2023 Volume 63 Issue 12 Pages 1965-1971
Recently, there has been a rapid increase in the discovery of ancient iron smelting furnaces on the Korean peninsula, which are assumed to have been in operation for a long period of time from the Proto Three Kingdoms Period to the Three Kingdoms Period. The iron smelting furnaces are concentrated in the central and southeastern regions of the peninsula, which has led to discussions regarding their timing and regional characteristics. In this study, we focused on the deep circular pit (Furnace pit), which corresponds to the underground structure of the furnace, and various types of pits (Forepit) for collecting pig iron and slags that flowed out from the furnace. We examined the regional and chronological characteristics of the iron smelting furnaces by reconstructing the three-dimensional structure of the furnaces. As a result, it was confirmed that the iron smelting furnaces changed from a semi-underground type, in which the furnace floor was located below the ground surface, to an above-ground type, in which the floor was located at the same level as the ground surface, and that the forepits changed from a horizontal to a vertical structure. These changes were the result of technological transformation and the ingenuity of the craftsmen who carried out the iron-making operations, in response to the higher quality and quantity of the iron produced.
Research on ancient iron production in the Korean peninsula has made great progress with the excavation of the Gyeongju Gwangseong-dong and Jincheon Seokjo-ri sites in the 1990s. While earlier studies had discussed iron production based solely on metallurgical analyses of iron products and iron ores,1) the excavation of production installations at these two sites led to research on smelting, melting and casting furnaces.
Though the situation in North Korea is still unclear, 102 smelting furnaces have been found at 21 sites in South Korea so far. The oldest smelting furnaces date from the early 2nd century in the proto-Three Kingdoms period. Although the technological origins of these furnace are still under investigation, they are generally considered to have been introduced from the Yan Kingdom of the Warring States period or later from Han dynasty in China.2,3) Furthermore, it is clear that the number of smelting furnaces increased with the start of the Three Kingdoms Period. Their distribution can be roughly divided into two regions: central and southeastern Korea. Furnaces of the period discharged pig iron and slag and were broken up after operation, with steel, wrought iron and slag being taken out of their insides. Therefore, what remains at iron production sites are deep circular pits dug directly under the furnaces and other pits of various shapes to collect the pig iron and slags that flowed out of the furnaces. In this paper, the former will be referred to as furnace pits and the latter as forepits - following the terminology of R. Pleiner.4) In addition, traces of blow pipes, furnace walls and slags that had been disposed of were also found at smelting sites. Most studies of iron production sites in the Korean peninsula have focused on the furnace pits and forepits, followed by blow pipes. After a comprehensive examination of these, the planar and three-dimensional shapes of smelting furnaces and the layout for blow pipes have been reconstructed, and experimental archaeological research in collaboration with metallurgists and smelters has also made progress.5,6,7)
This paper focuses on smelting furnaces in the central and southeastern parts of Korea during the Proto-Three Kingdoms and Three Kingdoms periods with a particular focus on furnace pits and forepits, and aims to clarify regional and chronological differences in smelting furnaces by examining the structure of the furnace pits. Furthermore, the study also aims to point out changes in smelting process and the associated technologies in view of the changes in furnace pits’ layout.
Iron production sites in Korea from the Proto-Three Kingdoms to the Three Kingdoms Period, ranging from the 2nd to the 7th century AD, have been identified at 21 locations, and the total number of smelting furnaces discovered amounts to 102. In the Central region (present-day Chungcheong-do and Gyeonggi-do), there are 9 sites with 61 furnaces, and in the Southeast region (Gyeongsang-do), there are 6 sites with 41 furnaces (Fig. 1). Although it is possible that other sites will be discovered in other areas in the future, the general distribution formed by the high concentration of smelting sites in these two regions is unlikely to change. The main artifacts excavated in connection with smelting furnaces are fragments of the furnace walls, blow pipes, iron slags and charcoal. Ceramic sherds that could be used to identify the period are almost inexistent. Radiocarbon dating of charcoal is thus the only way to date these furnaces. Using the Oxcal program, multiple measurements taken at a single site were combined with the R_Combine function to determine the range of overlap in the datings, and extract a statistical mean distribution. The derived results are then arranged in an ordinal sequence, as shown in Fig. 2. Based on these results, the duration of the operations at iron smelting sites in the two regions is shown in Table 1.
