Japanese swordsmithing is the labour-intensive bladesmithing process developed in Japan beginning in the sixth century for forging traditionally made bladed weapons (nihonto) The Development of Controversies: From the Early Modern Period to Online Discussion Forums, Volume 91 of Linguistic Insights. Studies in Language and Communication, Author Manouchehr Moshtagh Khorasani, Publisher Peter Lang, 2008, , 9783039117116 P.150] The Complete Idiot's Guide to World Mythology, Complete Idiot's Guides, Authors Evans Lansing Smith, Nathan Robert Brown, Publisher Penguin, 2008, , 9781592577644 P.144 including katana, wakizashi, tantō, yari, naginata, nagamaki, tachi, nodachi, ōdachi, kodachi, and ya (arrow).
Japanese sword blades were often forged with different profiles, different blade thicknesses, and varying amounts of grind. Wakizashi and tantō were not simply scaled-down katana but were often forged without a ridge (hira-zukuri) or in other ways very seldom done on katana.
Traditional methods Steel production
The steel used in sword production is known as , or "jewel steel" (tama – ball or jewel, hagane – steel). Tamahagane is produced from iron sand, a source of iron ore, and mainly used to make samurai swords, such as the katana, and some tools.
The smelting process used is different from the modern mass production of steel. A clay vessel about tall, long, and wide is constructed. This is known as a tatara. After the clay tub has set, it is fired until dry. A charcoal fire is started from soft pine charcoal. Then the smelter will wait for the fire to reach the correct temperature. At that point he will direct the addition of iron sand known as satetsu. This will be layered in with more charcoal and more iron sand over the next 72 hours. Four or five people are needed to constantly work on this process. It takes about a week to build the tatara and complete the iron conversion to steel. Because the charcoal cannot exceed the melting point of iron, the steel is not able to become fully molten, and this allows both high and low carbon material to be created and separated once cooled. When complete, the tatara is broken to remove the steel bloom, known as a kera. At the end of the process the tatara will have consumed about of satetsu and of charcoal leaving about of kera, from which less than a ton of tamahagane can be produced.The Tale of the Tatara A single kera batch can typically be worth hundreds of thousands of dollars, making it many times more expensive than modern steels.
Japanese tatara steelmaking process using ironsand started in Kibi Province in the sixth century and spread throughout Japan, using a unique Japanese low box-shaped furnace different from the Chinese and Korean styles. From the Middle Ages, as the size of furnaces became larger and the underground structure became more complicated, it became possible to produce a large amount of steel of higher quality, and in the Edo period, the underground structure, the blowing method, and the building were further improved to complete tatara steelmaking process using the same method as modern tatara steelmaking. With the introduction of Western steelmaking technology in the Meiji period, tatara steelmaking declined and stopped for a while in the Taisho period, but in 1977 The Society for Preservation of Japanese Art Swords restored tatara steelmaking in the Shōwa era and new tamahagane refined by tatara steelmaking became available for making Japanese swords.History of Iron and Steel Making Technology in Japan ーMainly on the smelting of iron sand by Tataraー. Mitsuru Tate (2005). Tetsu-to-Hagane Vol. 91. The Iron and Steel Institute of Japan.たたらの歴史 たたら製鉄の進歩 (Progress of Tatara Iron Making). Yasugi Cityたたら」の発祥と発展 (Changes in Japanese Tatara Iron Making Technology). Yasugi Cityたたら製鉄の歴史と仕組み. Nagoya Japanese Sword Museum Nagoya Touken World
Currently, tamahagane is only made three or four times a year by The Society for Preservation of Japanese Art Swords and Hitachi MetalsTatsuo Inoue; Science of tatara and Japanese sword ICBTT2002 during winter in a wood building and is only sold to master swordsmiths.
Construction
The forging of a Japanese blade typically took many days or weeks and was considered a sacred art, traditionally accompanied by a large panoply of Shinto religious rituals.Irvine, Gregory. The Japanese Sword: The Soul of the Samurai. London: V&A Publications, 2000. As with many complex endeavors, several artists were involved. There was a smith to forge the rough shape, often a second smith (apprentice) to fold the metal, a specialist polisher, and even a specialist for the edge. Often, there were sheath, hilt, and handguard specialists as well.
Forging
The steel bloom, or kera, that is produced in the tatara contains steel that varies greatly in carbon content, ranging from wrought iron to pig iron. Three types of steel are chosen for the blade; a very low carbon steel called hocho-tetsu is used for the core of the blade (shingane). The high carbon steel (tamahagane), and the remelted pig iron (cast iron or nabe-gane), are combined to form the outer skin of the blade (kawagane)."A History of Metallography", by Cyril Smith Only about 1/3 of the kera produces steel that is suitable for sword production.
