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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.30 No.9 pp.453-457
DOI : https://doi.org/10.3740/MRSK.2020.30.9.453

Manufacturing Technique of Gilt-Bronze Objects Excavated from Tomb No.1(Donghachong) in Neungsan-ri, Buyeo

Yong-bi Shin1,2, Min-hee Lee1, Gyu-ho Kim2
1Conservation Sciences Lab., Buyeo National Museum of Korea, Buyeo 33156, Republic of Korea
2Department of Cultural Heritage Conservation Sciences, Kongju National University, Gongju 32588, Republic of Korea
Corresponding author E-Mail : kimgh@kongju.ac.kr (G.-H. Kim, Kongju Nat’l Univ.)

July 15, 2020 July 15, 2020 August 30, 2020

Abstract


Tomb No. 1 (Donghachong) of the Buyeo Neungsan-ri Tomb complex (listed as UNESCO World Heritage Site), is a royal tomb of the Baekje Sabi Period. One wooden coffin unearthed there is an important relic of the funerary culture of the Baekje. This study examines the production techniques of gilt-bronze objects attached to the wooden coffin excavated from Donghachong. The base metal of the gilt-bronze object is pure copper, with single α phase crystals in a heterogeneous form containing annealing twins; Au and Hg are detected in the gilt layer. We suggest that the surface of the forging copperplate is gilded using a mercury amalgam technique; it is thought that the annealing twins of the base material formed during the heat treatment process for the sheet metal. The gilt layer is three to five times thicker for the gilt-bronze objects found near the foot of the coffin than those near the head. We estimate the plating process is carried out at least three times because three layers are identified on the plate near the head. Therefore, it is likely that the materials and methods used to construct the giltbronze objects found in different parts of the coffin are the same, but the number of platings is different. This research confirms the metal crafting techniques used in Baekje by the examination of production techniques of these gilt-bronze objects. Further, our paper presents an important example of restoration and reconstruction for a museum exhibition, through effective use of scientific analysis and investigation.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    The wooden coffins excavated in the tombs in Baekje historic areas were made to enshrine the deceased and are an important clue to understanding the funerary culture of the Three Kingdoms Period (ca. 57 B.C.E–668 C.E). Among those found in royal tombs in Baekje areas, a pair of wooden coffins from the Tomb of King Muryong in Gongju, one from the Twin Royal Tombs in Iksan, and one from Buyeo Neungsan-ri Donghachong are relatively well preserved. These wooden coffins are considered as important artifacts, showing the changes in funerary cultures by period and region. Fig. 1(a) shows the wooden coffin excavated from the Tomb of King Muryong, which was made with lacquered wooden panels, covered with silver plates around the corners, and ornamented with gilt nails.1) The ornamentation is more prominent on the sides than near the head or feet of the deceased. Fig. 1(b) shows that the wooden coffin of the Twin Royal Tombs in Iksan was decorated with giltbronze nails and gold leaf around the corners at the head and feet, and the lid and side panel were fixed with mortise and tenons decorated with flowery patterns.2) Unlike the wooden coffin from the Tomb of King Muryong, the ornamentation is concentrated near the head and feet, and it is supposed that there were differences in the manufacturing process. Fig. 1(c) shows the wooden coffin from Buyeo Neungsan-ri tomb No. 1. Its end pieces are ornamented with gilt-bronze panel, and the coffin boards are constructed or ornamented with nails with semicircle heads.3) The ornamentation method is different from the wooden coffin from the Twin Royal Tombs in Iksan, though they are similar in that their ornamentation is concentrated near the head and feet. Although most of the wooden coffin boards have been lost, the remaining gilt-bronze plates and nails showed the shape of the end pieces and the size of the coffin. It is noteworthy that different types of gilt-bronze plates and nails were used near the head and feet.

    Previous research on the wooden coffins from the Tomb of King Muryong and the Twin Royal Tombs in Iksan mostly focused on restoring the original shape from an archaeological perspective, and the analysis of the materials has been limited to some of the wood used in the coffin wood. Due to the limitations of cultural properties as an object, analysis of the materials used and manufacturing methods has been scarce. Against this backdrop, this study investigated and analyzed the materials and manufacturing methods used in the gilt-bronze objects attached to the wooden coffin from Neungsan-ri Tomb No. 1 (Donghachong), based on which we restored the wooden coffin to its original shape. As a result of our work, we reconstructed the coffin for a museum exhibition. This shows how scientific analysis and investigation can be used to produce museum exhibitions.