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Ancient smelting furnaces usually do not remain, as they were destroyed after smelting and buried underground for a long time. Features excavated at iron production sites consist of a deep furnace pit located beneath the furnace and an associated forepit. Furnace pits may be accompanied by an underground structure. These underground structures are dug and then completely filled with soil, gravel, charcoal and slags. Smelting furnaces were built on top of these pits, hence the diameter of the furnace’s base was larger than the diameter of these pits.
In other words, the diameter of the furnace is calculated by adding the thickness of the furnace wall to the diameter of the furnace pit, however, it is extremely difficult to determine the exact diameter of the furnace from available archaeological information.
Forepits, on the other hand, had the function of temporarily collecting pig iron and slags that discharged from the furnaces. Although there are no examples of blow pipes excavated in site, they are assumed to have been placed on the opposite side of the forepits. Often large-diameter ‘L’ shaped blow pipes have been excavated near the furnaces. It is assumed that a single blow pipe connected the bellow to the furnace.
The following section describes the smelting furnaces excavated in each area, with a focus on furnace pits and forepits. Furnaces with other functions are sometimes found adjacent to smelting furnaces and these will also be discussed when necessary.
3.1. Central Region1 smelting furnace (Fig. 3-1) and 2 smithing hearths were excavated at the site of Kagok-ri, Pyeongtaek, the oldest smelting site in the Central region. The diameter of the furnace pit is 110 cm and 50 cm deep. A pit filled with some sandy clay containing charcoal, sand, and slags, as well as layers of clay containing charcoal, burnt clay and fragments of walls was arranged under the bottom of the furnace. The forepit, directly connected to the furnace, was small and irregularly shaped with a narrow ditch extending from it. A blow pipe with a external diameter of 35 cm was excavated in its vicinity. This smelting furnace dates from the late 2nd to early 3rd century.
At the slightly newer Chungju Chilguem-dong site, as many as 27 smelting furnaces were excavated. This is evidence that smelting furnaces continued for a long period of time in a small area and traces of the furnaces are overlapping. The stratigraphic position of the furnaces can be divided into upper middle and lower layers (Figs. 3-2 to 3-5). Furnace pits measured 98 to 153 cm in diameter and were 16 to 75 cm deep; in some instances, structures were identified within the furnace pits, indicating that wooden posts were used. These wooden posts were probably disposed on the bottom and along the walls of the furnace pit, and then covered with clay to form the lower structure. On top of it was a further layer of sandy clay containing charcoal and slags. The furnace floor was set in this pit at a lower level than the ground surface. All forepits have a horizontal oval shape. Furnaces date from the early 2nd to the late 4th century. Furnaces with wooden structures are not discovered in the lower levels but appear from the middle levels.
12 smelting furnaces (Figs. 3-6 to 3-11) and 6 smithing hearths were found at the Songjeol-dong site in Cheongju. The diameters of the furnace pits ranged from 123 to 130 cm with depths of 65 cm remaining in the deepest example. The pits were filled with sandy clay, charcoal and slags. Furnace bottoms were disposed above them and are located below the ground surface level. At the Songdu-ri site in Jincheon, 25 smelting furnaces and 31 smithing hearths were excavated. Furnace pits are 167–208 cm in diameter and 32–136 cm deep. The bottom of the pit was filled with clay, charcoal and iron slags with the uppermost surface being the bottom of the furnace. In some of the furnaces, structures with wooden piles were discovered, similar to those at Chilguem-dong. The shape of the forepits at these two sites are more diverse with long horizontal oval as well as V- and L-shaped forms. They date from the first half of the 3rd century to the second half of the 4th century.
The Jincheon Seokjang-ri site (Fig. 3-12) is divided into two areas, A and B, where 3 and 1 smelting furnace were found in each area. Furnace pits measures 82–145 cm in diameter and 30–58 cm in depth. The bottoms of those pits were covered with sandy soil containing charcoal and clay. The furnaces bottom were set on it and located under the level of the ground surface. Imprints of wood observed inside of the furnace wall suggest that a wooden structure was embedded as the framework of the furnace. The planar form of all forepits is assumed to be an approximate rectangle with a long horizontal length. Although several other furnaces have been found at this site in addition to smelting furnaces above, it is not fully clear which process of iron production they correspond to. The furnaces date from the late 3rd to the late 4th century.
