Abstract
Iron making from iron sand in Chugoku district began soon after the introduction of iron ore smelting by the low shaft box-type furnace in the middle of the sixth century and it rapidly spread all over the country, but Chugoku district continued to be the center of its progress. The process consistently developed throughout the ancient and the medieval times towards more elevated furnace temperature by means of increased furnace volume, improved underground facilities for moisture elimination and increased blowing capacity of bellows. The development resulted in the birth of Tatara as the finally-evolved box-furnace on the one hand and in the change of the chief product of smelting from lumpy mixture of metal with slag or a large metal block involving some slag to molten pig iron (Zuku) on th other hand. The priority production of Zuku led to the invention of the two-stage refining process for its conversion into wrought iron (Ohkaji) in the 17th century. Zuku-making by Tatara and refining by Ohkaji formed Japanese indirect wrought iron making system.
Metal block (Kera), which was formed in Zuku making from a sort of iron sand hard to fuse (Masa) compared with another sort of iron sand (Akome), proved to contain about 50% parts of steel high in carbon content, which, naturally, were taken out by hammering after the rough crushing by falling-weight methd.
Because of economical disadvantages mainly due to inefficiency of iron sand smelting, Japanese indirect iron making system could not survive after 1923 in spite of several successes in developing new iron making process of blast furnace-type. However Tatara is preserved as a technical heritage and is still operated several times a year for direct steel making through Kera.
