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The Kera-oshi Method

Next we'll examine actually goes on at the tatara when using the kera-oshi method.

The kera-oshi method is used primarily along the northern side of the Chugoku mountain range where masa iron sand can be found. Given that the process from start to finish takes some 70 hours spanning three days and nights, it is also called the mikka-oshi, or “three day-pressing” method.

The process begins by putting in komori iron sand, which has a low melting point and excellent reducibility. Charcoal is added next. The mixture is burned, producing iron slag. At this point, heat retention inside the furnace improves thanks to the exothermic reaction (this is called the komori phase).

If the heat of the furnace is raised further, not just iron slag but also pig iron can be made (this is called the post-komori phase).

Kera is obtained as the mixture of masa and iron sand is gradually increased. The furnace's state becomes quite active and the flames shine an extremely bright golden yellow. While the furnace gradually erodes on the one hand, the kera grows on the other (this is called the agari phase).

The amount of masa iron sand in the furnace is increased further, causing the kera to grow ever larger. At some point in this stage, the walls of the furnace will have become quite thin and narrow, making them unable to withstand any further work, and so the tatara operations are brought to a close (this is called the kudari phase).

The entire process outlined above is referred to as hitoyo, literally, “one generation.”

Some 2.8 tons of kera and another 0.8 tons of pig iron are produced from the 13 tons of iron sand and approximately 13 tons of charcoal that are used in one typical hitoyo. Accordingly, if one looks at this from a modern steel-making perspective, the 28% yield rate for the iron sand used in the iron obtained represents an extremely poor value.

The best quality parts selected from the resulting kera are called tama-hagane(“jewel steel”). They were used as the basic material for high-quality edged tools such as Nihonto (Japanese swords). The tama-hagane acquired from 2.8 tons of kera is limited, accounting for one ton or less of the material. Accordingly, one can probably understand from this that tama-hagane was an extremely precious material.

Working at the tatara

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