    2. Analysis Methods

    We conducted an analysis to identify how the gilt-bronze objects were manufactured. To this end, we examined the stored fragments and some leftover specimens from the preservation process. The specimens were fixed with epoxy resin at their cross section and ground sequentially, with sandpaper (No. 400, 800, 1000, 1200, 2400, 4000) and abrasive cloth (3 μm, 1 μm). The cross section of the ground specimen was observed using a metallurgical microscope (DM 2500M, Leica, Germany) and scanning electron microscope (SEM, VEGAIILMH, TESCAN, Czech) before its components were analyzed using an energy-dispersive X-ray spectroscopy (EDS) attachment (QUANTAX, Bruker axs, Germany). In addition, we used a stereoscopic microscope (Leica MZ-12, Germany) to magnify and observe the surface of the artifacts to investigate how the gilt-bronze objects were manufactured and combined.

    3. Results and Discussion

    3.1 Base metal and gilt layers in the gilt-bronze objects

    Fig. 2 shows that the base metal, identified from the cross section of the specimen, was found to contain nothing but Cu, confirming that the base metal under the gilt layer was pure copper with no other alloyed metals. Fig. 3 shows that the observation through the metallurgical microscope revealed that the base metal had single α phase crystals in a heterogeneous form containing annealing twins. We believe that the base metal of the gilt-bronze object was first shaped before being forged.4) Further, we suggest that the annealing twins of the base material formed during the heat treatment process for the sheet metal.

    Table 1 shows how Cu was detected from the base metal, while Au and Hg were detected from the gilt layer; it shows that the objects were gilded with mercury amalgam.5) Fig. 4 shows that Cu, Au, and Hg were detected in the gilt layer. Mercury amalgam gilding, mostly used for creating gilt-bronze objects in ancient times, is a process which involves coating the surface with gold-mercury amalgam, heating to temperatures of 350 ~ 400 °C, and evaporating the mercury.6) Fig. 5 shows the Au-Hg phase diagram, illustrating that in goldmercury amalgam, gold reacts with mercury to generate intermetallic compounds (e.g., Au2Hg) and solid solution (with an Hg content of 16.7 % or higher).7) As the grain size of gold becomes smaller, the more chelate complex with mercury is formed. In addition, since only Au was detected from the cross section of the specimen, it can be presumed that high-purity (almost pure) gold was used for gilding.8)

    Table 2 and Fig. 6 show the gilt layer for the giltbronze ornament near the head is 3.79 μm thick at its maximum while the one near the feet is a maximum of 8.42 μm thick; the latter is three to five times thicker than the former. As for the gilt-bronze nails, the gilt layer is 27.5 μm thick max near the feet, greatly different from those near the head, which are 2.83 μm thick max. It can be concluded that the gilt-bronze plates and nails were gilded thicker near the feet than near the head. Fig. 7 shows that the lid near the head, with a relatively thinner layer of gilding, is found to have three layers of plating; hence plating is likely to have been carried out at least three times. In addition, the gilt layer near the head has smaller variances in thickness than near the feet, which shows a rather heterogeneous gilded surface.

    3.2 Combining methods and restoration of the giltbronze objects

    Fig. 8 shows that in the wooden coffin excavated in Donghachong, the gilt-bronze objects were used to attach the gilt-bronze plates and nails to the wooden coffin to fix and prevent distortion of the wooden panels, as well as for an ornamental effect. Gilding is observed in the front and around the holes in the gilt-bronze plates, and the gilt-bronze nails have the trace of gilding inside the nail heads near both the head and the feet, which implies that the objects were gilded after ornamentation, including cutting holes.

