The Tetsuzan hisho ranks “iron sand as first, the mountain source as second, and the furnace site as third,” revealing how important tatara operations are. In terms of the raw materials used, charcoal closely follows iron sand in importance. This is because even when the iron sand is of good quality, the iron will not come out if the charcoal is bad. Even if the iron sand is inferior to some degree, the iron will boil provided the charcoal is good.

Charcoal for general use is separated into two types: light, white charcoal that burns at a high temperature (approximately 1,000°C) and soft, black charcoal that burns at lower temperatures (from 400°C to 800°C). The types of charcoal used in the tatara, however, are a little different. They are called osumi (“greater charcoal”), which is the charcoal used for smelting the iron in the furnace at the tatara, and kozumi (“lesser charcoal”), which is the charcoal used for the smithery work. Osumi is made using a method resembling that for black charcoal. However, it burns at an even lower temperature, contains numerous partly heated elements, and its quality as charcoal is coarse. This is due to having a low fixed carbon component (less than 60%) and a high volatile matter component (more than 30%) that together create superior conditions for increasing the force of the fire. According to the Tetsuzan hisho, pine, chestnut, maki pine, and beech were good for making osumi. Shide birch, kobushi magnolia, and cherry were bad, while shii oak and crape myrtle were the worst. Sawtooth oak, nara oak, and scrub were favorable. For kozumi, pine, chestnut, horse chestnut, and Japanese cedar were all superior, while shide birch, shii oak, maki pine, kashi evergreen oak, and mochinoki oak were considered inferior. Kozumi is made by piling up wood into pit dug into the earth and lighting it on fire. When the fire burns out, brush wood, bamboo grass, and earth are tossed onto it and it is left to smolder. The yield from the live wood used is about 10% for kozumi, 20% with osumi.

The photographs below present cross-sections of typical charcoal, making it easy to understand why pine burns so easily.

Pine (excavated from the Shirogasako ironworks ruins in the city of Miyoshi, Hiroshima Prefecture)

Shide birch (excavated from the Tamanomiya No. 1 tatara ruins in the town of Tamayu, Shimane Prefecture)

Mizumaki pine (excavated from the Kadosaki ruins in the town of Oasa, Hiroshima Prefecture)

NOTE: The three photographs are all shown at the same magnification.

During the peak period, one tatara site on average would go through 60 production cycles annually. This would consume approximately 810 tons of charcoal, requiring 60 hectares of forest. Because it takes approximately 30 years to grow the trees needed, a miner required about 1,800 hectares of forest for a single tatara and equivalent multiples if he had several tatara. Mine owners started as the owners of large amounts of wet paddy land. They received permission to assert exclusive possession over forests held by the domain, then would lumber and raise these forests in a planned fashion, based on subsidies from the domain. In the later Edo period, the Chugoku mountains became a supply base of sort that produced around 90% of the iron used throughout Japan.

The person responsible for burning the charcoal to be used in the tatara is called the yamako (lit., “mountain child”). The mine owner arranges for the yamako to be assigned to a portion of the mountain, and the yamako devotes himself exclusively to the task of burning charcoal. The job of the yamako offered considerable profits if he was skilled at his job. A person who could not make his living as a yamako was said to be a “good-for-nothing, and should not be employed.”

Charcoal includes 2% to 3% ash. Some 30% to 40% of that is CaO, and approximately 3% is phosphorous. Given that the amounts of these elements vary based on the kind of tree being used, they also affect the quality of the tatara charcoal.

page top