing and striking became one motion, the basis of battojutsu and modern iaido. Smiths signed the tang on the side facing away from the wearer, so a signature's position shows whether a blade was a tachi or a katana.

Smiths folded tamahagane steel made from iron sand, then coated the blade with clay, thick on the spine and thin on the edge, heated it to nearly 1,400 degrees Fahrenheit and quenched it in water. The edge became hard martensite, the spine stayed softer, and the uneven cooling bent the blade into its curve. After Toyotomi Hideyoshi's 1588 sword hunt and the Tokugawa peace, the paired daisho became a badge of samurai rank and blades turned ornate. The Meiji government disbanded the samurai and banned carrying swords in 1876, smiths turned to farm tools, and World War II brought oil-quenched factory blades. Today a licensed smith needs a five-year apprenticeship and may make at most two longswords a month.

• The visible frosty band on a polished blade is a cosmetic finish called hadori, while the true hamon is a fine crystalline line seen only by tilting the steel in light.
• A finished sword can pass through up to eight specialists, including the togishi who polishes for weeks and the sayashi who carves the scabbard.
• Connoisseurs judged a blade's shape, the grain of its steel, and particles called nie and nioi described as stars and mist.
• Mass-produced wartime swords are not legally Japanese swords and can be confiscated and destroyed if brought into Japan.
• Sword making was banned from 1945 to 1953, and in 2014 Kawachi Kunihira recreated the lost Kamakura-era utsuri reflection and won the Masamune Prize.