Materials and preparation:
The first step is to choose high quality materials from which the blade will be made. The steel from which modern craftsmen make most weapons contains 99.99% pure electrolytic iron (tenkai-tetsu), sponge iron (kangai-tetsu) or the most popular substance – tamahagane steel.
Tamahagane is the traditional steel used to make swords. It is obtained in a kiln (Tatar), from charcoal and satetsu – iron sand, which is located on the river bottoms. Satetsu is in the form of iron trioxide (Fe2O3) and in order to produce steel, carbon must be added and oxygen must be taken away. This is done in the furnace.
The kiln is a niche made of clay. The thickness of the walls is about 30 cm. The main part of the furnace is located underground and is made of clay, stone and wood. The upper part of the tartar is repaired before each subsequent melting. This is what a Tatar looks like:
It takes at least 5 days to get the right steel. One day is needed to build a Tatar. After the construction of the new walls, a fire is set on the bottom. The fire for three hours, constantly lit with charcoal and oak wood. The resulting coals are covered with satu iron sand, and the top is covered with charcoal. The addition of satetsu is repeated every half hour for three days at a temperature between 1200ºC and 1500ºC. At this temperature, the oxygen supplied by the blacksmiths reacts with the carbon from the coal to carbon monoxide (CO). Carbon monoxide reacts with iron oxide to form pure iron and carbon dioxide (CO2), which is released as a gas.
This process plays the role of basic purification, as impurities are removed in the form of slag. In some ordinary furnaces, the process ends here, but in Tartar, the carbon from the charcoal continues to interact with the iron until an inhomogeneous mixture of iron and carbon, called tamahagane, is obtained. When cooled, the tamahagane steel is broken into usable pieces and sorted by quality and carbon content.
The carbon in the molten tamahagane can vary from 0.6 to 1.5%. The high-quality pieces are easily distinguished from the low-quality ones by their bright silver color. The carbon content in them should be about 1.0 to 1.2%. Blacksmiths carefully examine each piece of tamahagane and choose only pieces with a lower carbon content for the individual parts of the sword (in the sketch) – shingan – or soft core and kavagan – the hard shell. Despite the careful and thorough processing of the steel, most of the blacksmiths receive are not fit to produce quality swords. If the amount of carbon is too high, the steel will be too brittle and will not alloy and forge easily. If the amount of carbon is too low, the steel will be too soft and the blade will not be sharp enough. In both cases the result will be a low quality blade. For this it is necessary to make a second purification (irrigation) of the steel in order to adjust the exact carbon content needed to make a quality sword. The higher carbon pieces of steel are heated in the tartar, while the air from the bellows is not forced to climb upwards, passing through the steel. The movement of the air flow and the high temperature remove the excess carbon in the form of carbon dioxide. The low-carbon parts are melted in a furnace with a lot of charcoal, thus increasing the carbon content. The steel is placed in the upper part of the furnace and continue to add charcoal until it reaches the lower part. It is then believed that the carbon content has increased enough.
Once the metal has the required amount of carbon, the actual forging process can begin. The tamahagane is heated again and arranged in rough, quarter-inch layers. Then the layers are separated again and parts for the soft core (shingling) and for the hard shell (kavagan) are selected from them. The selected pieces are shaped like a block, which is wrapped in rice paper and covered with clay pulp, which acts as an insulator to maintain the shape during heating. Then it is placed in the hearth of the forge, where the temperature reaches 1300ºC. When the metal reaches the appropriate temperature, it is removed from the furnace and forged until a whole piece is obtained from the layers. The process continues until the piece doubles in length.
