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 shape and a very functional appearance. Its purpose is to cut well, and it is well designed for that. It is usually single edged, and is very slender and graceful, with some curvature. In addition, a well-made Japanese sword also feels well-balanced and very comfortable in the hands.
The steel on the surface of a Japanese sword (the jigane) has a definite color and texture. As the steel has a rather dark appearance, a good polish is required to bring out the details. In examining a well-made and well-polished sword, a clear color and fine texture can usually be seen. In addition, there is usually a distinct surface pattern that results from the repeated folding of the steel during forging. This pattern, which is called the “jihada,” will vary from sword to sword depending on exactly how the smith made the steel for a particular sword, and on what kind of steel he used. The surface texture, color, and jihada are all features that should be carefully observed when evaluating a sword.
The appreciation of a Japanese sword also encompasses the properties inherent to the steel itself; that is, the different forms of steel and crystalline structures in the steel. When looking at Western swords, in contrast, the hilt, engraving, and other embellishments can be an integral part of the sword, and are considered alongside the blade itself when evaluating the sword. In this respect, appreciating and evaluating a Japanese sword is different from appraising other swords or edged weapons. Japanese swords are examined and appraised by looking only at the bare, unmounted blade.
JIHADA PATTERNS
The process of repeatedly hammering out and folding the steel over onto itself produces a pattern, or jihada, in the steel surface. Jihada can vary extensively due to differences in the forging techniques of different swordsmiths. A sword must also have a very good polish for these patterns to become easily discernible. The patterns shown here can be seen on swords from different smiths of various historical periods.
MASAME HADA 柾目肌
This is a straight pattern.
ITAME HADA 板目肌
This pattern resembles wood grain.
