Tatara and Tamahagane
The use of traditionally made tamahagane steel in the blade is one of the essential features of the Japanese sword. In the last few years, this steel has been referred to as "jewel steel". This designation probably originated outside Japan. This may be due both to the beautiful appearance of the broken pieces of traditional steel and its stunning silver crystalline structure with cavities coloured in shades of blue, purple and gold, but also to the legendary reputation created in Japan. Many sources present tamahagane as a steel of exceptional properties that make it a true Japanese sword. So what is Tamahagane?
If we were to try to define what tamahagane steel is, the definition would be something like this: 'Tamahagane is steel produced by the technique of reducing iron ore on charcoal in a tartar furnace'.
The very words Tamahagane and Tatara are the Japanese names for the steel made in this way and the furnace built for this purpose. In the case of steel, the specific feature is the raw material. The iron ore used is the black iron sands of satetsu. Behind this name are the words sa (sand) and tetsu (iron, steel). Translated as iron sands. This special raw material is nothing more than eroded iron ore (magnetite). Tiny particles of the ore are washed down from the hills into the rivers when it rains. Because they are heavier than most of the surrounding sediment, they are deposited in layers where the water flow is slower. This allows them to be mined. Their collection and cleaning from the surrounding sand particles is currently done by using a magnet to which the magnetite sand attaches. The main historical reason for the use of iron sands in steelmaking in Japan was the lack of other sources of good quality iron ore. One of the important claimed characteristics of this raw material is the alleged chemical purity of the iron sands found in Japan. This information is only partly true. There are many areas in Japan where iron sands occur, but only some of them are free of undesirable elements such as phosphorus and sulphur. Thus, for the production of tamahagane used for sword blades, ore from such deposits was used which contained ore free of chemical impurities of the highest possible purity. This is similar for ore deposits outside Japan. In Europe, for example, manganese is often found in iron ore. Yet in Sweden, for example, it is possible to obtain magnetite without these impurities with almost perfect chemical purity.
Steel production (reduction)
The actual process of converting iron ore into steel is called reduction. It takes place in a vertical shaft furnace. Charcoal is used as fuel. Like the iron ore, this must be chemically pure. For example, oak wood contains sulphur, which contaminates the steel during the reduction process. The clay from which the furnace is built can also be a source of chemical contamination of the steel produced. At high temperatures inside the furnace, the inner layer of clay melts and releases the chemical elements it contains into the reducing environment (gases inside the furnace). These can then bind to the steel and chemically pollute it.
Before the development of blast furnaces capable of producing large quantities of steel virtually continuously, the possibilities for steel production were very limited. Vertical shaft furnaces with a forced air supply at the bottom were used to reduce iron ore. Simply described, this was a cone-shaped furnace. In the lower part of the furnace, above the bottom, there was a bellows through which air was pumped into the furnace. The furnace was alternately filled with charcoal and ore. This process was quite lengthy and produced a limited amount of steel, depending on the size of the furnace. Also, the quality of the steel produced was not consistent. In steel production in a smaller furnace with a feed volume of about 100 kg of charcoal and 30 kg of iron ore, the reduction takes about 3 hours and the product is 10-15 kg of steel (tamahagane) depending on the quality of the iron ore used and the optimization of the process. The piece of steel produced in this way is only high carbon steel with a carbon content of between 0.9-1.5% at optimum furnace parameters and ideal reduction process. This means that all the steel is usable for the production of hagane, the steel forming the sheath and blade edge. When using a larger furnace (wider inner diameter), a larger amount of steel is obtained. However, the reduction process is longer. Reduced steel above 15 kg has a more pronounced difference in carbon content between the surface and the core. The larger the bark, the lower the carbon content inside. Steel with a carbon content below 0.5% is not malleable. However, steel with a carbon content below 0.8% is no longer usable for cutting steel, because the carbon content is further reduced during the forging process. The ideal carbon content of tamahagan for the production of cutting steel is about 1.2-1.0%.