Modern metallurgy and methods of industrial metalworking allow us to study Japanese swords in great detail. All, however, are far from the idea that it is possible to produce a katana with the same high aesthetic value in a laboratory. Unlike modern industries, the steel from which Japanese swords are made is never actually heated enough to melt. Always a certain amount of steel remains uneven throughout the process. It is this quality that gives the necessary characteristics for each sword. The blacksmith constantly adjusts his techniques to the steel and shapes the sword according to his own ideas and ideas. Such a thing cannot be achieved in mass production in a factory, for example. Today’s science has proven that forging Japanese swords, contrary to popular belief, is not alchemy, but a craft that requires a lot of knowledge and experience. Metallurgical analysis can only confirm the wisdom of the decisions made over hundreds of years of trial and error by blacksmiths. It was not until the thirteenth century, without the luxuries of precision measuring instruments and machines, that swords reached a level of elegance and structure that would probably never be surpassed. The methods of production were determined only by tradition.
To a large extent, modern swords are made in the same way as in feudal Japan. Each blade has a number of unique metallurgical and mechanical properties. These properties appear after a series of complex forging and hardening processes. Before the methods of manufacture become clear, it must first be clear how the design of the blade allows it to be such an effective weapon. The length of the blade itself reaches 24-30 inches, which makes it shorter and lighter than tachy blades. It also makes it easier to handle with one hand, both by infantry and by riders. Each sword has a slight curve that runs across the blade, known as a sori (or dawn). This allows the samurai to draw his sword and strike his opponent with one movement, as often the outcome of the battle was decided by which of the two opponents would draw his sword faster. Each shinsakuto katana has a hard shell of high carbon steel and a core of more easily bendable low carbon steel.
Forging (China) of cavagana (hard shell):
When the double-elongated steel with a suitable amount of carbon is ready to be forged in kavagan, shita-gitae (basic forging) is performed. The process consists of repeatedly folding the metal block so as to form specific layers that are unique to each blade. It starts by heating the extended unit to the appropriate processing temperature. The chisel is then used until the block is almost divided into two equal parts, which are folded so that the metal regains its original length. It is processed again with a hammer and when the two parts are completely joined, the metal is brought back to double length. One bend takes about 30 minutes of precise forging. The number of folds of the metal depends on the individual style of the master. While the hot metal is being processed, it cools down quickly and work on it gradually becomes more difficult. Forging lasts only 3-4 minutes until the color of the steel from yellow-white becomes bright red. It is then placed back in the hearth of the forge and reheated. Each fold requires two or three warm-ups. When the steel is heated, the blacksmith periodically rolls the block in rice straw ash and fills it with clay paste.
This procedure is necessary due to the high temperatures and oxygen-rich air obtained from the blacksmith’s bellows. Under these conditions, carbon can easily be removed from the steel and it becomes unusable. This significantly reduces the oxidation and decarbonization of the metal. However, only half of the initial amount of tamahagane is used in shita-gita. A steel block with dimensions 25x2x3.5 cm, weighing from 1 to 1.5 kg is obtained. But at this stage the metal is not ready yet. It contains too much carbon, which is also unevenly distributed. To do this, the master divides the metal into three equal parts and returns them to the furnace. Four parts are needed to reach the length of the katana.
The pieces are then reunited in a metal block and the second forging process, known as age-gitae (final forging), begins. The metal is subjected to additional bending. This again loses about half of the starting material. A steel block with a weight of 0.9 to 1.6 kg with a carbon content of 0.6-0.7% is obtained. Most of the carbon in the tamahagan – about 0.3% – is lost during the first stage of forging, when the crude steel particles increase in area, they are stacked together and rolled up. Each subsequent folding causes the loss of another 0.03% of carbon, destroying the larger carbon crystals. The result is a steel with a carbon content of approximately 0.7%. Forging and folding have distributed carbon more evenly and helped to remove unnecessary impurities and slag.
Two special characteristics of kavagan depend on the quality of the steel and on the characteristic way of bending during shita-gita. These are jitsu – the quality of the steel obtained – and jihad – the characteristic pattern on the surface. They are influenced by many factors such as the direction of the folds, the force of the hammer blow, the union of the different blocks, etc. These are diagrams of the different structure obtained as a result of different bending of the metal.
Shingan forging (soft core):
While short blades such as tanto and wakidzashi are made entirely of kawaga, long blades such as katana also contain a soft core – a shingle that allows them to be both flexible and durable.
