direct descendants of such Kamakura-period swords.
Many details can be seen when examining the hamon. The nioiguchi—the boundary defining the hamon, also called the “habuchi”—should be clear and unbroken along its length. Hamon often have extensions or projections called “ashi” going from the habuchi toward the edge of the sword, which help define the details of the hamon. Ashi usually form straight lines roughly perpendicular to the boundary of the hamon. They can be very short and almost invisible, or very prominent and long. Their presence indicates that the hamon steel is well interlaced with, and bonded to, the steel in the body of the sword.
This sword from the Kamakura period has a well-developed choji hamon.
HAMON PATTERNS
OSHIGATA AND HAMON
A swordsmith needs years of training to be able to master the extremely difficult process of making a hamon, and no two smiths carry it out in exactly the same way. In fact, a properly made complex hamon can function as a smith’s “fingerprint.” There are many smiths, or groups of smiths, whose work can be identified from their hamon alone.
Photographs of swords usually show only the outline of the hamon against the body of the sword; fine details are generally not visible. Traditional texts on Japanese swords are illustrated with oshigata, tracings of the shape of the sword with the hamon drawn in by hand in fine detail. Since the hamon is such an important aspect of a Japanese sword, the best way to present it in a text is to have a photograph of the sword alongside an oshigata of the same blade showing the hamon in detail.
In the illustration at right, a full-length photo of a tanto (dagger) is presented alongside its oshigata. The photo shows the overall shape and form of the blade, as well as the color and some details of the steel surface. However, few details of the hamon are visible beyond its general outline. The oshigata shows a thick, complex hamon boundary with an intricate shape, along with details such as ashi (lines of softer steel extending into the hamon to the cutting edge). When hamon are described or discussed, the details shown in the oshigata are used to describe the hamon and compare it to others.
Photographs of swords can show the hamon outlines, but the intricate and complex details in the hamon usually are not visible. Traditionally, an oshigata is drawn by hand on a tracing of the blade’s outline to show all the important details. Here, a photograph and oshigata of the same tanto are shown side by side. Although the photograph shows several important features of the tanto, the hamon is only visible in outline. The accompanying oshigata of the same tanto shows the hamon in detail. Presenting a photo and an oshigata together is the best way to show a sword in a publication.
THE STEEL AND STRUCTURE OF THE JAPANESE SWORD
Because the properties of the Japanese sword depend on the steel it is made from, it is helpful to learn something about the unique qualities of traditional Japanese steel. Broadly speaking, steel is a combination of iron and carbon. Japanese steel is made in a traditional Japanese-style smelter called a “tatara,” using satetsu, an iron ore that is found in sand form. The steel that comes from the tatara as a result of this process is called “tamahagane.”
When the satetsu is smelted in the tatara, the resulting steel has a high carbon content—up to 2 or 3 percent. However, to make a functional and practical sword, steel with a final carbon content of 0.6 to 0.7 percent is ideal. Thus, the swordsmith’s first task upon receiving tamahagane from the tatara is to refine the steel and reduce the carbon content to this level.
The goals of the smith are to make the steel more homogeneous, to produce a uniform carbon content, and to remove impurities in the steel. This is accomplished by hammering out the tamahagane into a thin plate, then breaking up the plate into small pieces about an inch (2–4 cm) in size. These pieces are then stacked, heated, and hammered out into a billet. The billet is then repeatedly refolded over onto itself.
Throughout the process of hammering and folding the steel, the smith reduces the carbon content by about 0.2 percent with each fold. The steel is repeatedly folded until it reaches the appropriate carbon content of 0.6 to 0.7 percent. The smith can judge the carbon content of the steel by its behavior during the folding and forging process. Once the steel reaches the proper carbon level, it is forged out into the shape of a sword, and some filing and grinding work is done to further refine the shape.
The steel has to be worked in this manner for several reasons. First, the process optimizes the carbon content. Second, it removes slag and impurities; furthermore, the steel is more homogeneous after the forging process. The original tamahagane is very inhomogeneous, and would not make a good sword. In addition, the forged steel is tougher; that is, it is far more ductile than the original tamahagane and less likely to bend or break under use. Once the steel has been sufficiently worked and the sword has been shaped, the process of making the hamon, which requires a high level of carbon in the steel, can be carried out.
The basic principle involved in making a hamon is fairly straightforward, and utilizes an important property of steel: when steel is heated to a high temperature and then rapidly cooled, its crystalline structure will change, making it much harder than it was before. In practice, however, the details are critical; it took the Japanese perhaps five hundred years to progress from making the earliest basic hamon to being able to create the characteristic sophisticated hamon we see today.
Making a hamon requires some important conditions: the steel must be very pure and have almost no elements in it except iron and carbon. The carbon content must be fairly high: as stated earlier, 0.6 to 0.7 percent is ideal. The area of the steel where the hamon is to be produced must be heated to a critical point—typically close to 1470°F (800°C), the temperature at which steel loses its magnetism. Once it has reached the proper temperature, the steel must be cooled rapidly, usually by immersing it into a tank of water. The process of forming a hamon by heating and quenching the blade is called “yaki-ire.”
There is another very important element in making a hamon. The hardened steel that composes the hamon must be restricted to the edge of the sword. If the entire sword is hardened, it will be very brittle and likely to break in use. Only the area along the edge of the sword is to be heated and rapidly cooled. To solve this problem, Japanese sword-smiths developed a method of hardening only the edge of the sword by coating the blade with clay in a complex pattern before beginning the heating and cooling process. Because the entire blade must be heated during this process, the clay coating is added in such a way as to allow the edge to cool very rapidly while at the same time slowing down the rate of cooling on other parts of the sword. The slower cooling rate on the body of the blade will prevent it from hardening during this process. Ide ally, the result will be a hard edge with an intricate hamon pattern and a relatively soft body that will remain ductile and tough. If the hamon is properly designed and made, it will be unlikely to suffer much damage in use. In addition, a properly designed hamon will limit the size of nicks and damage that the edge will suffer during use.
The diagram on page 47 shows why it is possible to make a hamon in high-carbon steel. The horizontal axis of the iron-carbon diagram shows the percentage of carbon in a pure iron-carbon mixture, while the vertical axis shows the temperature. When iron and carbon are combined, the percentage of carbon and the temperature of the compound determine the form that the steel will take. Each form has different properties, the most important one for a sword being how hard the metal will
