The forging of a Japanese sword is a subtle and careful process, an art that has developed over the centuries as much in response to stylistic and aesthetic considerations as to technical improvements. To fashion these blades, the smith not only must possess physical strength, but also patience, dexterity, and a refined eye for the limits of the material and the beauty of a finished sword.
Japanese smiths traditionally use tama-hagane, steel produced in a tatara smelter from iron-rich sand. Modern smiths making Japanese swords in the traditional manner still use this type of steel today, now produced in the last operating tatara smelter, located in Yokota, Shimane Prefecture. However, the tatara smelting process, though efficient, is not perfect, and tama-hagane is full of impurities and lacks a consistent dispersal of carbon content, the vital ingredient for turning iron into steel. Too little carbon and the metal will be soft, too much and the metal is brittle.
Kitae: Forging the Blade
In order to correct and compensate for the quality of the tama-hagane, the folding technique of kitae was developed. First the smith selects suitable pieces of tama-hagane and forge-welds them into a single block. This block will form the outer skin of the finished blade. Next the smith begins the laborious process of hammering out and folding the block back on itself. The process yields two important results. First, impurities are worked out of the steel and the carbon content is homogenized throughout the metal. An experienced smith can control with great accuracy the quality of the steel in this way.
Second, the folding produces the jihada, or patterns, for which these blades are so famous. Each time the block is hammered out and folded back, layers are formed. By folding only fourteen times, over 16,000 layers are produced. When the blade is finished, the jihada is visible in the ji, the surface between the edge and ridgeline. The smith can choose specific jihada, such as masame (a straight grain parallel to the edge) or ayasugihada (concentrically curved grain) (2001.574), simply by varying the direction of folding. The block can be folded repeatedly in the same direction, in alternate directions, or crosswise, each method producing a different style of jihada.
The outer skin, called kawagane, is then wrapped around a softer iron core, or shingane. This combination gives the blade both the flexibility and the strength to resist breakage under stress. Additionally, the harder kawagane is better suited to sharpening than the more ductile core. The two layers are heated and hammered out into a long bar. This welds the layers together and forms the blank from which the finished sword is made. Once the blade has been forged into its basic form, the smith uses files and planes to bring out the final shape, followed by a rough polish. At this time, all the distinctive characteristics of the sword are present—a clearly defined profile, point, and ridgelines, the tang, and an even, level surface. All that remains is for the smith to prepare the edge.
Yaki-ire: Hardening the Edge
The hardening of the edge is in many ways the most important, and the most difficult, aspect of the sword-making process. It is the hardening of the edge that gives the blade its ability to take and retain amazing sharpness. To begin with, the blade is coated in yakibatsuchi, a mixture of water, clay, ash, and other ingredients. Every smith has his own special recipe, often a closely kept secret. The yakibatsuchi is applied over the surface, thicker along the spine and thinner at the edge. Working in a darkened forge room using only the light of the glowing coals, the smith carefully heats the blade. As the temperature rises, crystal structures within the metal begin to change. The smith carefully observes the color of the glowing blade, and when the critical temperature is reached the sword is quickly quenched in a trough of water.
At the critical temperature, around 750°C, the structure of steel changes to austenite, a phase where carbon thoroughly combines with iron. When the blade is quickly cooled by quenching, austenite changes to martensite, the hardest type of steel. However, where the thick yakibatsuchi was applied, the blade will cool more slowly, turning not into martensite but instead forming ferrite and pearlite, which are softer and more flexible. Like the kawagane and shingane, this combination of hard edge and softer body is what gives the blade its desirable qualities.
The hardening of the edge also creates a visible change in the surface of the metal. Depending on the way in which the clay mixture was applied, a variety of effects can be produced. This edge pattern is called the hamon, and is one of the most important aspects in the aesthetic appearance of a blade. Like the jihada, each of these patterns has a specific name. Suguha, for example, is a very straight hamon, while sanbonsugi describes a zigzag line in clusters of three.
After the hardening of the edge, if the smith is satisfied with the appearance and quality of the blade, it is then passed on to the polisher, who will give the blade its final mirrorlike polish, and other craftsmen who will make the scabbard and sword mountings. Complete mountings (36.120.417,418) have many elements, including metalwork such as tsuba (36.120.79) and menuki, lacquered wood, silk cords and wrapping, and ray-skin grips. Though these are all works of art in themselves, the blade remains the true centerpiece of the finished work, an example of the ingenuity of centuries of Japanese smiths and their desire to achieve the perfect blend of technology and art.