A Study on the Microstructure and Corrosion Characteristics of Early Iron Age Bronze Mirrors Excavated from the Korean Peninsula
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applied sciences Article A Study on the Microstructure and Corrosion Characteristics of Early Iron Age Bronze Mirrors Excavated from the Korean Peninsula Nam Chul Cho 1,*, Min Kyeong Jang 2 and Il Kwon Huh 3 1 Department of Cultural Heritage Conservation Sciences, Kongju National University, Gongju-si 32588, Korea 2 Collections Management Division, Seoul Baekje Museum, Seoul 05540, Korea; [email protected] 3 Conservation Science Division, National Museum of Korea, Seoul 04383, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-42-850-8541 Abstract: Bronze mirrors, considered important grave goods, were widely used before glass mirrors in ancient times. Most excavated bronze artifacts are covered with corrosive materials and lose their original colors. More importantly, identifying corrosion characteristics and the manufacturing techniques used for these artifacts are essential for proper artifact preservation. In this study, Early Iron Age bronze mirrors excavated from the Korean Peninsula were examined to determine their microstructures, corrosion characteristics, and production techniques using various analytical methods, such as Micro-Raman spectroscopy and field emission electron probe microanalysis. As a result, sulfides containing iron suggested chalcopyrite use during production or that the sulfides originated from copper, iron, and sulfur residual matte. The analysis also detected corrosion layers with high tin oxide (SnO2) levels and selective corrosion in the α + δ eutectoid phase on the artifact’s surface. In the corrosive layer, cuprite, malachite, and cassiterite corrosion products were detected, Citation: Cho, N.C.; Jang, M.K.; and nanocrystalline SnO2 was identified as a characteristic of long-term soil erosion. Identifying Huh, I.K. A Study on the these artifacts’ corrosion characteristics and manufacturing techniques is essential and can greatly Microstructure and Corrosion contribute to proper artifact preservation. Characteristics of Early Iron Age Bronze Mirrors Excavated from the Keywords: Early Iron Age; bronze mirror; artifact preservation; production techniques; corrosion Korean Peninsula. Appl. Sci. 2021, 11, 2441. https://doi.org/10.3390/ app11052441 Academic Editor: Emanuela Bosco 1. Introduction Most currently excavated bronze artifacts are corroded, showing a loss of their original Received: 9 February 2021 surface colors and being covered with corrosion products. Prolonged burial of bronze Accepted: 5 March 2021 artifacts leads to erosion, resulting in morphological changes to stable states and forming Published: 9 March 2021 various corrosion products. Some representative bronze corrosion products found in ancient bronze artifacts include cuprite (Cu2O), tenorite (CuO), malachite (Cu2CO3(OH)2), Publisher’s Note: MDPI stays neutral and paratacamite (Cu2Cl(OH)3). Here, the corrosion products’ type and shape are affected with regard to jurisdictional claims in by various factors, such as the artifact’s composition, manufacturing technique, size, published maps and institutional affil- and burial environment [1]. Furthermore, because of differing conditions, the burial iations. environment can be inferred based on the corrosion products. However, unlike artificial corrosion tests, the corrosion characteristics of naturally corroded bronze artifacts do not appear uniformly. Therefore, data on corrosion characteristics obtained from relics with known excavation sites can play a crucial role in artifact authenticity, preservation, and Copyright: © 2021 by the authors. long-term corrosion research. Licensee MDPI, Basel, Switzerland. The Wanju Sinpung ruins and tomb complex (dated approximately 300 to 200 BCE), This article is an open access article excavated from 2010 to 2011, is a site representative of Korea’s early Iron Age that shows distributed under the terms and the transition and social development of tombs. As a result, weapons (bronze swords and conditions of the Creative Commons arrowheads), tools (chisels and burin), and various