The traditional construction of a Japanese sword, such as the katana, wakizashi, tanto, naginata, or no-dachi, is a process refined from perhaps thousands of years of experience. The construction of a blade might take up to a year to complete, passing through the hands of many professionals, each whom might spend a decade or more in apprenticeship to learn their craft, and fetch a value of tens of thousands of dollars if constructed by the masters of the art.
The primary challenge in sword forging is the delicate balance between sharpness and durability. High-carbon steel can retain a sharp edge for an extremely long time, but is very brittle. A sword made entirely out of it would snap after only a little use. Low-carbon steel has a great deal of "give" and is very durable, but will dull very easily and often bend. Thus, to get "the best of both worlds", a sword is made by welding a durable, low-carbon steel core, called the shingane, to a high-carbon steel outer jacket, called the kawagane, which will retain a fine edge.
While Japan switched to the conventional style of Western steel production after World War II, the traditional Japanese method for steel production is still used for swordsmithing. Currently, all the steel used in swordsmithing is produced in a smelter in the Shimane prefecture. Only two or three tons are produced per year, at roughly a cost of two hundred times that of normal steel.
Traditional Japanese steel, known as tamahagane, is made from a black sand called satetsu. The sand is formed by the erosion of iron ore deposits and is found as a sediment in streambeds. Charcoal added to the sand during smelting adds the necessary carbon to make the iron into steel. As the charcoal is not uniformly distributed in the smelting process, the resultant chunks of steel will contain anywhere from 0.6% to 1.5% carbon content. Part of the necessary training of a Japanese swordsmith is to identify the carbon content of the various chunks by color and texture, and steel is usually chosen of roughly 1.0-1.5% carbon content as much of the carbon will be burned or worked out of the steel during the forging of the sword.
Forging the Blade
First, the softer core of the blade is made. A low carbon chunk of tamahagane, roughly the size of a brick, is heated until softened. The steel is hammered out until it is elongated in one direction, then folded back over itself crosswise. This folding process is repeated roughly 10 times, driving out any impurities in the steel. After this, the steel is hammered into the long, thin core, slightly smaller than that of the finished blade.
The outer jacket of the blade is made in a similar fashion, but with tamahagane of a higher carbon content. The outer jacket is folded many more times, resulting in about 30,000 folds in the final blade, which create the "grain" of the sword.
After the jacket is made, it is re-heated and carefully wrapped around the core. The core and jacket must be heated selectively to weld the two together into a single blade, while avoiding any sort of dirt that might create an air bubble between the two layers, ruining the sword. A slight curve is added at this time, which will be further exaggerated during the heat treatments and tempering.
Heat Treating the Blade
At this point, the blade is in a rough and unfinished form. The outer steel jacket, while high in carbon, is composed of steel in forms known as ferrite and pearlite, describing the way the carbon is dissolved with the iron. The ferrite and pearlite forms of steel are not hard enough to take a sharp edge, but can be converted into a form known as martensite, which can take an edge, by heating up the steel and then rapidly cooling it. The problem is that if the core of the sword were to turn into martensite, the sword would become too brittle to use. Thus, a special method is used to rapidly cool the edge of the sword, converting it into martensite, while allowing the core to cool slowly and return to the ferrite and pearlite forms.
First, the swordsmith uses a sharp two-handed drawknife to remove irregularities and unevenness in the sword. He then uses a metal file to shape the back and edge of the sword. Finally, he rubs a coarse stone over the sword, creating a rough texture that adheres well to a specially prepared clay which will aid in the heat treatment process.
The clay is thickly painted on to the back and sides of the sword, and thinly onto the edge. This insulates the core of the sword while exposing the edge, so that the edge of the sword can cool rapidly and form martensite while the core will cool more slowly. The smith paints the border between the thin and thick clay artistically, as the line created by the meeting of the martensite and ferrite/pearlite is very distinct, known as the hamon, and is unique for each blade.
The sword is carefully heated in the forge to the critical temperature where the steel can be converted into martensite. This process, known as yaki-ire, is entirely gauged by the color of the blade, and thus is always done at night. When it has reached the right temperature, the sword is plunged into a trough of cold water, quickly cooling the edge of the blade and allowing the core to cool slowly. Nearly half of all blades made will crack during the yaki-ire due to the rapid change in temperature.
Finally, blades that do survive the yaki-ire are heated again, this time to a much lower temperature, which does not affect the properties of the steel but relieves the stresses built up during the heat treatment process.
At this point, the blade is completed as far as the swordsmith is concerned. The blade must then be carefully polished by a separate professional polisher who can bring out the detailed features of the blade, such as the grain and the hamon.
Source:Southern California Naginata Federation, http://www.scnf.org