Large-scale iron smelting features and evidence for change in technology, unfolding in one of Japan's largest iron producing regions.

Distribution of features Seven ancient iron smelting furnaces, and 26 charcoal-making kilns were found. In addition, in the woods on the eastern side of the investigated area, there are 31 depressions where smelting furnaces are thought to have existed, and another 23 have been found which are inferred as charcoal kilns. Three iron smelting furnaces and 16 charcoal kilns have also been found nearby at the Tategoshi site.

Annular feature (ca eighth century, latter half) Twenty meters in diameter, it is inferred that six chimney-shaped furnaces were operated in its central portion, dug down about 1.5 m. The photo on the left was taken from within the feature. Chimney furnaces are generally built singly, utilizing differences in elevation in the natural topography, but at the Yokodaidō Iron Smelting Historic Site they were built in concentrated fashion in this large-scale, man-made earthwork.

Charcoal making kilns (ca eighth century, latter half) Sixteen charcoal making kilns were built in concentrated fashion at the central portion of the investigated area. Charcoal was used as fuel for melting the iron sand; chambers of these tunnel-shaped kilns were dug underground, with their floors sloping upward in a climbing kiln structure.

Nara period bellows pipe (ca eighth century, latter half) This type of bellows pipe was solely for use with a chimney-shaped furnace; it is a large earthen cylinder for conveying wind produced by the foot-operated bellows into the furnace interior. Long examples were 70 cm or more in length, with internal diameters of around 8-10 cm. A single item was used each time the furnace was operated, and is thought to have been fixed to the furnace wall with clay. The item in the photo is 51.3 cm long, with an internal diameter of 8.1 cm.

Artist's reconstruction of a chimney-shaped furnace (ca eighth century, latter half) To the rear of the cylindrical bloomery (furnace), about 1 m in diameter, was attached a foot-operated bellows. (Illustrations by Suda Hiroyuki)

Box-shaped furnace and artist's reconstruction A box-shaped furnace was built at the black area in the center of the photo where charcoal has accumulated. Immediately to one side, part of the furnace wall was found where it had collapsed. The external dimensions of the furnace were about 210 by 90 cm, and its height is inferred to have been 150 cm or more.

Early Heian period furnace wall with attached tuyeres (ca ninth century, first half) Tuyeres were attached to the bottommost tier of the furnace wall. The technique of attaching tuyeres, or the bellows nozzles for a blacksmith's forge, to air holes in a smelting furnace was especially advanced in the Pacific coastal region of Fukushima and Miyagi prefectures in the eighth and ninth centuries. At the Yokodaidō Iron Smelting Historic Site there was distinct differential use of clay melting at lower temperatures (1,200-1,300oC) for the furnace walls, and more highly heat-resistant clay for the tuyeres. Width: 33.5 cm.

Heian period ink-inscribed pottery (ca ninth century, first half) This is a Haji ware plate with an attached pedestal. On the bottom side the character (zai, wealth) is written in ink. At the Yokodaidō Iron Smelting Historic Site, Haji ware with the characters (yamatsukuri, lumbering), (ie, house) (ta, rice paddy) were also recovered. Diameter of bottom: 8.2 cm.

Lumps of iron bloom Nearly all of the iron produced at the Yokodaidō Iron Smelting Historic Site contained about 2-4% carbon in addition to the iron. Such recovered iron lumps, from around the first half of the ninth century, were shaped into a variety of iron implements after being wrought to remove the impurities. The item at upper right is approximately 3.8 cm long.

The Yokodaidō Iron Smelting Historic Site, and the distribution of government office and fortress sites in the ancient T?hoku region

Yokodaidō Iron Smelting Historic Site, Minamisōma City, Fukushima Prefecture

Discovery of a large-scale iron furnace group

The Yokodaidō Iron Smelting Historic Site, located in the Pacific coastal region of Fukushima prefecture, is an iron smelting site from the Nara to the Heian periods (eighth to the latter half of the ninth centuries). Chimney-shaped bloomeries (furnaces) were found within an annular feature (a doughnut-shaped mound of earth) of the Nara period, and to its south and southeast were a group of Heian period iron furnaces, dumping grounds for slag, and charcoal-making kilns. The site is situated on a hilltop 40-50 m in elevation, about 7 km distant from the coastline.