In documents on tamahagane production in Japan, large tatara is usually presented. A furnace of rectangular base with a wall height of about 1.5 m and a series of pumps bringing air into the furnace from both long sides is filled continuously for three days and nights with about 15-20 tons of coal and iron sands. The result is a steel furnace weighing about 2.5 tonnes. The difficulty of the whole process is extreme. The design of the furnace, its construction, the amount of material used and the length of the steel reduction process itself, removing the huge bark from the furnace and cutting it into smaller usable parts, are all very demanding. Although the quantity of steel obtained is large, the quality of carburizing varies considerably. It contains a significant proportion of low-carbon steel, which, when used for hagane, needs to be remelted and carburised by the oroshigane method after sorting.
A significant advantage of the steel from the large tatara is its purity after breaking and sorting. The steel pieces are not contaminated superficially with slag and coal residues as is the case with the steel from the small furnace. This makes it somewhat easier to work with such material when preparing steel for the sword.
Effect of steel on blade metallurgy
For some reason, steel from smaller furnaces is significantly more sensitive in creating metallurgical effects. Using high quality, pure tamahagane made by Shimane, I can't create utsuri and generally the blade is poorer metallurgically. Also, the color of the steel is silver, similar to blades made from modern high carbon steel. On blades made by me from tamahagane in small tatara, the utsuri is very pronounced, usually not utsuri. The most pronounced metallurgical effects are achieved when I combine small furnace tamahagane and oroshigane in a 1:3 ratio. The small furnace tamahagane produces a silver-coloured steel with a hint of blue. Oroshigane gives the steel a distinctive blue colour. However, this is just my experience and the experience of other artisans may be quite different.
Steel for Japanese swords and its production are shrouded in an aura of mystery. The current production of tamahagane for swordsmiths in Shimane, Japan was revived in the second half of the 20th century. It was built on essentially non-existent and incomplete information about historical steel production methods in Japan. The absence of historical written records of steel extraction technologies made it impossible to draw objective information. The whole process was thus based on an antique picture scroll depicting a tatara. The scroll's scenery included deities watching the steel-making process. I have a bit of a feeling that this is how tamahagane production is presented even today. In the world of Japanese swords, myths and legends about the uniqueness of tamahagane and its production are perpetuated and fed. Completely neglected is the importation of steel into Japan, used for sword making in various historical periods. In reality, however, high-quality steel for weapons was an important traded raw material in both Japan and Europe. In fact, the technological uniqueness of the reduction of ore to steel in the tartar is nonexistent. The same method was used in all cultures capable of producing steel. The exception is perhaps the large volume of tatara. However, I have considerable doubts about its use in the normal production of steel in ancient history, given its many disadvantages and the difficulty of the process. Artisans around the world have typically tried to work cheaply, efficiently, and to simplify the technology, not complicate it. Using smaller kilns would then make more sense. A comparison of the metallurgy of ancient and modern swords also leads me to this opinion. The tamahagane steel itself is so exceptional only in its authenticity. In terms of metallurgy and metallurgy, it's a primitively reduced unalloyed high-carbon steel. Its chemical purity is influenced by the chemical purity of the raw materials. Its designation as 'Jewel steel' is more romantic in nature and is based on its often beautiful appearance in the raw state than on any real uniqueness as a raw material. Technologically, the Japanese historical methods of the process of obtaining steel by reduction in a shaft furnace do not differ significantly from similar historical methods used throughout the world. This is also true of the methods of refining inferior or substandard tamahagane by the oroshigane method. On the contrary, outside Japan there were more advanced refining technologies producing steel of significantly higher quality. For example, the crucible wootz (bulat). The tamahagane processing technique of reloading to clean, homogenize and reduce carbon content was a commonly used method outside Japan. Also, the composition of steel of different grades and carbon contents (shingane, hagane, kawagane...) in a packet before drawing it into a bar from which the blade was subsequently shaped was a technique known and commonly used. Many Celtic or Viking swords were much more sophisticated in their material composition and workmanship and more technologically sophisticated in their execution than the blades of Japanese swords. It is also worth noting that metallurgy and steelworking were already at such a high level in Europe several centuries BC.