2 smelting furnaces were excavated at the Daehwa-ri site in Chungju (Fig. 3-14). Diameter of the furnace pits is 135–160 cm and its depth is 30–41 cm. The bottom of the furnace pits was covered with cobbles. Sandy soil was piled above them, and the furnace floor was laid out on top. However, even in this furnace, the floor is below the surface level of the ground. The shape of the forepits became longitudinally oval or wide ditch-shaped. Three smelting furnaces were found at the Gusan-ri site in Jincheon-gun. Diameter of the furnace pits were 120–130 cm and the depth 20–45 cm. In one of them, clay was pasted on the bottom of the pit and 15–40 cm-sized cobbles were paved above it. A sand and clay were piled on top of the cracked cobbles broken and the furnace bed was set on the upper layer. The level of the furnace floor was lower than the surface level of the ground.
Although the form of the forepits presents an approximate rectangular shape, it is longitudinally elongated. The furnaces date from the late 4th to the late 5th century.
3.2. Southeast RegionIn the Southeast region, there is the Gyeongju Hwangseong-dong site, where large-scale iron tool production was conducted from the 1st to the 3rd century BC. 29 melting furnaces (Fig. 4-1), a fining furnace (Fig. 4-2) and more than 26 forging hearths have been discovered. It is unlikely that the pig iron used for the numerous melting and refining furnaces was transported from faraway to this site. A lot of large L-shaped blow pipes (Figs. 4-5 to 4-6) excavated are similar to those used for known smelting furnaces. Noteworthy is the Dalcheon site in Ulsan, approximately 30 km from this site. A large number of iron ore mining pits were found here (Figs. 4-10 to 4-11), as well as potteries from the Proto-Three Kingdom Period. Based on the evidence found at these two sites, it can be well assumed that iron was being produced in the Southeast region during this period.
The earliest smelting furnaces in the southeast region at present are found at the Hagye-ri site in Gimhae (Fig. 5-1) and the Bonglim-dong site in Changwon (Fig. 5-2), where one smelting furnace was found in each. All forepits show a longitudinal oval shape. The furnace pit at the Hagye-ri site is 87 cm in diameter and 48 cm deep; the pit was filled with burnt sandy soil containing charcoal, over which clay was pasted to form the furnace floor. The floor was set below the surface of the ground. The diameter of the furnace pit at the Bonglim-dong site is 116 cm, but only 6 cm of the deepth remained. 2 smithing hearths were also found in the vicinity of the furnace pit. The period of these sites corresponds to the first half of the 4th century and the first half of the 5th century.
7 smelting furnaces have been investigated at the Sachon site in Milyang (Figs. 5-4 to 5-5) and three of them are preserved in good condition. Furnace pits are 70–112 cm in diameter and 60–73 cm deep. It was dug in by two successive degrees, and cobbles was arranged on the upper degree in two or three piles. The lower part of the pit is alternately filled with soil, charcoal and iron slags. 18 smelting furnaces were also found at the Imcheon Keumgok-ri site in Milyang (Figs. 5-6 to 5-8), located approximately 500 m northeast of the Sachon site. The furnace pits are 70–112 cm in diameter and 48–73 cm deep. The uppermost part of the pit is lined with cobbles, and no iron or slag deposits were observed on their inner surfaces (Fig. 6). This indicates that this part of the cobbles was not a part of the inner furnace, but rather of the underground structure. This feature is similar to that of the Sachon site. The planar form of the forepits is predominantly elliptical, but in some cases they are irregularly shaped following destructions during operation. In the vicinity of the smelting furnaces, 16 smithing hearths and a refining furnace were also excavated. Period of these two sites corresponds to the first half of the 6th century and the first half of the 7th century.
As indicated in the previous chapter, there is some discrepancy in the dating of smelting furnaces in the Central and Southeast regions, making it difficult to discuss strict regional characteristics. However, a comprehensive examination of the characteristics of furnaces in both regions reveals the general trends and the technological evolutions in smelting furnaces of the Korean peninsula. This is clearly shown in the level of furnace beds, the lay out of the furnace pit, and the form of the forepit.
