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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 be.

      Steel assumes various forms, including martensite, austenite, ferrite, pearlite, and cementite, as it is heated and cooled. The relatively soft body of the Japanese sword is largely composed of ferrite and pearlite, which form below 1340°F (727°F). The iron-carbon diagram shows a line labeled “critical temperature” that goes from 1340°F to over 1650°F (727°C to over 900°C), depending on the carbon content of the steel. If the steel is heated to this critical temperature or higher, it will lose its magnetism and take the form of austenite. However, a sword cannot remain at these high temperatures in normal life or use. If the blade is heated to above the critical temperature and cooled very rapidly, the austenite structure will break down and transform into another structural form called martensite. Unlike austenite, martensite can exist at room temperature; it is also very hard. Martensitic steel along the edge of a sword creates an optimal cutting edge.

      To create a hard martensite edge while simulta neously leaving softer ferrite and pearlite in the sword body, the blade must undergo the yaki-ire process. The major problem is to insure that the edge area cools much more rapidly than the body of the sword, so that the edge becomes martensitic steel while that in the body of the sword remains in the form of ferrite and pearlite. As mentioned above, this is accomplished by coating the blade with a layer of clay that is very thin over the edge area and relatively thick over the body of the sword. The areas covered with the thicker layers of clay require only a few thousandths of a second longer to cool than the edge, but this is sufficient to leave the steel in a softer state after yaki-ire.

      If the sophisticated clay coating functions as it is supposed to, this heating and quenching process will result in different hardnesses in the different parts of the blade. While the pattern of the clay affects the shape of the hamon, it does not exactly resemble the final hamon. Only after a great deal of experience and training will a swordsmith be able to create a clay coating that will give him exactly the hamon he wants. Similarly, it takes years of experience for the smith to be able to heat the blade to the precise temperature needed in order to produce the desired hamon.

      In addition to the process already described, the construction of a Japanese sword usually requires an additional step: the forging of a softer steel core into the center of the sword. This core serves as a kind of shock absorber to protect the blade from extreme stresses and fracturing. Thus, the process of constructing a properly made Japanese sword results in a composite structure containing three types of steel:

      • The soft steel core (the shingane);

      • The hard, high-carbon outer steel jacket that forms the surface of the sword (the kawagane);

      • The hardened edge formed of martensitic steel (the hamon).

      The structure, composition, and metallurgy of the Japanese sword are unique. First, the quality of the steel makes possible its slender and graceful shape. The cutting edge has a visible pattern along the edge, unseen among other swords in the world, indicating that the edge has been hardened to a far greater degree than the rest of the sword. The steel itself shows signs of the forging necessary to make such a high-quality steel, and a pattern (jihada) can usually be seen on its surface. These features are all essential to the appreciation and evaluation of a Japanese sword.

      A piece of tamahagane, the high-carbon steel used to make a Japanese sword. Tamahagane is smelted from iron ore found in sand form.

      This iron-carbon diagram shows the names of the different crystalline or structural forms taken by steel depending on the conditions. The two variables here are the temperature (shown on the vertical axis) and the carbon content (shown on the horizontal axis). To form the hamon, the blade must be heated above the critical temperature contour, where it loses its magnetism. A typical Japanese sword, with a carbon content of 0.6 to 0.7 percent, must be heated to around 1380°F (750°C) or higher to form a good hamon on cooling.

      Yoshindo applies a two-component clay coating to a new sword to form the hamon. The pattern of the clay can be very detailed; it is the experience and skill of the swordsmith that will determine what the resulting hamon will look like.

      Cross-section of a typical Japanese sword. The blade has a core of soft steel (shingane) at its center, and is wrapped in a hard high-carbon steel jacket called the “kawagane.” The yaki-ire process results in a cutting edge made of martensitic steel, which is far harder than the steel in the body of the sword. Thus, a properly made Japanese sword is composed of three different types of steel.

      JIHADA AND JIGANE

      There are customary ways of viewing and appreciating the Japanese sword, as previously described. The sword’s singular appeal as an art form derives from the fact that innumerable features to be appreciated and evaluated lie in the blade and the steel itself. Although steel is used in other art works, generally speaking the shape is the feature of prime importance in such pieces. In appreciating the Japanese sword, there are many other elements to observe as well. Of course, the shape itself is a consideration: a good sword has a graceful

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