Southern California Naginata Federation

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Naginata Blade Construction

Part One: Forging Techniques

Table of Contents

  Introduction
  Blade Properties
  Preparation of the Raw Steel
  Forge Construction
  Preparation of the "Core" Steel
  Preparation of the "Jacket" Steel
  Final Steps
  Related Articles of Interest  
  Upcoming Articles  
  References

Introduction:

Throughout history, the Japanese sword has had no equal. Its quality, beauty, and strength far surpass even the legendary Damascus and Toledo blades, and its cutting edge is said to be sharper than any other blade in the world. To this day, metallurgists have never been able to exactly duplicate the steel found in these marvelous works of art. The sword is known as the "soul of the Samurai", and in Japanese mythology it was one of the three sacred gifts given to the emperor by the Sun Goddess as a token of legitimate authority. What were the properties of the sword that gave it such metallurgical and spiritual significance? In this series of articles, we'll take a look at the methods used to manufacture sword and naginata blades, as well as their accessories (kodogu). We'll also learn about their proper care and handling. In this first section, the kitae, or forging process is discussed.

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Blade Properties

For battlefield applications, the steel used in naginata and sword blades was required to have two distinct properties. First, it had to be flexible enough to withstand direct impacts and thrusting types of cuts without breaking. Conversely, it had to be hard enough to retain its notoriously sharp cutting edge. How could one piece of steel be both flexible AND hard? Only the Japanese were able to develop forging and heat treating methods which resulted in steel that had BOTH properties.

Careful metallurgical examination of a Japanese sword reveals that it is composed of not one, but TWO distinctly different types of steel. It contains a soft "core", called shingane, which is made of low carbon steel. Wrapped around this is a harder "jacket" made of higher carbon steel, or kawagane. It is important to note that the finished blade is NOT a laminate, but instead consists of two separate pieces of steel which, through the forging process, have been welded together.

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Preparation of the Raw Steel

All steel is composed of iron and carbon. The source of the iron ore used in Japanese sword steel is a special type of black sand called satetsu. This black sand is formed by the erosion of natural iron ore deposits and is found mixed in with silts and sediments in streambeds. Today, all satetsu used in the manufacturing of blades is refined in a smelter (tartara) in Shimane Prefecture. During smelting, most of the impurities are removed from the molten iron. Charcoal is added during the smelting process, and is the source of the carbon needed to transform the iron into steel. The end result is tamahagane; the raw steel used to make blades. The amount of carbon in the tamahagane usually varies between 0.6% and 1.5%. It is the amount of carbon in the steel which will determine its hardness, so the swordsmith needs to select the tamahagane very carefully. Most smiths prefer to start with tamahagane which contains 1.0% to 1.5% carbon, knowing that some of the carbon will be lost during the subsequent forging process.

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Forge Construction

The forge consists of a "furnace", built into the earthen floor of the smithy, that's heated by special charcoal capable of burning at high temperatures. An air bellows, which is an integral part of the forge, provides a continuous flow of oxygen to the charcoal. This is an important design feature unique to Japanese bellows. It allows the volume and speed of the air flow, and as a result the temperature of the coals, to be precisely controlled. Directly adjacent to the forge is an anvil and a hydraulically powered air hammer. In earlier days, one or two apprentices did the hammering.

Before beginning his work, the swordsmith purifies himself and prays before the Shinto shrine located in the smithy. He prays for divine guidance, and that his efforts will please the deity. The finished blade is said to contain the soul of the swordsmith, as well as the soul of all future owners of the blade who will become its caretakers. It is for this reason that the blades are thought to have spiritual significance.

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Preparation of the "Core" Steel

The softer (low carbon) core steel, or shingane, is made first. A piece of tamahagane, about the size of a brick, is heated in the forge until softened. (At no time in the forging process is the tamahagane ever heated to its melting point.) It's then hammered until it becomes slightly elongated. At this point, the smith folds it in half crosswise and the entire procedure is repeated approximately ten times. The process of heating, hammering, and folding drives out impurities present in the steel. After these steps are completed, the core is heated and hammered into the shape of a long, thin metal wedge. It's then set aside while the jacket steel, or kawagane, is prepared.

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Preparation of the "Jacket" Steel

For the jacket steel, the swordsmith carefully selects tamahagane which is harder (higher in carbon content). Again, a piece about the size of a brick is repeatedly heated, hammered, and folded over upon itself. The jacket steel undergoes this process many times more than the core , resulting in steel which is composed of nearly 30,000 "folds", or layers. These layers produce the subtle and beautiful grain (jihada) that will become visible on the surface of the blade once it's polished.

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Final Steps

In the final stages, the jacket steel is hammered out until it measures slightly longer than the piece of core steel made previously. It's then re-heated and wrapped around the core. The smith carefully heats and hammers the two pieces together until they form a solid metallurgical bond. Extreme care must be taken during this process to ensure that no gaps are left between the jacket and core steels, and that no dirt or debris is trapped there. To do so would result in air bubbles or voids which would seriously weaken the blade and render it worthless in battle. After the two pieces of steel are joined, the smith continues to heat and hammer the blade until it measures close to the desired length. At this time he also adds a small amount of curvature to the blade, keeping in mind that this curvature will increase during heat treatment and subsequent tempering.

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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:  

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