4.1. Level of Furnace FloorThe smelting furnaces in the Central region are generally formulated as having their floors all located in pits and at a level below the surface of the ground at the time. In contrast, newer furnaces in the south-east region have their floor located at a level close to the ground surface. Each smelting furnace could be called a semi-underground or an above-ground type8) as shown in Fig. 7. In other words, the smelting furnaces in the central region would discharge pig iron and slag from below the surface, whereas the newer furnaces in the south-east region would discharge them from the surface level. In terms of the working position of smelters, the former would have to bend over considerably because the openings for discharging the pig iron and slags were located relatively lower. In contrast, the latter opening has a higher position, which reduced the need to stoop which is assumed to have made the work itself easier. This can be evaluated as a sign of ingenuity oriented towards ease of smelting operations.
However, not only the external appearance of the smelting furnace and the working position of the smelters as discussed above, but also the changes in the level of the furnace floor were of significance in relation to the quality of the iron produced. As for the semi-underground smelting furnaces in the Central region, the lay out of the bottom of the furnace pit transformed gradually.
It started as a type with only clay piled up on the bottom, then changed to a type accompanied by wooden structures and finally to a type with cobbles on the bottom. Furthermore, furnace pits of above-ground smelting furnaces appearing in the latest phase in the Southeast region, and were filled with clay, then charcoal or slags.
Complex structures built beneath smelting furnaces are also evident in Japanese smelting furnaces, such as the well-known tatara. These are regarded as an elaborate device to block out humidity in the ground.9) They have become significantly larger and more complex, especially since the late Classical Period. It could be said that underground structures of the furnaces for the purpose of moisture protection developed in the Korean peninsula from the Proto-Three Kingdoms period to the Three Kingdoms period. It should be noted, however, that the function of the underground structure is not only to prevent moisture, but also to retain heat of the furnace bed. As the furnace bed cools down, the temperature of the molten iron and slag also declines, preventing them to flow out of the furnace. Advances in underground structure can be assessed as reflecting improvements in iron production.
4.3. Form of the ForepitA forepit of the smelting furnace discovered at the oldest Kagok-ri site in the Central region had an irregular shape with a narrow ditch attached to it. Forepits from subsequent phases have a variety of planar shapes, with the most common examples being oblong oval, rectangular, fan-shaped and V-shaped. It was shown that forepits with oblong oval or rectangular shapes appeared in the new phases at the Daehwa-ri and Gusan-ri sites. This change in the form of the forepits from horizontal to elongated can be considered as a transformation of the working space used in moving the pig iron and slag from the furnace’s opening. In other words, in the early stages, discharging products flowed in a lateral direction, but gradually they were moved only in the direction of the opening.
On the other hand, a forepit of the oldest Hagye-ri site in the Southeast region already had a longitudinal oval shape. Although there are some exceptions in the case of the Imcheon Keumgok-ri site, many smelting furnaces from later periods were accompanied by oblong oval forepits. Therefore, the movement of pig iron and slag in the direction facing the opening was selected from an early stage in the Southeast region. This difference in the form of the forepit represents a change in the movement of the smelter and it can be assessed that the direction of work at the front of the furnace became convergent in the two regions.
Smelting furnaces of the Proto-Three Kingdoms period and the Three Kingdoms period were examined above covering the Central and Southeast regions of the Korean peninsula. Development of the furnaces’ structure, such as the level of the furnace bottom and underground facilities for humidity-protection, were revealed as well as changes in the discharging methods of the pig iron and slags, and the direction of their discharging. The various elements of smelting furnaces can be simplified and conceptualised as shown in Fig. 8.
As already pointed out, there is still a lack of data on smelting furnaces in the South-east region of the Proto-Three Kingdoms period and the Central region of the late Three Kingdoms period. This problem remains to be clarified with the increase in excavations.
Furnaces for casting, melting, fining and processing the pig iron produced in smelting furnaces, are also unevenly distributed in the South-east region. In relation to the ironwork production, the detailed aspects of these furnaces need to be clarified. More meticulous research and further studies are expected in the future.
