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December 2007

December 2007 -- Snow



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  • Japanese Blades: Landscapes in Metal
    by Pierre Carles

    What is so special about Japanese blades

    Last month, we discussed the different parts of a Japanese sword, how they fit together, what they are named, and which key visual elements must be present in an illustration to create a believable representation. Still, we missed the real beauty and mystery of the very heart of the sword: the blade itself. However well-decorated, richly gilded or engraved the mounting may be, all this is nothing compared to where the true value of a Japanese sword lies: in the hands of its craftsman, and in the way he imparts to the blade an amazing, unique and deadly beauty... to a point where orthodox connoisseurs have old blades mounted in the most blank and understated mounting, so as to let the blade express its inner beauty without the visual disturbance of rich clothes. Part 2 of this feature will discuss the hidden beauty of the blade, and I hope you enjoy the journey as much as I did discovering these well-hidden secrets over the years.

    As the title of this section states, the first question is: What is so special about Japanese blades? Swords have been forged almost everywhere in the world, at all times since the Hittites discovered steel more than 2000 years B.C. What makes Japanese blades so different?

    A rapid visual inspection (shown in Part 1 of this feature) reveals two elements found in no weapon other than Japanese: first, the hamon, that clear separation between the white area near the edge of the sword and the rest of the sword; then, the jihada, the grain of the metal. Both these elements are specific to Japanese blades, and are the visible results of an amazingly complex and well-optimized forging process. The hamon has already been introduced in the first part of this feature. The jihada was described above as the “grain” of the metal. Yes, metal in a Japanese sword has grain, very much like watercolor paper, which illustrators know well about. Just like in the case of paper, Japanese blades can exhibit an incredible variety of grains. But let us illustrate this with a photograph, worth a thousand words. This is a close-up of a 17th-century tanto, showing the two above-mentioned elements: hamon and jihada:

    The sharp color difference between the white edge and darker bulk part of the blade (hamon) is clearly visible, much like in the photographs we saw in Part 1. But with this new close-up view, the grain of the metal (jihada) can now be seen as well: an intricate pattern of wavy lines, reminiscent of wood, water on a stormy sea or a sand-covered shoreline. This is the grain of the blade, as specific and particular to the sword as fingerprints are to a human. Such grains can be very different in shape, strength and pattern, and every type has a colorful name of its own (which would be too lengthy and too technical to list here). A slightly different type of jihada is visible in the next blade, not a sword, but a modern (traditionally-forged) Japanese knife:

    The hamon and jihada, as said above, are the visual testimony of the forging process, and in this process we find the real magic of the sword. But let us first talk about the base material of which the sword is made: steel.

    Japanese traditional steel

    To forge a sword, one needs steel, an alloy of iron and carbon. Iron is found naturally in the form of iron-sand, an ore made of iron, iron oxides and various mineral elements, which looks very much like a red-black sand. Carbon is nearly everywhere around us (and inside us too), in various forms, but the base source of carbon in the traditional processing of steel is charcoal: the very charcoal that serves as the fuel used to melt the iron.

    In traditional Japanese metal-processing, steel was (and is still) produced in a large furnace called tatara. A tatara is basically a rectangular open furnace, about fifteen-feet long, five-feet wide and 3-4 feet tall, built of clay and constructed with various openings at its bottom so as to recover molten steel and to allow air in. The first interesting thing to note is that a tatara is an expendable furnace: it is built for the production of a finite quantity of steel out of an initial stock of iron ore (about 8 tons of ore are needed to produce 1.5 tons of steel). Once the steel is produced and cooled down, the tatara is destroyed to recover all the metal. So the very first task in a traditional Japanese forge is to build a tatara. In that furnace, charcoal and rice straw are burnt, piled in layers with iron ore, and the burning can last for days (more than 12 tons of charcoal is burnt to produce that 1.5 tons of steel). Amazing temperatures of 1500° Celcius (2700° Fahrenheit) are reached, and the molten iron is chemically combined with the carbon from the charcoal and straw to give steel. Several qualities of steel are produced throughout the process, from its early to late stages. One can easily imagine how much wood and straw must be used to fuel such a large furnace for several days without stop, and how much manpower is needed too (since a constant air current must be provided to the fire through the use of bellows). Speaking of the burning of so much wood, I cannot help mentioning here Hayao Miyazaki’s beautiful animated movie, Mononoke Hime, where a wood spirit is faced with a clan deforesting the land to produce steel. The clan’s name? Tatara. ;)

    But back to steel. Out of the tatara comes a wide variety of steels of different carbon concentrations and impurity content. Experts are able to discern between “good” and “bad” steel, and to estimate the carbon content of this or that part to sort the whole batch out. Sword-makers then come and do their "shopping", choosing this or that part for the forging of this or that blade. A carbon content of 0.7% is usually optimal for the steel of a sword. The important thing to know, apart from the obvious issue of impurities, is that the more carbon steel contains, the harder it is. But since there is no such thing as a free lunch, the more carbon it contains, the more brittle it gets too, an unacceptable feature for a weapon. How can the sword-maker balance these two seemingly incompatible constraints: hardness to get a sharp sword, and resilience to make it resist strong blows? This is where the story gets really amazing.

