The formation of the Japanese katana is a true form of art, beautifully constructed to withstand the chaos of battle. It has been used by warrior classes across ages to protect the warrior to its highest potential in the heat of combat. The process from sand to shape is complex and requires an experienced swordsmith to forge this weapon.
Once the sand has been made into two types steel (tamahagane), wafers of the highest-quality metal are selected for the katana. They are separated into the higher-carbon steel (kawagane) and lower-carbon steel (shingane). This is where the forging process begins. The metal is stacked three to five inches high on a steel plate that has been welded to a long handle. This block, which can weigh four to five pounds, is wrapped in rice paper and coated in a clay slurry for insulation to help in maintain form when it is heated. When the temperature reaches approximately 1300 degrees Celsius it will glow yellow or white, indicating that it ready to be hammered into a single bar.
The founding forging or “shita-giate” is performed when the bar of steel has been created and deemed to have the proper carbon content for its intended purpose. The forging of the kawagane begins with heating the bar to the proper temperature and then striking its center with a chisel, making a notch dividing the bar into two even parts. One half of the bar is held against the edge of an anvil while the other is hammered until the halves form a 90 degree angle and the bar can be folded back upon itself. It is then hammered out again to form the original elongated bar, and the process is repeated multiple times.
Re-heating the metal can cause the oxygen-rich air to quickly remove carbon from the tamahagane, which would reduce it to pure iron and make it useless as a sword. Preventative measures to avoid this situation require the swordsmith to occasionally remove the bar from the forge, roll it in rice straw and recoat it in a clay slurry. It is still common for nearly half of the tamahagane to be lost in the process. The bar can weigh up to three and a half pounds at the end of the shita-giate. To ensure even distribution of the carbon throughout the bar, the swordsmith will cut the bar into four equal pieces and stack them upon each other. They are melted down once more before forming a single bar. This is called the “age-giate” or finishing forge.
The shingane is forged in similar fashion with the main difference being that additional folding is necessary to drive out the higher-levels of impurity in this lower-carbon metal. If the impurities are not removed the two steels will not fuse together properly.
Jitetsu and Jihada
Two important details that result from the folding techniques used in the shita-giate include the “jitetsu,” which means steel quality, and the “jihada” (also referred to as the hada), which is the surface pattern on the metal. This pattern will show on the blade’s surface once the final polishing takes place. Factors that determine the quality of the steel and the jihada include the weight of the hammer strikes, directions of the folds and the metal combination. This can add a significant amount of value to the katana for both artistic and functional purpose.
The two types of (tamahagane), are then fused through a process called tsukurikomi. Two and a half pounds of the kawagane is hammered and heated into a 15-inch long plate. It is then bent into a U-shape and a one pound bar of the shingane is inserted into its base. The shingane is crucial for shock absorption and does not run the entire length of the kawagane, leaving the piercing point for the sharper steel. The kawagane will completely enshroud the shingane. The tsukurikomi process is vital to both the functional and artistic value of the katana. If there are any gaps in the welding, the weapon is worthless.
Forming the Sunobe
After the tsukurikomi the swordsmith reheats the metal to begin forming the initial shape of the sword. This process of heating and hammering produces the “blank” or “sunobe,” which closely resembles the shape of the finished katana with nearly 90% of the swords full length and width but without curves or definition. At this point the swordsmith also defines the nakago or tang. This is the part of the steel that sits inside the hilt. The depth of the nakago will determine the sturdiness of the sword. It is crucial that that sunobe is uniform in thickness throughout.
Shaping the blade (hisukuri) is a process largely dependent on the temperature of the metal. The formation begins with the cutting edge (ha-saki) and is heated in and hammered in six -inch increments at around 1100 degrees Celsius. If the metal is overheated it could cause separation of the delicately melded kawagane and shingane. If it is too cool the sunobe could break. The work must be done quickly to avoid these problems. This process leaves the sunobe looking much like a sword, but still lacking sharp edges.
The process to give the weapon its hardened cutting edge is called shiage. The swordsmith uses a tool called a drawknife or sen to shave off irregularities and unevenness off the surface of the sunobe. A file is then used over the back edge followed by a rough grinding with a carborundum stone over the entire blade. Today the carborundum is commonly referred to as silicon carbide. After the shiage process, the katana is well defined and has taken the shape of the great weapon it will become.