Written by Jeremiah Bourque. Cover and sword construction graphics by Andrew Adams.
Historical 日本刀 ( “nihontou”, lit. “Japanese swords”) are not just pieces of art, but finely tuned, exquisitely crafted weapons of war. This short book is a celebration of the craftsmanship and dedication that went into the creation of these exquisite blades. One can simply look at these weapons and appreciate them for how they look, but knowing what lies under the surface is even more compelling. Together, the two form a strong foundation for true appreciation of the Japanese sword. Japanese swords are said to be the soul of the warrior. They are also said to hold the soul of the swordsmith. This should not be misunderstood by taking it too literally; it is largely a figurative expression. Nonetheless, a reader who comes to comprehend the effort and skill required to forge these blades will understand how the sword truly contains a piece of the soul of its smith, for the sword reflects the qualities of his spirit. I hope you will enjoy this book. Thank you very much. - Jeremiah Bourque
Section I: The Creation of a Japanese Sword 1. The Birth of Hagane (Steel) Japanese sword construction begins with the most basic of raw materials: satetsu (砂鉄), literally “iron sand.” As the name implies, this is iron-rich sand found in abundance in Japan. By contrast, actual iron ore is a scarcity on the Japanese islands. In modern times, researchers have found these iron sands to be nearly devoid of the impurities that weaken final steel products. Thus, what began as a historical accident and an invention born of pure necessity evolved into the production of steel superior to that smelted elsewhere. Today, virtually all steel outside of Japan is produced from iron ore (and therefore includes additional impurities), but Japan continues to use iron sands for producing steel for extremely refined tasks. For example, Kai Industries is a leading manufacturer of surgical blades made from steel produced from iron sand. Traditionally, hagane ( 鋼 ), or steel, was produced using a tatara ( 鑪 ), a furnace named for the word for “foot bellows” (also “tatara,” kanji: 蹈鞴). That is, the tatara is a furnace with a foot-operated bellows used to add air – and therefore oxygen – to the fire to further raise the temperature. Producing steel by this method is known as tatara smelting (たたら製鉄).
2. Selecting the Proper Hagane
The traditional tatara furnace fared no better than similar furnaces all around the world. That is to say, the process was unreliable; temperature fluctuations resulted in inconsistent carbonization of the iron. Some of the resulting metal alloy was wrought iron, iron with a very low carbon content; some was pig iron, with such a high carbon content that the metal was exceptionally brittle. Only a master smith, relying on long years of experience and the traditions handed down from master to apprentice for centuries, can reliably choose the pieces of hagane (é‹ź), or actual steel, that are useful for the construction of a Japanese sword. In most cases, this is not a single grade of steel; multiple grades are profitably employed, as we will see later. Incredibly, master smiths differentiate steel pieces by their visual characteristics. This is truly what one may call a skill, rather than any kind of science. The mind ponders how much trial and error must have occurred before the smiths of old times got it right on a consistent basis and were able to pass down knowledge to their deshi (ĺź&#x;ĺ?), or apprentices, over the centuries.
As smiths go, being part of a long and illustrious tradition was certainly helpful to one’s reputation. Even so, a smith’s reputation was ultimately personal; a track record of verified success made the great smiths revered within their trades. After all, selecting the wrong pieces of steel would cripple all later steps. Modern tatara smelting is more efficient and better controlled. While few people still make Japanese swords in Japan (most “Japanese swords” are made in China, from where the original techniques were imported), those that do are able to take a chunk of raw hagane, pound it to remove impurities and better distribute the carbon, and separate chunks into a pair of blocks suitable for sword manufacturing. While in old times, a smith would have his deshi do the pounding, a modern foot-operated pounding machine simply and effectively eliminates the need for extra manpower for this step.
