1. INTRODUCTION
Changes in the production capacity and technology of metals in ancient times influenced the development of ancient society such as politics, military, and agricultural culture. Therefore, iron-making technology and iron-making ability are one of the important factors influenced and influenced in the beginning and development of ancient society on the Korean Peninsula (Noh, 2000).
The iron culture of the Korean Peninsula was estimated to have been introduced from China around 4 century B.C. and had a great influence on the formation process of the Three Kingdoms in A.D.. The production and manufacturing technology of iron differs according to the era, society and class, spreads and develops through the expansion, movement, and exchange of powers. Therefore, the change in a group’s culture and power relationship can be speculated through the study of the material production process and technology of the excavated iron artifacts (Lee, 2002).
The physical properties of iron are determined by crystal structure and microstructure, and microstructure can be controlled by applying artificial treatment. Through this, it is possible to analyze the microstructure of the artifact and infer the treatment method applied at the time of processing (Scott, 1991).
Iron artifacts excavated from the Korean Peninsula are diverse, including a long sword, an iron axe, an iron nail, an iron chisel, and an iron head. The iron axe is similar to a contemporary axe and is named thus. Axes excavated from the tomb are categorized according to their forms and functions. A forged iron axe is generally understood to be used as a hoe as well. Regarding an iron chisel, it is presumed that there are morphological differences based on its function and the era it was used in, but it was mainly used to make grooves in wood (National Research Institute of Cultural Heritage, 2015).
The Guryong-ri site in Ungcheon, Boryeong is located at San 172 Guryong-ri, Ungcheon-eup of Boryeong-si, Chungcheongnam-do, and it was found during a cultural artifact excavation conducted between April 2016 and January 2017. Excavations were conducted at a total of 9 sites (Figure 1). Through the investigation, the sites of the Bronze Age, the Baekje Period, the Goryeo Dynasty, the Joseon Dynasty, and the unknown period were identified, and 975 artifacts, including earthenware, stoneware, and metals, were excavated (Baekje Cultural Properties Research Institute, 2019).
The purpose of this study is to estimate the production method of 5 iron artifacts excavated from the Baekje stone chamber tombs of the Guryong-site in Ungcheon, Boryeong (6-7 century A.D.) and understand the metallurgical characteristics of iron used as material. In addition, the results of this study are compared with previous research on surrounding sites to examine the characteristics of the sites.
2. SAMPLES AND ANALYSIS METHOD
The 3 iron axes and 2 iron chisels were selected as samples to identify the metallurgical features as iron artifacts excavated from Baekje Stone Tombs in Guryong-ri site, Ungcheon, Boryeong (Table 1).
Sample were collected to minimize the demage to the artifacts. The specimens were mounted with an epoxy resin, polished in order of density with sand paper of 100-4000 mesh, and washed with water. Then, they underwent fine polishing using abrasive cloth (MD-Mol, MD-Nap, Struers, Denmark) and an abrasive (DP-Spray 3 μm, 1 μm, 0.25 μm, Struers, Denmark). The polished samples were etched using Nital 3% (HNO3 3% + Ethyl alcohol 97%) solution.
The etched sampels were observed for microstructure and non-metallic inclusions under a metallurgical microscope (NEXCOPE, U.S.A.). After the surface of the samples was coated with platinum, microstructure and non-metallic inclusions were observed with a scanning electron microscope (SEM, MIRA3, TESCAN, Czech), and an energy dispersive spectrometer (EDS, QUANTA300, BRUKER, Germany) was used to analyze the chemical composition of the non-metallic inclusions.
In addition, Raman micro-spectroscopy (LabRamARAMIX, Horiba, Jobin-Yvon, France; Nd:YAG Laser 514 nm(He-Ne analysis)) was performed to identify the crystals of non-metallic inclusions. Raman shift, which has been cited in the reference, was used to determine the result of the analysis.
3. RESULTS
3.1. Iron axe (No. 1)
A part of the blade of the iron axe (No. 1) was lost, but overall the artifact is complete. The planar form widens from the blade tip toward the blade. The joint is wide and the line of the blade is arc-shaped (Baekje Cultural Properties Research Institute, 2019).
Figure 2A show the part of the iron axe (No. 1) from where the sample was collected. The left of Figure 2B looks toward the interior and the right toward the blade of the artifact. A difference in the carbon content and grain size can be observed. Because of high carbon content, the figure was dark at the blade, and become brighter toward the interior. The inner portion of the artifact shows relatively large ferrite and elongated non-metallic inclusions (Figure 2C, 2D).