The best known part of the manufacturing process is the folding of the steel, where the swords are made by repeatedly heating, hammering and folding the metal. The process of folding metal to improve strength and remove impurities is frequently attributed to specific Japanese smiths in legends. The folding removes impurities and helps even out the carbon content, while the alternating layers combine hardness with ductility to greatly enhance the toughness.
In traditional Japanese sword making, the low-carbon iron is folded several times by itself, to purify it. This produces the soft metal to be used for the core of the blade. The high-carbon steel and the higher-carbon cast-iron are then forged in alternating layers. The cast-iron is heated, quenched in water, and then broken into small pieces to help free it from slag. The steel is then forged into a single plate, and the pieces of cast-iron are piled on top, and the whole thing is forge welded into a single billet, which is called the age-kitae process. The billet is then elongated, cut, folded, and forge welded again. The steel can be folded transversely (from front to back), or longitudinally (from side to side). Often both folding directions are used to produce the desired grain pattern. This process, called the shita-kitae, is repeated from 8 to as many as 16 times. After 20 foldings (220, or 1,048,576 individual layers), there is too much diffusion in the carbon content. The steel becomes almost homogeneous in this respect, and the act of folding no longer gives any benefit to the steel.A History of Metallography by Cyril Smith – The MIT Press 1960 Page 53-54 Depending on the amount of carbon introduced, this process forms either the very hard steel for the edge (hagane) or the slightly less hardenable spring steel (kawagane) which is often used for the sides and the back.
During the last few foldings, the steel may be forged into several thin plates, stacked, and forge welded into a brick. The grain of the steel is carefully positioned between adjacent layers, with the configuration dependent on the part of the blade for which the steel will be used.
Between each heating and folding, the steel is coated in a mixture of clay, water and straw-ash to protect it from oxidation and carburization. This clay provides a highly reducing environment. At around , the heat and water from the clay promote the formation of a wustite layer, which is a type of iron oxide formed in the absence of oxygen. In this reducing environment, the silicon in the clay reacts with wustite to form fayalite and, at around , the fayalite becomes a liquid. This liquid acts as a flux, attracting impurities, and pulls out the impurities as it is squeezed from between the layers. This leaves a very pure surface which, in turn, helps facilitate the forge-welding process.Iron and Steel in Ancient Times By – Det Kongelige Danske Videnskabernes Selskab 2005 Page 65 Through the loss of impurities, slag, and iron in the form of sparks during the hammering, by the end of forging the steel may be reduced to as little as 1/10 of its initial weight. This practice became popular because of the use of highly impure metals, stemming from the low temperature yielded in the smelting process. The folding did several things:
It provided alternating layers of differing hardness. During quenching, the high carbon layers achieve greater hardness than the medium carbon layers. The hardness of the high carbon steels combine with the ductility of the low carbon steels to form the property of toughness. It eliminated any voids in the metal. It homogenized the metal within the layers, spreading the elements (such as carbon) evenly throughout the individual layers, increasing the effective strength by decreasing the number of potential weak points. Mathematically, the process by which the metal is homogenized through the folding is given by the baker's map. It burned off many impurities, helping to overcome the poor quality of the raw steel. It created up to 65,000 layers, by continuously decarburizing the surface and bringing it into the blade's interior, which gives the swords their grain (for comparison see pattern welding).
Generally, swords were created with the grain of the blade (hada) running down the blade like the grain on a plank of wood. Straight grains were called masame-hada, wood-like grain itame, wood-burl grain mokume, and concentric wavy grain (an uncommon feature seen almost exclusively in the Gassan school) ayasugi-hada. The difference between the first three grains is that of cutting a tree along the grain, at an angle, and perpendicular to its direction of growth (mokume-gane) respectively, the angle causing the "stretched" pattern.
Assembly
In addition to folding the steel, high quality Japanese swords are also composed of various distinct sections of different types of steel. This manufacturing technique uses different types of steel in different parts of the sword to accentuate the desired characteristics in various parts of the sword beyond the level offered by differential heat treatment. Globalizing the Prehistory of Japan: Language, Genes and Civilization, Volume 6 of Routledge Studies in the Early History of Asia, Author Ann Kumar, Publisher Taylor & Francis US, 2009, , 9780710313133 P.23
The vast majority of modern katana and wakizashi are the maru type (sometimes also called muku) which is the most basic, with the entire sword being composed of a single steel. However, with the use of modern steels, this does not cause the sword to be fragile, as in former days. The type is made using two steels, which are called hagane (edge steel) and shingane (core steel). Honsanmai and shihozume types add the third steel, called kawagane (skin steel). The many different ways in which a sword can be assembled varies from smith to smith. Sometimes the edge-steel is "drawn out" (hammered into a bar), bent into a U-shaped trough, and the very soft core steel is inserted into the harder piece. Then they are forge welded together and hammered into the basic shape of the sword. By the end of the process, the two pieces of steel are fused together but retain their differences in hardness. The more complex types of construction are typically only found in antique weapons, with the vast majority of modern weapons being composed of a single section, or at most two or three sections.