    Forging the blade

    The sword-maker has now carefully selected and purchased a batch of various steel nuggets. How to make a sword of that? The first step is to make the steel more homogeneous, with a fine-tuned carbon content, and as free of impurities as possible. These three objectives are achieved through the same process, a lengthy endeavor which can last for days:

    The sword-maker piles a precise quantity of small pieces of steel on top of a specially-forged steel plate attached to a long steel rod. The small pieces are wrapped in a special rice paper, and are heated in the craftsman’s furnace until they melt into a single steel block. That block, as said above, is highly inhomogeneous initially. The first step consists in stretching and folding that block again and again, each fold making the fine structure of the steel more homogenized than the last. It would require many drawings to explain that process, so a short video will be much more informative (and funnier too). Since my family would object me shaping steel in a furnace in our living-room for three days, I used modeling paste instead of steel to show the process: with two chunks of modeling paste, one grey and one black, it is easy to visually demonstrate that initial forging process. Of course, in the real process, only one color of steel is present, so the visual impression is less striking. The two colors here are thus for visual purpose only. But as will be seen later, they also help understand how the jihada ensues from the folding process, when the forged blade is finally polished.

    Video, part 1 (mp4 format, ~4 Megs)
    Video, part 1 (mov format, ~4 Megs)
    Video, part 1 (small version, mp4 format, ~1 Meg)
    Video, part 1 (small version, mov format, ~1.2 Megs)

    If you know a little about cooking, you will have immediately noticed that the above process is the very same process used to make croissants out of a dough. And this is where Japanese forge meets French cuisine. :)

    And if you know a little about mathematics, you will have immediately noticed also that the successive folding of an initial steel chunk is a very efficient mixing process. The first fold creates two layers out of the initial one, the next one creates four, then eight, and so on, each new fold multiplying the number of layers by 2. It is not rare that a sword-maker folds this initial steel chunk as many as 15 times, which, if you remember your math, leads to two times two times two … layers, where the number of times you multiply by two is 15. This is noted 2^15, and it is equal to … 32,768. Yes, the base steel used for the forging of a Japanese blade is a fine composite structure made of several tens of thousands of layers. Some sword-makers where said to fold their steel chunk as many times as 21 or 22 times, leading to several millions of layers.

    Through that process, the three objectives above are achieved: the final piece of steel has a more homogeneous composition in terms of carbon content than the initial one, the impurities have almost all left the piece, and the subtle dosage of rice straw burning around the steel throughout this process has helped control the average carbon content. If we cut our little piece of folded modeling paste, the fine layered structure is clearly visible:

    And think that this piece has been folded “only” 10 times (I was bored after that), which is still 32 times less layers than a piece folded 15 times.

    Now that the metal piece is ready, it will be used to shape the blade itself. But shaping a blade out of this piece would be too simple. Again, the process is more subtle than just that. You will remember the incompatible constraints of hardness and resilience. How to circumvent that problem? The idea is to use different steels for different parts of the sword: the edge of the sword is the part that needs to be as hard as possible; on the other hand its core has to be supple, so as to bend instead of just breaking when struck too strongly. So the base steel used to shape the sword is a composite structure itself: it has a soft low-carbon steel core, around which is wrapped the hard high-carbon layered steel block obtained through the initial process described above. Again, our modeling paste comes handy to illustrate that: imagine that the grey core is soft steel, and that the folded piece created in the above video represents the hard steel wrapping:

    Now the sword can really be put into shape. But wait! Where is the hamon, where is the jihada? They are still not visible at this point. And this leads us to the next step.


    A very early discovery of metal work is the quenching process, by which a forged piece of steel is made harder and more resilient. It consists in rapidly cooling the heated piece of steel, usually by plunging it into cold water. When the forged steel is cooled down slowly, it remains soft, and basically useless as a weapon. When it is cooled very rapidly, it becomes harder (but again more brittle). How is that possible? This has to do with the science of thermodynamics, which deals with the equilibrium states of materials. Do not stop reading at this point: we will not get too technical. But still, it is interesting to introduce the concept of metastability at this point.

    When a state of matter is stable, it means that the conditions of the material can be changed (like heating it, changing the pressure, and so on), and the material will remain more or less the same, and will return to its initial state once it gets back to its initial conditions. If you plunge an iron spoon in boiling water, it will heat up, then cool down once it is out, and nothing will have seemingly changed: the spoon was initially in astable state and returned back to it. But there are many examples in nature where the same process would lead to a very different result. Think of chocolate for instance. Have you ever noticed that when you leave milk chocolate to melt in the sun and re-solidify afterwards, it looses its original texture, and gets covered in a kind of distasteful white foam? This is because chocolate at room temperature is not stable but metastable: its original state was created at a higher temperature, where all the components gently melt together, and then were rapidly heated and cooled down in a complex thermal process in order to “freeze” this particular state and bring it back to room temperature, where it should normally not appear. When you let the chocolate melt and re-solidify, the solidification is slow, and that carefully “frozen” metastable state is lost: components separate and chocolate returns to its “normal” stable state at room temperature: an ugly-looking chunk of brownish matter covered with white foam. Yuck!

