20/11/2009
The authentic Japanese sword is made from a specialized Japanese steel called “Tamahagane” (The Japanesse Steel. Black sand) which consist of combinations of hard, high carbon steel and tough, low carbon steel. There are advantages and setbacks to both types of steel. High-carbon steel is harder and able to hold a sharper edge than low-carbon steel but it is more brittle and may break in combat. Having a small amount of carbon will allow the steel to be more malleable, making it able to absorb impacts without breaking but becoming blunt in the process. The makers of a katana take advantage of the best attributes of both kinds of steel. This is done by a number of methods, most commonly by making a U-shaped piece of high-carbon steel (the outer edge) and placing a billet of low-carbon steel (the core) inside the U, then heating and hammering them into a single piece. Some sword-makers use four different pieces (a core, an edge, and two side pieces), and some even use as many as five.
The block of combined steel is heated and hammered over a period of several days, and then it is folded and hammered to squeeze the impurities out. Generally a katana is folded no more than sixteen times, then it is hammered into a basic sword shape. At this stage it is only slightly curved or may have no curve at all. The gentle curvature of a katana is attained by a process of quenching; the sword maker coats the blade with several layers of a wet clay slurry which is a 
special concoction unique to each sword maker, but generally it is composed of clay, water, and sometimes ash, grinding stone powder and/or rust. The edge of the blade is coated with a thinner layer than the sides and spine of the sword, then it is heated and then quenched in water (some sword makers use oil to quench the blade). The clay slurry provides heat insulation so that only the blade’s edge will be hardened with quenching and it also causes the blade to curve due to reduced lattice strain along the spine. This process also creates the distinct swerving line down the center of the blade called the hamon which can only be seen after it is polished; each hamon is distinct and serves as a katana forger’s signature.
The hardening of steel involves altering the microstructure or crystalline structure of that material through quenching it from a heat above 800 °C (1,472 °F) (bright red glow), ideally no higher than yellow hot. If cooled slowly, the material will break back down into iron and carbon and the molecular structure will return to its previous state. However, if cooled quickly, the steel’s molecular structure is permanently altered. The reason for the formation of the curve in a properly hardened Japanese blade is that iron carbide, formed during heating and retained through quenching, has a lesser density than its root materials have separately.
After the blade is forged it is then sent to be polished. The polishing takes between one and three weeks. The polisher uses finer and finer grains of polishing stones until the blade has a mirror finish in a process called glazing. This makes the blade extremely sharp and reduces drag making it easier with which to cut. The blade curvature also adds to the cutting power.
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Smelting steel
The traditional katana sword is fashioned only from the purest steel, which the Japanese call tamahagane (“jewel steel”). Over three days and three nights, smelters using ancient techniques shovel roughly 25 tons of iron-bearing river sand and charcoal into the mouth of a tatara, a rectangular clay furnace built specifically to produce a single batch of tamahagane. Composed of carbon, the charcoal is as much a key ingredient in steel as a source of fuel for the furnace. The tatara will reach temperatures of up to 2,500°F, reducing the iron ore to steel and yielding about two tons of tamahagane. The highest quality tamahagane can cost up to 50 times more than ordinary steel made using modern methods. The traditional katana sword is fashioned only from the purest steel, which the Japanese call tamahagane (“jewel steel”). Over three days and three nights, smelters using ancient techniques shovel roughly 25 tons of iron-bearing river sand and charcoal into the mouth of a tatara, a rectangular clay furnace built specifically to produce a single batch of tamahagane. Composed of carbon, the charcoal is as much a key ingredient in steel as a source of fuel for the furnace. The tatara will reach temperatures of up to 2,500°F, reducing the iron ore to steel and yielding about two tons of tamahagane. The highest quality tamahagane can cost up to 50 times more than ordinary steel made using modern methods.
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Dissolving carbon
While fired at high temperatures, the tamahagane is never allowed to reach a molten state. This is to ensure that just the right amount of carbon will dissolve into the steel, and that the percentage of carbon will vary throughout the tamahagane (between 0.5 and about 1.5 percent). Katana-makers use two types of tamahagane: high-carbon, which is very hard and allows for a razor-sharp edge, and low-carbon, which is very tough and allows for shock absorption. A sword composed simply of one kind of steel or the other would either dull too quickly or be too brittle. On the third night of smelting, when the tatara masters break open the clay furnace to expose the tamahagane, they use the degree of ease with which the pieces of newly made steel break apart to discern their carbon content.
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Removing impurities
The best pieces of tamahagane are sent to a swordsmith, who heats, hammers, and folds the steel repeatedly in order to further combine the iron and carbon, and to draw out any remaining undissolved impurities, or “slag.” This step is as vital as it is tedious, because if other elements besides iron and carbon remain in the resulting sword, they will weaken it. Once the skilled smith has removed all of the slag, he can judge the carbon concentration of the tamahagane by the degree to which it yields to his constant pounding. One expert has likened eliminating slag from steel to squeezing liquid from a very hard sponge.
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Forging the sword
After the smith hammers all slag from the tamahagane, he heats the hard, high-carbon steel and shapes it into a long, U-shaped channel. He then hammers the tough, low-carbon steel, which he has shaped so it will make a snug fit into the channel and forges the two metals together. Both types of tamahagane are now exactly where they need to be: the hard steel forms the sword’s outer shell and deadly blade, while the tough steel serves as the katana’s core. This perfect balance of properties is what made the katana the samurai’s most durable and prized weapon.
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Coating the katana
While the katana’s body is now complete, the swordsmith’s work is far from over. Just prior to firing the sword a final time, he paints a thick, insulating mixture of clay and charcoal powder onto the blade’s upper sides and dull back edge, leaving the sword’s sharp front edge only lightly coated. This serves both to protect the blade and to give it its signature wavy design called the hamon, which later polishing will reveal.
The swordsmith then places the katana back into the fire to be heated to just below 1,500°F; any hotter and the sword might crack during the next step.
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Curving the blade
Next, the smith pulls the katana from the fire and plunges it into a trough of water in a rapid cool-down process called “quenching.” Because the sword’s back edge and inner core contain very little carbon, they can contract more freely than the high-carbon steel at the front edge of the blade. The difference in both the degree and speed of contraction between the two forms of tamahagane causes the sword to bend, creating the distinctive curve. This is a tricky stage, in which as many as one in three swords is lost.
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Polishing the blade
The katana, fully forged, now goes to a skilled sword polisher, who may spend more than two weeks honing the sword’s razor-sharp edge. He meticulously rubs the blade with a series of grinding and polishing stones, some valued at more than $1,000 each and often passed down through families for generations. Sometimes called “water stones,” these tools are typically composed of hard silicate particles suspended in clay. As the clay slowly wears away during use, more silicate particles are revealed, guaranteeing excellent polishing quality throughout the life of the stone. Each consecutive set of polishing stones contains finer and finer silicate particles and removes less and less of the steel.
