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Iron Produced from the Tatara

There is the kera-oshi method and there is the zuku-oshi method, and the products made from them are kera (a mass of steel) and zuku (pig iron), respectively.

After the kera has been broken up at a metal working area on site, the pieces are sorted through at the haganezukuri-ba (“steel-making site”). The best portions will become tama-hagane. The remainder, on the basis of such factors as size, surface fractures, and amount of iron slag in the compound, is sorted out into silvereye, manufacturing powder, bu-kera (kera with a carbon content between that of tama-hagane and iron), and kera-hoso (small clumps of bu-kera). The tama-hagane, silvereye, and manufacturing powder is packaged and shipped as is for sale. The remainder will be sent together with the pig iron to a metal-working shop where it will be finished into sage-hagane or knife steel and then shipped.

Spindles used for breaking up kera at the metal working site (Wako Museum).

The spindles are dropped from a high place and break up the kera.

The work performed at the metal-working shop differs from the forging that is done today. It includes an operation for fusing and removing the iron slag contained in the kera through forging it in a half-melted state. As such, it is a kind of smelting process and can also be described as a steel-working operation, in which the pig iron and kera scrap are decarbonized and turned into soft steel. Accordingly, the manufacturing method for the steel that reaches the metal-working shop from zuku-oshi can be said to be the Japanese version of an indirect iron-manufacturing method.

The kera-oshi method, by contrast, is a process for creating steel directly, since steel is produced directly from the iron sand. It is a traditional technology unique to Japan and is lauded as a national cultural heritage.

Varieties of tool used at metal working factories (Wako Museum).

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