    Chocolate is not the only example of a metastable state in every-day life. Diamonds are another (well, assuming you see diamonds in your everyday-life, in which case I want your job). Diamonds are made of pure carbon, just like coal. The difference between diamonds and coal is that diamonds should not exist at room temperature and pressure. The stable state of carbon under such conditions is coal, not diamond. So how can we have diamonds? Because they were formed several kilometers deep in Earth’s crust (where diamond is the stable form of carbon) and suddenly sent into the atmosphere by explosive volcanism billions of years ago. The rapid (supersonic) rise of diamonds from the deep to the surface was way too fast for the carbon atoms to rearrange into their normal stable form. So the diamond state was “frozen” by this rapid cooling, and we can now enjoy diamonds around us although they should normally not be here. But watch out: the same thing that happens to chocolate can happen to diamonds: when heated sufficiently high and allowed to cool down gradually, they will naturally return to their stable state: coal. Which is why you do not want to get a diamond anywhere close to a flame. Do not try this at home!

    So, how does that relate to swords ? Most will have understood already: by quenching the sword, a particular molecular organization of steel called martensite, obtained under high temperature, is “frozen” and stabilized at room temperature. Had the cooling been more gradual, steel would have returned spontaneously to its natural stable state at room temperature: pearlite, a softer state of lesser use for a weapon. So quenching is an essential process in forging a blade.

    But what is so special about the way Japanese sword-makers do it? Again, quenching the whole sword too abruptly makes it extremely hard but extremely brittle at the same time. Quenching it too slowly yields a piece of soft metal which will bend and warp at the first blow. The answer of Japanese sword-makers to that dilemma has been brilliant: adapt the rate of cooling to each part of the sword. In order to do that, the forged blade is first allowed to cool down gradually, leading to a soft pearlite structure. Once cold, the craftsman applies clay onto the sword, with a thick layer over its upper part and a thin layer near the edge. Then only, the sword is re-heated and suddenly plunged into cold water. Where clay is thick, the rate of cooling is slower: the metal in the upper part of the sword (away from the edge) returns progressively to its stable soft pearlite state. Where clay is thin (near the edge), the cooling is more abrupt: steel retains its structure of hard and brittle martensite, which can easily be polished into a deadly edge. This cooling process is referred to as “tempering” rather than “quenching”, suggesting a subtle temperature control rather than an abrupt and blind temperature quench.

    And what about the hamon then? Martensite happens to be whiter than grey pearlite: the hamon is just the visual separation between the parts of the blade that were cooled down gradually or rapidly, respectively. And since the visual aspect of a blade is the signature of the sword-maker, each sword-maker has its own way of applying clay onto the blade, drawing wavy patterns and thus giving various and highly specific shapes to the hamon line.


    Now that the sword is shaped and tempered, the “only” thing that remains to be done is polishing it. I say “only”, because polishing a Japanese blade is as much of a technical challenge as forging it. Polishing is handled by specialized craftsmen, separate from sword-makers, and polishing a single sword can takes weeks. As you may have guessed already, this is when the grain, the jihada, is finally made visible. By removing matter from the surface of the sword gradually, the polisher “eats” into all those tiny interwoven layers of folded steel, revealing an intricate pattern, much like the grain of wood is revealed through cutting. And now is time to take our modeling paste model again, and “polish it”. Only, with modeling paste, polishing can be made in a crude and simple way: by cutting through the matter with a knife. And through that cut, like in the case of wood, a fine and rich grain is revealed:

    Video, part 1 (mp4 format, ~2.5 Megs)
    Video, part 1 (mov format, ~2.5 Megs)
    Video, part 1 (small version, mp4 format, ~1 Meg)
    Video, part 1 (small version, mov format, ~1.2 Megs)

    Magic, isn’t it? Let us admire it again in a close-up photograph, and compare this new image to the previous photographs of real blades shown above:

    DIY Japanese blades! :)

    In the next and last part of this feature, we will finally recollect a brief history of Japanese swords. So bear with me!


    (1) Photograph courtesy of John Berta, subjected to copyright.

    (2) Author’s personal collection.

    The author feels extremely indebted to John Berta from Canada for authorizing the reproduction of the beautiful photographs of his personal collection of blades and mountings.

    Pierre Carles Pierre Carles is a freelance fantasy illustrator who occasionally works as a theoretical physicist in the University of Paris. A long-time lover of Asian culture, he has recently studied Japanese culture more in-depth, in most of its traditional aspects.
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