3. Tamahagane, the Jewel Steel In other parts of the world, sword forging via pattern welding employed a process of carbonization, adding carbon back to steel to bring the level of carbon up to standard, and forge welded this higher grade steel with lower grade, softer steel, resulting in a natural line (the “pattern”), even without the decorative finishing that became popular, turning the pattern into art. This is not, however, the process used in Japan. Proper selection of the pieces of steel – first by the iron smith, then by the swordsmith – ensures that a high grade of very pure steel is the foundation of the remainder of the process. While the iron smith does his best, the swordsmith is ultimately responsible for final screening of the steel. Once the proper pieces of steel are chosen, the smith and his deshi re-heat the steel and forge weld the pieces into a single block. (For the uninitiated, this means pounding the hot pieces together until they merge.) This work must be done briskly, lest the metal cool down and become unworkable. Pounding and re-pounding the steel into a rectangular shape distributes the carbon content evenly. The hardest steel used in Japanese swords – that is, the steel always used for the edge – is known as tamahagane ( 玉 鋼 ), or “jewel steel,” emphasizing its precious nature to both the smith and the end user.
4. Layers upon Layers It is at this stage that the steel is folded into layers. The ultimate number of layers is the result of simple mathematics, as follows:
The piece of steel is folded. Result: Two layers. The steel is folded again. Four layers. Folded again. Eight layers. Folded again. Sixteen layers. ...And so forth.
Tamahagane was folded by good smiths between 12 and 16 times. Twelve folds would produce 4,096 layers. Sixteen folds would produce 65,536 layers! Twenty folds would produce a million layers, beyond which no folding would bring any conceivable metallurgical advantage. So why layer the steel? Layers create two closely related effects. The first effect is to allow the layers to bend at rates that differ very, very slightly when lateral (sideways) stress is placed upon the blade. Layering the steel creates resilience (toughness) that makes each part of the blade more likely to bend, and spring back, but not break. Broken swords are not much good to the warriors holding them. The second effect is to allow the layers to cool at different rates during the tempering process. The faster steel cools, the harder it becomes; this is a product of the carbon that is mixed with the steel. The slower steel cools, the softer it remains. This creates a natural mix of hardness and flexibility. Kai Industries’ surgical blades, mentioned earlier, employ the same layering process today. This development is impressive enough, but Japanese sword construction gets even more advanced.
5. The Core of the Matter
A cutting instrument, including a sword, must fulfill certain basic parameters. 1. It must have, and keep, an edge. 2. It must not bend beyond a certain limit. 3. It must not break. Hardness solves problems #1 and #2, but #3 is a serious problem. The harder steel is made, the more brittle it becomes. This exposes the blade to chipping (when hitting something sufficiently hard) or simply breaking (when exposed to stress that exceeds its resilience). Layering helps, but Japanese swordsmiths felt they needed more. For the moment, let us examine the so-called kobuse method of forging.
This method employs tamahagane for the edge and the sides of the blade. However, the back and interior – in other words, the core – of the blade are composed of softer steel, known as shingane (“body steel”), subject to the same process of layering. Thus, Japanese swords forged in this manner have both the advantages of layering and a softer, more flexible core. The result is a sword that: 1. Keeps its edge. 2. Retains its structure while cutting. 3. Strongly resists breaking while cutting. This creates, in a highly efficient, relatively lightweight package, a blade that properly achieves its primary duty: to cut, and cut well, without breaking from the stress of doing so. Unfortunately, this method does have a serious drawback: edge on edge contact between Japanese swords is likely to chip both blades so much that the blades would have to be ground down to the core to eliminate the chip. This would create a portion of the blade that will not properly retain an edge, ruining the blade for any practical purpose. Thus, edge on edge parrying is not how samurai ever wanted to do business. If parrying is unavoidable, proper technique is to use the edge of the blade only. Ideally, the defender essentially slaps or swats the attacking blade out of the way. Knowing this, swordsmiths began modifying their creations to better resist damage from edge on side impacts. So how does a swordsmith insert this core? It’s actually quite simple, really: the tamahagane is forged into a rectangle with a U-shaped opening in the back. The shingane is inserted into the slot. The two pieces are forge welded together and beaten and drawn out into the form of a blade. This lays the groundwork for the final step in creating the blade itself: tempering.
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