Making a shingling is similar to making a kawagane – pieces of steel with a suitable carbon content are heated, forged and folded into a block. The blacksmith usually uses about a kilogram of tamahagane with an average carbon content of about 0.5%. Most often, additional bending of the metal is required, as in most cases it contains a large amount of impurities. They must be removed so that the two parts of the blade can be joined properly and to avoid defects and cracks on the surface of the finished sword. At the end of the process, the block weighs 200-250g and contains 0.2-0.3% carbon.
Steel base formation (zucchini)
There are two main methods (tsukurikomi) that are used to combine kavagan and shingle.
The first method – kobuse-gitae – is simpler. In it, the blacksmith heats up and forms a kavagan in a flat piece, then folds it into a U-shape and the hot shingle block is placed in the base. In this method, the core does not run the entire length of the shell and the pointed tip of the sword consists only of the solid part of the steel. The two pieces of semi-joined metal are returned to the furnace and heated to over 1300 ° C and the forging is continued so that the cavage completely envelops the soft core and forms the base of the sword. This process is very delicate and especially important for the successful forging of the sword. If the two parts are not completely fitted and there are gaps, the work is considered useless. If the inside is not completely covered, the final product will have weak spots and will also be considered useless.
The second method of zucchini is much more complicated. The process is known as hon-sanmai-gitae. It can use 2 to 4 parts of hard steel to form the shell around the core. For the blade, a harder and correspondingly richer carbon steel, known as hagane, is used. Each part must be masterfully glued to the others, for which separate processes are used for each of them. This method of obtaining a different construction changes the physical characteristics of the finished sword. Regardless of the methods of zucchini, the differences are determined by the styles of the individual masters.
Sunobe shaping (basic shape):
When kavagan and shingan are properly joined, the blacksmith reheats the metal and begins to form the basic shape of the sword. Sunobe is formed as the blacksmith lengthens the steel through a series of hammer blows. The result is close to the shape of the finished katana and is approximately 90% of its length and weight, but it is much denser and lacks the characteristic katana curve. At this stage, the tip and handle of the sword are also formed.
Blade shaping (Hisukuri):
The shaping of the blade begins with the processing of the sharp part. This is done by heating parts of the skeleton to 1100 ºC and gradually processing it. Slowly, the future blade sharpens and regains its final shape. The heating is carefully controlled due to the delicate nature of the metal. If the steel is overheated, a strong hammer blow can cause the core to separate from the casing, and if it is not hot enough, the surface can be damaged.
The tip of the sword (kissaki), the strip between the blade and the tip (shinogi) and the back of the sword (mune) are also formed during this stage. Properly covered, it will withstand a great deal of adverse conditions. Although it looks like a finished sword, at the end of the stage the blade is still quite blunt and a little thicker.
Coarse sawing (Shiage):
Once the blade shaping is complete, the blacksmith prepares the sunobe for the process that provides the hard blade to the final weapon. It starts with rough sawing and sharpening (shiage). A special tool called a sen is used for this purpose. In this way all irregularities on the surface are smoothed. A file is used for the back of the sword. Then the whole blade is sanded with a special stone, composed mainly of silicon carbide (SiC). At the end of the swing, all the characteristic lines of the sword are well defined, but the blade remains rougher. This is necessary for the next hardening process.
Creating Hamon (Tsuchioki):
There are many types of steel with specific physical and chemical properties. Different shapes of steel can be seen on the graph, depending on the temperature and carbon content.
The strength and hardness of steel depend to a large extent on carbon. It is known that different temperatures lead to different crystal structures in the atoms of carbon and iron. When the steel from which the swords are made is heated above its critical temperature – approximately 750ºC, it enters the austenite phase. Austenite has a structure that allows iron ions to combine with carbon ions. When austenite cools suddenly (by applying water, for example), its structure changes to another, called martensite. Martensite “locks” the carbon atoms. The result is the hardest form of steel.
