Japanese Sword Making
The Ancient Japanese Methods of Sword-Making
The samurai sword is one of the greatest fighting weapons of all time, carefully engineered for close-up combat, renowned for its deadly cutting edge. It is certainly an object of absolute beauty from both an aesthetic and a scientific point of view.
The great swordsmiths of Japan were steeped in the mysterious traditions of the metal. They were able to transform this mystery into something that was very real, the samurai sword. It is said that the sword is the soul of the samurai.
This remarkable weapon reveals a highly developed metal making skill that dates back more than 1,000 years to 900 AD. Embedded deep into the material of the blade are different types of steel, each contributing to the sword’s deadly effectiveness and its unique design.
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In our common understanding, we think of metals as strong, but in fact, metals have other important characteristics. Controlling these characteristics requires expert formulation. So how, in the absence of understanding this science, were the ancient craftsmen of Japan able to achieve such technical excellence?
In the small village of Shimane, in southwest Japan, they still make the metal ore for samurai swords the same way it has been done for centuries, in earthen smelting furnaces called tataras.
Akira Kihara is one of the last remaining tatara masters in the world. He won’t sleep for three days and three nights as he watches the furnace create steel that is the quality needed for the world’s sharpest swords.
This special steel called tamahagane is made from iron ore sand, collected in local rivers, and charcoal. Within an hour, the iron sand sinks to the bottom, to what we call the bed of fire. When you look at the bed of fire you can clearly see, from the color, whether or not it has become tamahagane.
The traditional Japanese smelting furnace is constructed out of clay with a row of inlets along the sides. Air is blown into the furnace with bellows that drive the temperature up to nearly 2,500 degrees Fahrenheit.
Steel is basically iron, and to that you add a little bit of carbon, and the carbon gives it much more strength than the iron would have by itself. These subtle chemistry changes can really make important adjustments to the overall properties of the metal.
By the end of the second day, the furnace would already eat up 18 tons of iron ore, sand and charcoal — the fire’s fuel and also the source of its carbon.
Steel is composed of the element iron and the element carbon. The ancient Japanese could physically see iron, but they had no idea what carbon was. They understood that charcoal was needed for the process, but they didn’t understand charcoal as carbon. Carbon wasn’t discovered as an element until just a few hundred years ago.
Today the recipe is no secret. The hard part is keeping the oven cooking just right. Throughout this smelting process, Kihara must tend to the furnace to make sure that nothing goes wrong.
Inside the furnace, the iron and carbon slowly form into the right mixture. Beneath the furnace, lies a chamber nearly 10 feet deep. Ventilation channels flank either side. Any moisture seeping in from the ground would lower the temperature, ruining the steel, and they would have to start all over again.
In the very hot center of the furnace, the iron and carbon combine. This process happens at the atomic level. By nature, iron atoms bond to one another in a specific geometric arrangement. When heated, that structure changes form.
Iron actually has two different forms that it takes: a low temperature form, in which the atoms are arranged in a particular way, and a high temperature form, in which they are arranged a little bit differently. There’s actually more space between the iron atoms at high temperature.
When this high temperature form cools quickly, it contracts, and the carbon atoms become trapped in between the iron atoms. If this happens, the iron can’t fully assume the structure that it would like to. It will be distorted by virtue of the fact that the carbon atoms are lodged into these spaces that are really too small for them to fit into. And that kind of form of steel — iron and carbon mixed together — is very hard and very strong.
Manipulating this heating and cooling process, as well as controlling the ingredients in the steel, allows metallurgists or ancient craftsmen to change the properties of the metal.
Metals are elements, so they are made up from single kinds of atoms. They have this ability to bend, to take on new forms and also to have their mechanical properties be controllable. And so by adjusting their chemistry and adding a little bit to them or taking something away, by controlling their heat treatment, you have a lot of control over their properties, and this makes them very versatile.
In the traditional smelting process, the metal ore never completely reaches a liquid state so that the steel ore, or tamahagane, will not be uniform in its mixture of iron and carbon. Some parts will have more carbon, some less. These different mixtures of steel will become very important in the engineering of the sword.
More carbon makes the steel harder so that it can hold a sharper edge, but too much carbon makes the steel brittle, and no samurai wants to be caught with a broken sword.
In engineering, there is a limit to how hard you can make the metal before it becomes brittle. The standard test for measuring this is called the Charpy test. A large pendulum is used to break a sample of metal. Hard metals don’t bend, so they break more easily. A metal that resists the energy or force of the pendulum and bends before it breaks is said to be tough.
