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Medieval Tatara

Concentration in the Chugoku Mountains and Furnace Growth

Production of iron during medieval times (approximately 1200 to 1600) came to be concentrated in the Chugoku region, particularly the mountainous areas where modern tatara iron manufacturing appeared. The primary source of iron was iron sand.

As evidenced by the Oya ruins in Hiroshima Prefecture, iron manufacturing entered a new era from the 11th to 13th centuries with the growth in furnace sizes and the development of underground structures. The floor of rectangular chamber-style furnaces began to resemble the bottom of a boat, while the main body of the furnace itself reached 2-meters long and 1-meter wide, bringing it closer in size to the early modern tatara. Moving from the latter half of the 14th century into the 15th century, we find the remains of furnace floors with this shape in such places as the Ishigami ruins in Hiroshima Prefecture, or the Shimoinasako ruins in Shimane Prefecture. The furnace shape is extremely close to that of the early modern tatara, and they have an underground structure. Changes in the length and width of furnaces for each time period are shown on the chart. The trend toward ever larger furnaces is apparent.

Changes in Furnace Length and Width

Improvements in Tatara Productivity

Demand for swords increased rapidly in the latter half of the Muromachi Period (1333-1573), and general demand for steel grew remarkably with the establishment of commoner society. Approximately 37,000 swords were exported every year to Ming China, said to slice through multiple layers of objects. There were demands for an increase in tatara productivity as a result, and with both increases in tatara size and concentration of production in the mountainous areas of the Chugoku region. Improvements to the bellows were also necessary, and trade with the Ming led to the use of reciprocating foot bellows operated by several people. Furnace temperatures also increased, and advances in the removal of carbon steel and pig iron led to larger production volumes.

Iron and Trade

The famous Chigusa and Dewa types of steel appeared at the start of the 16th century. These steels are believed to have been made through a process consciously intended to create steel in mass quantities, without pig iron or wrought iron. This is thought to mark the start of the kera-oshi manufacturing method, and to form the background that made trade with the Ming possible.

During the Warring States period (1467-1568) Japan iron supplies became insufficient. It was around this point that nanban (“southern barbarian,” so called because Europeans came to Japan from their bases to the south in the Philippines and Macao) iron came into Japan via the Portuguese, with swords and firearms made in earnest. Nanban iron was also referred to as “Indian iron,” and was used for approximately 90 years from the Keicho (1596-1616) to Hoei (1704-1711) eras. After unifying Japan, Toyotomi Hideyoshi made war on Korea, a move that is said to have encouraged the spread of the new Chiba and Dewa steel mass production methods throughout the country.

The famous swordsmith Horikawa Kunihiro made use of these new types of iron (Chiba and Dewa steel), along with innovative forging techniques to produce a new type of sword. In Japanese, swords that were made before his time (before the Keicho era) are referred to as koto, or “old swords.” Those made after him are called shinto, or “new swords.” Accordingly, there are differences in the unprocessed metal used in koto and shinto. Put another way, is a difference in the manufacturing technique.

The shinto is made using mass-produced steel, represented by Chiba steel. Although it is not certain, it is thought that koto were produced using steel made by decarburizing either pig iron or ordinary iron (reducing the carbon in wrought iron to make steel).

Iron Manufacturing Technology

While the development of the tatara furnace from ancient to medieval times has been detailed above, we have not yet answered the question of what kind of iron was manufactured in those furnaces.

Did they mainly make pig iron, decarburize it through a large-scale blacksmith manufacturing process, and turn it into iron and steel? Or was wrought iron made into kera (mixture of raw steel and slag), put through smelting and forging (pounding the kera at a high temperature and squeezing out the iron slag)? This is not something we can determine with certainty, just as we don't know what method was used to manufacture koto.

The large-scale forging seen with the early modern tatara is thought to date to the middle of the Edo Period (around the latter half of the 17th century), when mass production of iron was established. Given that numerous blacksmithing furnaces have been detected alongside medieval tatara, and given also that blacksmithing furnaces have been discovered with well-developed underground structures, the progress in large-scale blacksmithing technology is easily seen.

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