The composition of non-metallic inclusions was identified through SEM-EDS analysis (Table 2, Figure 3). Point A-2 was presumed to be a glass matrix with high CaO and TiO2 content. Point B-1 was identified to be fayalite of 58.25wt.% FeO and 21.29wt.% SiO2. Point B-2 was identified as wüstite of 100.00wt.% FeO.
The TiO2 content was high at 29.73wt.% at point A-1; thus, Raman micro-spectroscopy was performed. As a result (Figure 4), a Raman shift of 484, 556, 681 cm-1 were detected, confirming it as ulvöspinel (Fe2TiO4).
3.2. Iron axe (No. 2)
The iron axe (No. 2) narrows down from the fore-end toward its blade tip and gradually widens toward the blade. Traces of wooden substance can be found inside the tip of the blade, and the line of the blade is arc-shaped (Baekje Cultural Properties Research Institute, 2019).
Figure 5A show the part of the iron axe (No. 2) from where the sample was collected. The left and right sides of Figure 5B show the blade and the interior of the artifact, respectively. Three layers were found due to intensity differences along the direction of the non-metallic inclusions. At the center of the sample, small grain-sized pearlite was observed. Carbon content decreases near the surface; thus, light-colored ferrite with a large grain size was observed. Moreover, non-metallic inclusions exist as long lines (Figure 5C, 5D).
The composition of non-metallic inclusions was identified through SEM-EDS analysis (Figure 6, Table 3). Point A-2 and B-1 were identified to be wüstite containing FeO as the main component. Point A-1 and B-2 was detected at 15.02wt.% and 20.26wt.% P2O5.
To identify the crystal, Raman micro-spectroscopy was conducted on Point B-2. As a result (Figure 7), Raman shift of 153, 196, 295, 393, 554, 606, 826, 932, 970 and 1063 cm-1 were detected, identifying them as polycrystals of fayalite (Fe2SiO4) and sarcopside (Fe3(PO4)2).
3.3. Iron chisel (No. 3)
The blade of the iron chisel (No. 3) is missing, and the head is square-shaped. The handle narrows toward where the missing blade is presumed to have been connected (Baekje Cultural Properties Research Institute, 2019).
Figure 8A shows the part of the iron chisel (No. 3) from where the sample was collected. The upper and lower portions of Figure 8B were the surface and the interior of the artifact, respectively. Ferrite grown in an isometric form was observed near the surface of the artifact, toward the interior of the artifact, the color becomes brighter, and large-grain ferrite was found (Figure 8C, 8D).
The composition of non-metallic inclusions was identified through SEM-EDS analysis (Figure 9, Table 4). Points A-1 and B-1 were identified to be wüstite, Feo content of 89.92wt.% and 99.62wt.%, respectively. Points A-2 and B-2 were found to be glass matrixes with high CaO content, Al2O3, P2O5 and K2O were also detected.
3.4. Iron axe (No. 4)
The iron axe (No. 4) narrows down slightly from the fore-end toward the blade tip of the axe, and gradually widens toward the blade. The line of the blade is arc-shaped (Baekje Cultural Properties Research Institute, 2019).
Figure 10A shows the part of the iron axe (No. 4) from where the sample was collected. The upper and lower portions of Figure 10B were the surface and the interior of the artifact, respectively. Because of severe corrosion, only the microstructure of the artifact’s interior can be observed. Mixed ferrite and pearlite were found together overall (Figure 10C, 10D).
The composition of non-metallic inclusions was identified through SEM-EDS analysis (Figure 11, Table 5). At point A-1, 96.57wt.% of FeO and 0.96wt.% of Cl were detected, which are identified to be iron oxide produced by corrosion. Points A-2 and B-2 were assumed to be the glass matrixes, and CaO, Al2O3, and TiO2 are also detected.
At point B-1, 28.13wt.% of CaO and 33.16wt.% of TiO2 were found; thus, Raman microspectroscopy was conducted for identification. As a result (Figure 12), Raman shifts of 178, 248, 286, 336, 470, 504, and 777 cm-1 were detected, confirming it as perovskite (CaTiO3).
3.5. Iron chisel (No. 5)
The head of the iron chisel (No. 5) is missing. The handle narrows down to where the blade had been connected (Baekje Cultural Properties Research Institute, 2019).
Figure 13A show the part of the iron chisel (No. 5) from where the sample was collected. The upper and lower portions of Figure 13B were the surface and the interior of the artifact, respectively. Ferrite was observed near the surface of the artifact, and it was bright in color as there was less carbon content. The interior of the artifact become darker, and pearlite increases. In addition, non-metallic inclusions were elongated, and crystal grains were found in various sizes (Figure 13C, 13D).
The composition of non-metallic inclusions was identified through SEM-EDS analysis (Figure 14, Table 6). Point A-1 was presumed to be wüstite with FeO as the main component, and points A-2 and B-1 were estimated to be fayalite due to the high content of FeO and SiO2. Points A-3 and B-2 were assumed to be the glass matrixes, CaO, Al2O3, K2O were also detected.