In the case of tough metal, instead of just breaking, it actually bends. And, so, what you’ll see is a lot of stretching on one side of the metal until, eventually, it’s been torn apart. It actually looks a fair amount like if you take a piece of taffy or a Tootsie Roll — and as you know the Tootsie Roll or the taffy will stretch first and then it will come apart — and you will see the effect of that stretching. You want very large energy absorption to have a tough metal. You want it to be able to take a lot of damage before it breaks.
Working with metals has always been dangerous. At the smelting furnace, there is a Shinto shrine devoted to a sacred deity the workers believe helps keep them safe. These elaborate rituals are not just about safety, but, also about quality control.
In modern industry, today, we depend very strongly on quality control. The ancient Japanese swordsmiths used religious ritual to make sure that each and every time that they manufactured this same object, it was done exactly the same way. If you think that what you are doing has religious significance, you pay extra attention to it. This reverence helps explain why the sword is so important in Japanese culture.
They’d say that the sword is the soul of the samurai. In almost all pre-modern cultures, matter and spirit were interfused, and so it wasn’t so hard for them to imagine that an object could have this kind of power — what’s called the luminous power or an otherworldly power. And in Japan, the gods could be found in any natural object, including waterfalls or trees or a mountain. So it wasn’t a big leap for them to think that an object as powerful and as reverently made as a sword could have some sacredness to it.
It is that sense of the sacred that drives Master Kihara. After 36 hours of feeding the voracious fire, everyone is exhausted, but Kihara carries on, never sleeping, always keeping vigil.
Using only the ancient methods, Kihara monitors the process by watching and listening to the fire. On the morning of the fourth day, Kihara can tell by looking and listening inside the core of the tatara that it is time to break up the furnace and extract the steel ore.
Once the ingot is cooled, it is broken into small pieces. This process helps to sort the steel. Pieces that break off easily are more brittle; parts that are more difficult to break apart are tougher. The most skilled masters are those that can deliver the best quality of both types.
Kihara will then choose which of these pieces is good enough to go to the sword maker, who will give this raw steel its famous shape.
To create its unique shape, the raw steel must be forged into the blade. And while the work of a blacksmith seems pretty straightforward — brute strength that muscles metal into shape — the real secret is what is going on deep inside the metal.
Sakurai is a small town, not far from the fabled city of Kyoto and home to Gassan Sadatoshi, one of Japan’s most renowned swordsmiths. His family has been in the business for generations.
When the pieces of raw steel arrive from the smelters, Gassan carefully examines the tamahagane to see if it is up to scratch for his next sword. Gassan can judge from the texture and color of the raw steel just how much carbon is in it. Brighter pieces have more carbon.
The great swordsmiths of Japan were far more than just blacksmiths, they were more like alchemists. They were steeped in the mysterious traditions of the metal: how it was melted, how it was molded, how it was beaten. They may not have understood the chemical composition of it, but from years of practice, years of apprenticeship, and years of tradition, passed on from master to pupil, they were able to perfect the art of sword making.
Gassan needs to find the best quality pieces of both the harder and softer types of the tamahagane because part of his art is to find a way to combine them so that the sword will have the advantages of their different characteristics.
There is really still only one way to learn the art of sword making, through apprenticeship — no easy career path. They rise at dawn, help with household chores, and work with their master six days a week. It takes years to learn correct forging technique.
The untreated metal is protected from rusting before the process begins. Heating softens the steel for hammering so that the pieces can be formed into one. The hammering also drives out most of the remaining impurities, called slag. Molten slag can be seen in some of the dramatic sparks as it is squeezed out of the steel by the hammering.
The steel is pounded flat and then folded time and time again to thoroughly mix the iron and carbon. Gassan gauges the concentration of the carbon by the way the steel bends. Another thing that is happening in this brutal pounding of the steel is that the shape of the metal, all the way down to the atomic level, is changing.
The material actually gets harder as it’s hammered. Many people have the experience of bending something, for example copper tubing, and feeling it get harder. This is because every time you bend it, it creates microscopic defects inside, and the more defects it has, the stronger the material gets.
Now, the smith, ultimately, would like to keep changing the shape of the sword, and so this hardening is not really what he wants. So he can put that sword back in the flame, heat it up, and the heat will heal the defects. At that point the material will go back to being relatively soft and formable, and the cycle can be repeated over and over again.
These defects also play a key role in giving metals one of their unique characteristics, their bendability. This bending happens at the atomic level.
