Naginata Blade Construction

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Part Two: Heat Treatment and Tempering Methods

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Introduction:

After forging is completed, the blade is close to the desired shape and length but does not yet have the metallurgical properties needed to make it useful in battle. The edge, although made of the higher carbon "jacket steel" (kawagane), is still not hard enough to retain its legendary sharpness. In this article, the heat treatment and tempering processes will be discussed. During that time, the microscopic crystalline structure of the jacket steel will be transformed; resulting in a superb blade capable of withstanding rigorous battlefield use.

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Shiage: Rough Grinding and Shaping

The first step in preparing the blade for heat treatment is shiage, or rough grinding and shaping. The smith uses a special two-handled drawknife; pulling it along the length of both sides of the blade. The drawknife, or sen, is quite sharp and is usually made from a piece of a sword or naginata blade. This removes any surface irregularities or unevenness. Then, a standard metal file is used to shape the back and edge, although at this point the edge is not sharpened. Finally, a coarse carborundum stone is rubbed along the entire blade to make its surface slightly rough. This is necessary for proper adhesion of the special clay which will be applied later during the heat treatment process.

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Blade Metallurgy

Metallurgical analysis of the naginata blade at this point shows that it is composed of a mixture of ferrite and pearlite or pearlite and cementite; depending on the carbon content. These terms are used by metallurgists to describe the extent to which carbon has dissolved into the iron crystals, as well as the way the crystals are shaped.

During heat treatment, the blade will be heated to a temperature slightly above that which is referred to as the "critical temperature" (Tc). When steel is heated to its Tc (approximately 1,300 degrees F), the ferrite and pearlite are transformed to austenite. Austenite is composed of iron and carbon, just as ferrite and pearlite are, but differs from them in the manner in which the iron crystals are shaped as well as the amount of carbon which is dissolved into them. If the blade is allowed to cool slowly back to room temperature after being heated above the Tc, the steel will simply change back to ferrite and pearlite. Unfortunately, neither of these materials are hard enough to be sharpened to a razor-like edge. For this, the steel along the edge of the blade needs to have its crystalline structure transformed to a different material known as martensite.

Martensite is formed when steel, heated above its Tc, is cooled very rapidly by immersing the red-hot blade in water. Although martensite is very hard and capable of retaining a sharp edge, it's also very brittle. If the entire blade were made of martensite, it would be useless in battle. Therefore, the "core" must be allowed to cool slowly so that it can revert back to the original ferrite and pearlite structure, which is soft enough to be flexible. Conversely, the edge must be cooled very quickly so that it transforms into the hard martensite. The swordsmith, by controlling the rate of cooling of different parts of the blade during the water quenching process, will produce a blade which has these opposing properties.

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Tsuchioki: Application of the Adhesive Clay Mixture

Before heat treatment is begun, a special type of adhesive clay mixture is applied to the entire blade. A relatively thick layer is placed along the top and upper sides, and a much thinner layer along the edge. This clay will act as an insulating "blanket" during the quenching process; allowing those areas covered by a thicker layer to cool much more slowly.

When applying the thin layer along the edge, the swordsmith usually applies it in an artistic pattern. The martensite, which will be formed when the edge steel cools very quickly, will be readily visible after the blade is polished. This artistic pattern of martensitic steel is called the "hamon". Uniquely patterened on each blade, it will greatly add to the sword's beauty and worth. After the swordsmith has finished applying the clay mixture, the blade is set aside for about an hour to allow it to dry thoroughly.

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Yaki-ire: the Water Quenching Process

The smith then inserts the clay-coated blade into the forge and begins heating it. The blade is slowly pulled back and forth through the hot coals until it's been uniformly heated above the critical temperature. Because the smith judges the temperature of the steel solely by its color, yaki-ire is always done at night. Working in the darkened smithy, the swordsmith can accurately gauge the steel's temperature. Steel which has been heated to a temperature just above its Tc will be bright red-orange in color.

When the smith determines that the blade has reached the desired temperature, he plunges it into a trough of cool water. Those areas covered by a thicker layer of clay will cool slowly, allowing them to simply revert back to ferrite and pearlite. The edge, covered by only a thin layer of clay, cools very quickly and is transformed into martensite.

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Yaki-modoshi: Blade Tempering

Due to stresses induced during rapid cooling of the red-hot steel, less than 50% of blades survive the yaki-ire process without cracking. Those that do survive must then be tempered (stress relieved). During tempering, the blade is heated again, this time to a much lower temperature (approximately 300 F) and re-quenched. This lower temperature, since it's well below the critical temperature, won't alter the molecular structure of the steel. Instead, it will simply help to relieve any internal stresses which have built up.

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Sorinaoshi: Adjusting the Curvature

After tempering, the clay is removed from the blade, and any fine adjustments curvature (sorinaoshi) are made . This is done by heating and gently hammering those areas which need to be corrected. At this point, any decorative grooves or ornate carvings (horimono) are also added. The smith then turns the blade over to the polisher for polishing.

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In Part 3, the polishing process will be discussed. It's during this time that the true beauty and luster of the blade will become visible, as well as the subtle features present in the steel such as the grain (jihada) and heat-treated edge (hamon).

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Written by Sue Kent
Southern California Naginata Federation

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Related Articles of Interest: 

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References:  

  A sincere "Thank You" to Mr. Dennis Heise for his technical assistance.

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