The Nara period annular feature was around 20 m diameter in scale, made by digging approximately 1.5 m down in the center and piling the dirt in a ring around the circumference. Six chimney furnaces were placed inside. It is thought that the large-scale earthwork was made in order to set up the furnaces in planned fashion.

Iron technology changes in the Heian period

Five meters south of the annular feature, approximately 60 tons of material related to the operation of box-shaped furnaces, such as furnace wall fragments and iron slag, were recovered from a Heian period dumping ground. The furnaces producing this discharge were on the eastern side of the investigated sector, and it is anticipated that a large-scale group of box-shaped furnaces unfolds over the surrounding area. It is thought that the main basis of iron production changed in the Heian period, from the Nara era chimney furnaces to smelting in these box-shaped facilities.

In addition, 26 subterranean kilns for making charcoal have been ascertained. These were built in concentrated fashion on a single slope, with the shape, size, and method of construction being very similar. From this it is inferred that the same craftsman was most likely involved in all of the kilns, which were operated over a short period of time.

The coastal area of Fukushima prefecture is rich in iron sand which serves as a source of iron, and in ancient times was one of the foremost iron-producing regions in the entire country. At the Yokodaidō Iron Smelting Historic Site, furnaces for smelting iron, dumping grounds for slag, and kilns for making charcoal are preserved as a set in good condition, and it can be called a vital site for considering aspects of ancient iron production such as the introduction of chimney-shaped furnaces. (Kadowaki Hidenori)

Ancient policy towards the Emishi and the productive system of Mutsu and Hitachi provinces

In the Ancient period, people called "Emishi" by the central authority lived in the Tōhoku region. There were frequent clashes between the central administration, which aimed to expand its sphere of influence, and the Emishi who did not comply, and many such incidents are seen in entries of the Nihon shoki chronicle from the latter half of the seventh century.

Disputes between the political center and the Emishi grew increasingly violent in the Nara period. As if to stake out their newly won territories, the central administration established fortresses (jōsaku), government offices combined with military installations, in the central portions of lands it wrested from Emishi control, and also at the front lines of the conflict.

In 774, the dispute between the two sides escalated into a prolonged war. The fighting which came to be known as the "Thirty-eight-year War" started when Ōtomo no Surugamaro, appointed "Barbarian-subduing Generalissimo" by Emperor Kōnin, struck back at Emishi who invaded Monou castle, and lasted until 811 when the then Generalissimo Fun'ya no Watamaro pleaded with the Emperor to put an end to the conflict.

The period when iron production was conducted at the Yokodaidō Iron Smelting Historic Site, from the latter half of the eighth to the latter half of the ninth centuries, coincides with this time of ruthless battle between the central authority and the Emishi. The Pacific coastal region of Fukushima prefecture yields iron sand in plentiful amounts, and at the time was among the first rank of iron-producing zones in Japan. Large-scale iron smelting and casting sites have been verified not only at Yokodaidō, but repeatedly in this area. It is thought that in order to sustain long-term organized iron production, which requires large-scale forestry, the central government was involved.

Iron was produced in great quantity as the raw material for iron tools used at fortresses on the front lines during the period of conflict with the Emishi. Through clarification of these sites, conditions of this conflict that cannot be learned from historical documents come to light. (Ōmi Toshihide)

Adapted from Hakkutsu sareta Nihon rettō 2011 [Excavations in the Japanese Archipelago, 2011] (Bunkachō [Agency for Cultural Affairs], ed., Asahi Shimbun Publications, 2011).