The samurai needs a sword that is both hard and flexible, but remember that there is a tradeoff. Hard metals, with more carbon, hold an edge but a brittle one can break. Tough metals bend, but can’t hold an edge. Smelting creates pieces of both types which are then purified and shaped.
The crucial moment is when both types of steel are forged together. The hard steel is pounded flat and then folded into a u-shape. The tougher low-carbon steel is heated to a glowing red and inserted into the core. This relatively simple procedure is the culmination of a great deal of work and key to the ultimate success of the sword.
The metal the samurai needs is placed where it is most needed. The hard steel is wrapped around the outside to allow for the cutting edge to be razor sharp, and at the core, where the sword needs to absorb the impact of those deadly blows and be more flexible, the tougher steel is used.
The success of the ancient swords’ engineering was judged by a rating system that was as ghoulish as it was efficient. Sword testing and the criminal justice system of ancient Japan went hand in hand, so to speak. Depending on the severity of someone’s crime, they could have an appendage chopped off. Stealing might lead to the loss of a hand; bigger crimes had more severe penalties.
By modern standards, testing a sword using actual body parts is, of course, distasteful. But from a practical point of view, very effective, because that is really what the blade is designed to do.
A blade that would go through a body at its midsection was certified as a better blade than would go through, for example, somebody’s wrist, or ankle. A blade that could go through two bodies, or three bodies, or four bodies, or even five bodies, as some inscriptions read, was believed to be the best blade. And those that are in existence today are in the best Japanese museums that you can find; a “five-body” blade.
At Gassan’s forge, a much more delicate work is underway. The final stage of the forging process will harden the steel to hold its razor sharp edge. It is done through a dramatic step of heating and quickly cooling the steel to lock in the carbon. This process, called quenching, will accentuate the hardness of the high-carbon steel but has little effect on the tough, more flexible, low-carbon steel.
Gassan prepares the sword for the dramatic quenching stage by painting on a secret mixture of clay and charcoal powder. This will insulate parts of the blade allowing some parts to cool even more quickly than other parts. In time, it will also reveal one of the most unique signatures of the samurai sword.
The sword is now ready for this crucial moment. Gassan darkens the forge so that he can judge, by the color of the metal, exactly when the sword edge has reached 1,500 degrees Fahrenheit. Too hot and the steel might crack, too cold and the quenching would fail.
The most critical time in the whole sequence is this quenching operation. As many as one out of three blades is lost at this point in time.
Gassan is ready to pull the sword at a moment’s notice.
When the smith plunges it into water, the part with low carbon, inside the core of the blade, shrinks because there’s really not much carbon in there to be trapped. The part on the outer surface though, is filled with carbon, and so it’s prevented from shrinking as much as it would like to. The two types of steel contracting at different rates forces the blade to curve, giving the samurai sword it’s distinctive shape.
The final stage of this thousand-year-old craft will give the sword its legendary fighting edge. It will also make or break the value of the sword.
Master Takeshi Hon’ami is a 14th generation sword polisher. He will spend several weeks bringing the forged steel to a brilliant luster. It takes a lifetime to master this trade.
Hon’ami’s polishing stones are rare and can cost a thousand dollars or more. After 10 days of polishing, he moves on to one of the finest stones, the jizuya, so small it is sometimes no bigger than a grain of rice.
Although he keeps the edge of the katana, or sword, turned away from his body, it takes extreme concentration to rub these stones against some of the sharpest steel in the world.
It is only through his intensive polishing that the full beauty of the sword is achieved and a hidden beauty is revealed. Just before quenching, clay was painted onto the blade. Only now can the full effect of this be seen. Embedded in the steel is a visible line called the hamon. This wavy line is considered a great art form and is created by the skillful manipulation of the steel’s inner structure.
The creation of this line doesn’t just show the skill of the swordsmith, it’s also an expression of his creativity, and it gives every sword a unique character.
The cutting edge is, of course, what the samurai sword is all about, and honing the sword’s sharp tip is the last step in the process.
It has taken 15 people nearly six months to make this one sword. Blacksmith Gassan, finally, can now see the results.
“A sword that I spent so much time on, to see it finished, with all its characteristics brought fully to light, it leaves me speechless.”
Now finished, it is perhaps appropriate that even though the sword will never be used for its original purpose, it has not lost any of its value in Japanese culture. Exceptional samurai swords can sell for hundreds of thousands of dollars as art objects and antiques.
So, today, the samurai sword has moved from the bloody battlefields to the rarefied world of art collectors, perhaps another sign of the legendary resilience that was engineered into the sword more than 1,000 years ago.
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