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Concept
Japanese swordsmithing
Summary
Japanese swordsmithing is the labour-intensive bladesmithing process developed in Japan beginning in the sixth century 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.
Tatara (furnace)
The steel used in sword production is known as tamahagane, 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.
Official source
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