Forging a Japanese Sword
A major development in Japanese swordsmithing came in the mid-1500s, when new tatara ironmaking methods were developed to produce high-grade tamahagane steel. Japanese swords had been made with refined iron for centuries, but the amount of carbon in tamahagane steel is ideal for making blades that are both sharp and supple. Even today, it is the preferred material of Japanese swordsmiths.
When forging a sword, swordsmiths first heat and flatten the steel to a thickness of around 5 millimeters. The metal is then broken into pieces and sorted by hardness. To make the cutting edge of the sword, the hardest pieces are stacked and remelted into a single ingot. This ingot is then repeatedly folded, often as many as 15 times, producing a piece of steel with nearly 33,000 layers. These layers appear on the finished blade as a grain pattern that varies according to the forging technique.
Next, the ingot is wrapped around a core made from softer, more pliable steel, and they are hammered together. The swordsmith then hardens the blade through a quenching process called yaki-ire. The blade is first coated with uneven layers of clay, then heated to around 800 degrees Celsius and plunged into cold water. The layers of clay cause the metal to cool and contract unevenly, giving the blade the distinctive curve for which Japanese swords are known.
The style in which the clay is applied determines the appearance of the edge pattern (hamon) that runs down the blade. It can be straight, curvy, jagged, or any other shape the swordsmith desires. The swords in this display show how a swordsmith can produce different results and appearances using the same basic materials.