Traditionally refined steel and modern steel
Traditionally reduced steel and Modern steel is produced in blast furnaces. In these, coke was usually used as fuel. The fundamental difference is that the steel in the blast furnace passes through a completely liquid state. During this phase, the slag separates from the steel and accumulates on the surface of the liquid steel. The liquid state of the steel allows the casting of ingots of essentially homogeneous and pure steel.
In a shaft-type tatara furnace of smaller dimensions, liquid steel can also be produced. However, this method is not much used in sword making. A large tatara produces steel in the form of inhomogeneous ingot. The steel forms a spongy, semi-liquid structure in the reduction process. Thus, from a metallurgical point of view, the resulting steel is of a lower quality than that produced in blast furnaces. However, blades made from recoated modern steel are, in my opinion and experience, different. Visually, they are somewhat sterile. There is an aesthetic value in reloading modern steel. It creates a "snake", a folded steel structure. Mechanically, creating a laminated structure can increase the toughness of the blade. Also, the folding of modern steel reduces the carbon content of the steel to the desired values, ideal for a sword blade. Paradoxically, the technique of folding and layering breaks the homogeneity of the modern steel used. It could be said that this makes the resulting processed steel similar to the recoated traditional steel. With traditional steel, the folding has the opposite effect, turning inhomogeneous steel into relatively homogeneous steel. The steel particles in the snake structure are richer and more pronounced compared to modern steel. However, a similar effect can be achieved by using modern steels with different carbon contents in the construction of the base package at the beginning of the overlay process.
However, more significant differences can be noticed after the quenching process. I have conducted a number of comparative experiments during my twenty years of experience in the sword and knife making industry and have made several findings. Blades made of modern steel are more prone to cracking during the hardening process. I also tested the resistance of the steel during repeated hardening. I repeatedly hardened two similarly processed blades made from modern and traditional steel. Between each cycle, I always heated the blade to a temperature of about 900°C and allowed it to cool slowly. This loosened it and removed the original hamon line and tension in the steel. The modern steel blade made it through two cycles. During the third hardening, longitudinal cracks appeared in the steel in the area of the hamon line. The traditional steel blade withstood 7 cycles of hardening without defects. After the seventh hardening I did not continue the experiment.
Another significant difference is the sensitivity of the steel and its ability to form different metallurgical structures. Modern steel produced nice hamon lines, distinctive nioi, kinsuji, sunagashi in repeated experiments. Surprisingly, modern steel was also capable of producing utsuri, although its expressiveness fell far short of the utsuri qualities of traditional steel. They almost never produced nie. Traditional steel produced such effects of nie, not utsuri, koshiba. Kinsuji and sunagashi were also present. However, these are effects caused more by the reloading technique, and hardening only partially affects them.
There was also a difference in the hardness of the steel. In modern steel, hamon feels harder at the same measured values (e.g. 62 HRC). When grinding on an arato stone, the hamon/ji contrast is more pronounced than the equally hard hamon of a traditional steel blade. The hamon has a blue colour, almost glassy and dark blue when polished. It is also more prone to breakage. At this high hardness, the steel crumbles on the blade when polished. (micro-breakage) To achieve optimum properties comparable to traditional steel blades, it is better to temper a modern steel blade at a temperature of approx. 250°C to a hardness of around 60 HRC.
It is difficult to objectively compare the visual properties of modern and traditional steel blades. Blades made of modern steel may be less metallurgically active. However, the end result in this respect is more dependent on the skill of the polisher. I believe that as long as the swordsmith and polisher are both skilled craftsmen, one cannot tell the difference between a blade made of tamahagane and modern steel. In my opinion, the high proportion of oroshigane in traditional steel blades gives the blades a more "Koto" character. But it always depends on the steel processing and the hardening method.
The last difference between modern and traditional steel is corrosion resistance. In both cases it is unalloyed high carbon steel. For example, modern tool steel 19 191 (cz) is similar in chemical composition to tamahagane. However, despite its similarity, modern steel is more susceptible to corrosion than traditional steel. I don't know why this is so.