Only the blade (yakiba) of the katana is made of martensite. It is easy to shape and can be sharp enough, but it is too hard and easily breakable to provide the flexibility and durability needed to take a direct hit. These qualities are more inherent in the softer forms of steel – ferrite and perlite. Japanese blacksmiths discovered methods that allowed both the hardness of the blade and the flexibility of the sword itself. This is obtained by heat treatment, in which only the metal in the blade is changed from perlite to martensite. The transition zone between these two phases is clearly visible in the finished sword, so a lot of effort is put into getting an aesthetically beautiful pattern. This pattern is called ham and is considered the most important aesthetic part of the blade. Initially, the blacksmith invented the design of the hamon.This process begins with a clay mixture (tsuchi-even – in the photo), which will be applied to the blade before heat treatment. Usually the mixture contains equal parts clay, charcoal ash to control the temperature, powdered sand, prevent breakage and other blacksmith-specific ingredients. Add water and treat until sticky enough to be applied to the surface. The mixture acts as an insulator. The blacksmith applies it on the surface of different thickness – in thinner layers more martensite will be formed. Therefore, the layers on the blade are thinner. The thickness of the layers will determine the final pattern. As martensite is very brittle, the blade may be damaged. To do this, the blacksmith leaves very thin strips of the mixture on the surface of the blade, perpendicular to it. In this way, very thin veins of perlite remain behind the martensite (ashi – in the last photo) and if a small crack appears on part of the blade, it will spread only to the area with perlite and greater damage will be prevented.
Tempering of the blade (yaki-ire):
When the tsuchi-even dries completely, the sword is heated to a red-orange color (above 700ºC) and then quickly immersed in water. Iaki-ire is usually performed at night. This allows the blacksmith to better determine the temperature by the color of the metal.
The first step is to properly fill the furnace so that each part of the metal is heated to the same degree. When the furnace is ready, the blacksmith places the blade in the recess of another steel block and fastens them together with a leather strap. The sword is placed in the furnace several times on different sides. Once heated to the specified temperature, it is quickly immersed in water. There are many forms and styles of performing yaki-ire. After this process, the blacksmith again passes the blade through a low-temperature furnace (about 160ºC). This is part of the thermal process and is called yaki-modoshi. This reveals some of the residual highlights of the main cooling and is often repeated several times.
The blacksmith must be very careful while performing these operations, because instead of a more complicated ham, it can disappear completely or break down. After the last cooling, the dry clay is removed and the metal is checked for cracks. The blacksmith then applies a 2% solution of nitric acid and ethanol to the surface to clearly highlight the pattern design.
Adjust the curve. (Sorinaoshi):
During yaki-ire, the temperature difference between the blade and the rest of the sword is the reason for the characteristic curve of all katanas. Due to the slower cooling of the base of the sword, shrinkage is caused, which remains even after the work is completed. The effect of this phenomenon is to increase the curve of the sword. Therefore, blacksmiths usually form the initial shape of the sword with only a small curve. Then small changes in the curve are required. If the curve is too large, the blacksmith can reduce it using his hammer, thus affecting the length. If it is too small, hold the back of the blade in the parts where the curve should be larger, to a heated copper block.
More work on the katana:
These are just the first of many steps needed to create a katana. Forging is followed by rough polishing of the blade and indentation of decorative elements and thread. The distinctive sign or signature (mei) of the master is encrusted. Subsequently, the blade is passed on to other skilled craftsmen.
It is first transferred to a polisher, which completes and cleans the blade. This emphasizes the details, color and quality of the metal. The results can be seen after many days of work, when the extremely sharp blade of the katana, with well-defined jihad and jitsu, is ready. Other craftsmen take care to place the copper ring (habaki – in the photo above) and the decorated blade guard – tsuba (pictured below). After that, the work passes into the hands of carpenters who make the sheath and the handle. Finally, the sword returns to the blacksmith.
The creation of these weapons is still part of a respected tradition with the same rituals as were performed in the days of the samurai. Many blacksmiths spend more time perfecting their trade and very few receive the title of master. The whole production process requires many years of experience and knowledge. Despite the fact that many of these ancient weapons are now considered only works of art, the beauty of the katana hides its deadly nature.
You can read more about the samurai sword here.
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