    According to the measurement of the gilt-bronze ornament, the panel of the head side was 60.0 cm wide at the top, 4.5 cm wider than the bottom panel, which is 55.5 cm wide. Thus, the bottom of the wooden coffin was made at a 50° angle, and the top at a 130° angle, to fit the curvature of the lid. The lid was about 4.4 cm wider than near the head, giving perspective to the appearance. The total height, including the lid and the body, is 70 cm near the head, but it is not clear in the head direction, which only features the end pieces of the lid. The top side of the end pieces on the left and right is an arc with an oblique angle to fit the curvature of the lid in a fan shape, and the bottom side is squared. Unfortunately, the coffin wood has mostly been lost and it was difficult to confirm the length of the wooden coffin. But it is estimated to be 60 cm wide and 210 cm long, based on the platform on which the coffin was placed in the stone chamber of Donghachong tomb. The type of wood used was found to be Japanese umbrella pine, Sciadopitys verticillata, as in the Tomb of King Muryong and Twin Royal Tombs, and the wooden coffin was found to have been lacquered.9) Fig. 9 shows how, based on these findings, the original shape of the wooden coffin was estimated in order to produce a reconstruction. The reconstructed coffin was introduced in a special exhibition at the Buyeo National Museum, “Baekje Neungsan-ri Tomb No. 1 Donghachong.”10)

    4. Conclusion

    We identified the ancient manufacturing methods and metal craftwork techniques employed, based on an observation of the coffin’s structure, component analysis, and examination of its morphological characteristics. In addition, we measured the wooden coffins and investigated the traces of combination. Based on these findings, we restored the objects and reconstructed the wooden coffin for an exhibition. The findings of this study can be summarized as follows.

    The base metal of the gilt-bronze plates and nails is pure copper, with single α phase crystals observed in a heterogeneous form containing annealing twins, and Au and Hg were detected from the gilt layer. Hence, we suggest that the surface of the forging copperplate was gilded using a mercury amalgam technique, and also that the annealing twins of the base material formed during the heat treatment process for the sheet metal. The gilt layer is three to five times thicker for the gild-bronze plates and nails near the feet than those near the head. The lid ornamentation contained three layers of plating near the head, which means plating is likely to have been carried out at least three times. Based on this result, it can be presumed that the gilt-bronze ornaments were plated with the same materials and the same method, but varied in the number of times they were plated. The size of the wooden coffin is estimated to be 60 cm wide and 210 cm long based on the measurement of the giltbronze objects and the coffin platform of the stone chamber. We used these findings to restore the wooden coffin and the gilt-bronze ornaments, excavated from Neungsan-ri Tomb No.1, Donghachong, and built a reconstruction for a museum exhibition. This investigation provides a scientific research methodology that could be used to restore and preserve lost artifacts.

    Figure

    MRSK-30-9-453_F1.gif

    Photo showing the appearance of the wooden coffin; (a) King Muryeong’s Tomb, (b) Twin Tombs of Iksan, and (c) Donghachong Tomb (replica).

    MRSK-30-9-453_F2.gif

    Images of optical SEM and EDS showing (a) the analysis area of the object (b) the EDS spectrum of the object.

    MRSK-30-9-453_F3.gif

    Images from the metallurgical microscope (a) showing the analysis area of the object (b) from the rectangle in Fig. 3(a).

    MRSK-30-9-453_F4.gif

    Backside illumination and SEM mapping image of the gilt-bronze ornaments. (a),(b): nails, (c),(d): coffin lid plate.

    MRSK-30-9-453_F5.gif

    Assessed Au-Hg phase diagram.

    MRSK-30-9-453_F6.gif

    Metallurgical and SEM micrographs. (a), (b) gilt-bronze plate from the head, (c), (d) gilt-bronze plate from the foot.

    MRSK-30-9-453_F7.gif

    Metallurgical and SEM micrographs of gilt-bronze layers.

    MRSK-30-9-453_F8.gif

    Photo showing the appearance of the gilt-bronze ornaments. (a): Head side, (b): Foot side.

    MRSK-30-9-453_F9.gif

    Photo showing the replica of a wooden coffin from Donghachong Tomb.

    Table

    The chemical composition of gilt-bronze ornaments analyzed by EDS. (Unit: wt%)

    The gilt-bronze thickness measurement results. (Unit: μm)

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