4. DISCUSSION AND CONCLUSION
The study observed and analyzed the microstructure and non-metallic inclusions of 3 iron axes and 2 iron chisels excavated from the Baekje stone chamber tombs of the Guryong-ri Site in Ungcheon, Boryeong (6-7 century).
Ferrite and pearlite were observed in the iron artifacts excavated from the Guryong-ri site in Ungcheon, Boryeong, suggesting that hypoeutectoid steel was used as the material. In most artifacts, non-metallic inclusions in the form of long lines are identified because of the forge welding process. In the case of the iron axe, the grain size was larger or smaller than that of the iron chisel, and more non-metallic materials were also confirmed to be elongated. In addition, differences in carbon content were observed in some artifacts depending on the direction of the sample For the iron axe (No. 1), the carbon content increases near the surface; thus, it was carburized to strengthen the surface. The carbon contents of the iron axe (No. 2) and the Iron chisel (No. 5) decrease near the surface, indicating that these artifacts underwent a decarbonization process from molding. Though the size of grains varies because of the difference in the degree of processing, no signs of heat treatment have been found.
The metallurgical characteristics of these artifacts were examined by comparing them with the sites in the coastal area around the samples, which are he Bujang-ri site in Seosan (4-5 century) and the Bongseon-ri site in Seocheon (4-6 century). It is found that the iron artifacts excavated from the Bujang-ri site in Seosan were slowly cooled from high temperatures and underwent procedures such as molding, quenching, decarbonization or tempering based on their use (Son, 2011). The iron artifacts excavated from the Bongseon-ri site in Seocheon were molded into iron tools, but martensite was formed in some artifacts due to quenching. These artifacts went through carburizing and decarbonization processes (Cho et al., 2014). This confirms that heat treatment technology existed in the surrounding sites of the previous period, and the artifacts of the Guryong-ri site in Ungcheon, Boryeong didn’t undergo heat treatment considering their purpose of use.
As for the chemical composition of the non-metallic inclusions, P2O5, CaO, TiO2 were detected in addition to FeO and SiO2; crystals of wüstite, fayalite, ulvöspinel, sarcopside and perovsikte were also identified. This means that the iron bloom produced by direct smelting was used to manufacture the artifacts. Further, to examine the P2O5, CaO and TiO2 that were found in high contents, they were plotted out in ratio to SiO2 (Figure 15). As SiO2 is an oxide that is hardly reduced when directly smelted, the ratio is similar to that of an ore (Cho, 2015).
As the P2O5/SiO2 ratio (Figure 15A) was widely distributed compared with that in other site, the refining technology was identified to not have been uniform, In addition, the TiO2/SiO2 ratio (Figure 15B) was found to be higher than that of other site. This proves that a titanium-containing ore was used to manufacture the artifacts, unlike in other surrounding sites. TiO2 is usually caused by raw materials, but it is not detected in all artifacts currently analyzed, so it is likely that it was affected by various factors such as furnace walls in addition to raw materials. When the CaO/SiO2 ratio (Figure 15C) and the (Al2O3/SiO2)/(CaO/SiO2) ratio (Figure 15D) were 0.42 or more and 0.83 or less, respectively, it can be understood that an additive has been included (Kim, 2014; Lee, 2017). As the results from the samples fall within the range, it confirms that the iron artifacts of the Guryong-ri site in Ungcheon, Boryeong contain additives that are similar to the surrounding sites. It is possible that CaO containing raw ore was used because CaO was affected not only by the use of slag former but also by the ingredients of raw ore.
Therefore, the iron artifacts excavated from the Baekje stone chamber tombs at the Guryong-ri site in Ungcheon, Boryeong, were manufactured by folding and tapping hypoeutectoid steel using the iron bloom manufacture through the direct smelting method, and they underwent the carburizing process according to their purpose of use. Moreover, compared with the sites of Bujang-ri site in Seosan and Bongseon-ri site in Seocheon, which are closely located to the Guryong-ri site in Ungcheon, Boryeong, the show similarities in that the direct smelting method was used, and additives were added. However, it is confirmed that the refining techniques used to manufacture the iron artifacts were not uniform. In addition, it is highly likely that ore different from that of the surrounding sites was used for production, but a more comprehensive comparative study with the surrounding sites is needed in the future.
This study examined the iron artifacts from the Guryong-ri site in Ungcheon, Boryeong, identified the metallurgical characteristics and compared them with artifacts from surrounding sites. It is theorized that if further related research is conducted in the future, the characteristics of each region and period will be more clearly identified.
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