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The History of the Tatara

Types of Iron Sand

Biotite granite is the parent rock of the masa iron sand used in the kera-oshi process. Iron forms 1% to 2% of biotite granite, which is further characterized by its lustrous black color and a rather large grain (50-100 mesh) that allows it to be easily broken up. Akome iron sand for zuku-oshi use comes mainly from diorite. Six to nine percent of diorite is iron, which is rather high. Diorite's other characteristics include a fine grain, red coloring that comes from the presence of gang, and a solidity that makes it difficulty to break up.

Various oral traditions have come down about how to differentiate between the types of iron sand. For example, one method calls for taking a handful of iron sand, working the sand against the palm of the hand to get a reaction, and sprinkling it over flames. If it produces a crackling sound and pops, then the iron sand is of high quality. If it feels like ash in the hand and does not react against the palm of the hand even when worked against it, then it is low quality. The iron will not boil, and it will not serve as sword steel. Another method calls for using the color and weight after crushing it up as the factors for making a judgement. According to Shimohara Shigenaka (1738-1821), author of Tetsuzan hisho (a private reference work on iron mines), iron sand is the most important factor in producing good iron. “The basis for opening an iron mine is to find good iron sand. If this one element is bad, then the mine will not do,” he wrote.

Both masa and akome iron sand are referred to as yama, or “mountain” iron sand because they are taken directly from weathered parent rock (and hence was in a location where it would be exposed to the elements, as on a mountain). However, there are other types of iron sand, referred to in Japanese as kawa (river) and hama (beach) iron sand based on the location from which they are collected. Because these types of iron sand have been carried and filtered out by currents of flowing water, and then are deposited on land, they are thought to be the product of the mixing of various types of iron sand. Generally, however, most of this sand is made up of elements with a high titanium component. These kinds of sand are used mainly in the zuku-oshi method and as komori- and agari-stage iron sand for the kera-oshi process.

There are numerous explanations for why masa iron sand is used with the kera-oshi method and akome with the zuku-oshi one. Conventionally, it is thought that pig iron (zuku) can be easily produced from akome iron sand because it contains high concentrations of hematite (Fe2O3), and is easily reduced. However, as the chart below shows, the results achieved when conducting reduction tests show that masairon sand has better reducibility. Given that melting point research shows that masa iron sand melts at 1,420°C and akome at 1,390°C, it may be that this disparity is reflected in the degree of ease with which pig iron and kera are produced.

Reduction time and Reduction Rate of masa iron sand, akome iron sand and hama iron sand

Tatara iron-manufacturing prospered in the Chugoku mountain ranges because superior iron sand—masa iron sand that contained few impurities—could be collected there, and high quality wrought iron and steel has been manufactured there since ancient times.

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