Japanese Sword Construction

Japanese swordsmithing is the labour-intensive bladesmithing process developed in Japan for forging traditionally made bladed weapons ( ''nihonto'') 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 other such forms which were very rare 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. A single ''kera'' 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 Metals 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. 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. 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

HOME


Content is Copyleft
Website design, code, and AI is Copyrighted (c) 2014-2017 by Stephen Payne


Consider donating to Wikimedia



As an Amazon Associate I earn from qualifying purchases