bronze artifacts (bronze mirrors) were Attribution (CC BY) license (https:// discovered [2]. Specifically, bronze mirrors describe the temporal and spatial transition creativecommons.org/licenses/by/ of Korean bronzeware, which researchers use to explain the development of bronzeware 4.0/). Appl. Sci. 2021, 11, 2441. https://doi.org/10.3390/app11052441 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 2441 2 of 14 arrowheads), tools (chisels and burin), and various bronze artifacts (bronze mirrors) were Appl. Sci. 2021, 11, 2441 discovered [2]. Specifically, bronze mirrors describe the temporal and2 spatial of 13 transition of Korean bronzeware, which researchers use to explain the development of bronzeware manufacturing techniques [3]. In this study, various analytical methods were applied to manufacturingthe techniques bronze [mirrors3]. In this excavated study, various from the analytical Wanju methods Sinpung were ruins applied to investigate to the their micro- bronze mirrorsstructures, excavated fromcorrosion the Wanju characteristics, Sinpung ruins and to manufacturing investigate their processes. microstructures, corrosion characteristics, and manufacturing processes. 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials The first bronzeThe mirror first (Ga-2)bronze was mirror excavated (Ga-2) was on the excavated bedrock on at the bedrock site’s center, at the with site’s center, with the mirror sidethe facing mirror upward. side facing From upward. the brim From to thethe upperbrim to handle, the upper only handle, a part only of the a part of the arti- artifact remained,fact and remained, its surface and had its asurface relatively had regular a relatively corrosion regular layer. corrosion A second layer. mirror A second mirror (Ga-31) was found(Ga-31) in scattered was found pieces, in scattered and its surface pieces, had and a regularits surface layer had of a black regular corrosion. layer of black corro- Meanwhile, a thirdsion. Meanwhile, mirror (Ga-35) a third was mirror found (Ga-35) erect in was the pit’sfound left-hand erect in side,the pit’s with left-hand the side, with mirror side facingthe mirror toward side the facing tomb’s toward center. the However, tomb’s cent twoer. knobsHowever, were two partially knobs were lost. partially lost. The left-hand knobThe left-hand had traces knob of ahad mold traces mounted of a mold for boring mounted holes for during boring the holes molding during the molding process; the presenceprocess; of the fine presence patterns of indicated fine patterns that itindica wasted used that for it ceremonial was used for purposes ceremonial purposes (Figure1)[4]. (Figure 1) [4]. Figure 1. Bronze mirrors from the Sinpung Site in Wanju labeled (a) Ga-2, (b) Ga-31, and (c) Ga-35. Figure 1. Bronze mirrors from the Sinpung Site in Wanju labeled (a) Ga-2, (b) Ga-31, and (c) Ga-35. 2.2. Metallography Samples were2.2. Metallography obtained from areas already damaged during excavation, areas that lacked patterns, or areasSamples that were could obtained not be restored. from areas The already samples damaged were mounted during withexcavation, an areas that epoxy resin forlacked metallographic patterns, or study areas and that polished could not using be restored. silicon carbide The samples grinding were pads mounted with an (220, 500, 1200,epoxy 2000, andresin 4000 for grit)metallographic with water- study and diamond-water–based and polished using silicon suspensions carbide (3 grinding pads and 1 mm, respectively)(220, 500, 1200, on a microcloth2000, and 4000 pad. grit) The sampleswith water- were and etched diamond-water–based with a 3% ferric suspensions chloride (FeCl3(3+ and HCl 1 + mm, ethyl respectively) alcohol) solution on a formicrocloth 2 to 3 s during pad. The the samples final step. were Meanwhile, etched with a 3% ferric the microstructuralchloride observation (FeCl3 + HCl was + performedethyl alcohol) using solution a metallurgical for 2 to 3 s microscopeduring the final (Leica, step. Meanwhile, DM4 M). A brightfieldthe microstructural (BF) setting observation was used to was observe performed the microstructure using a metallurgical and secondary microscope (Leica, copper, while surfaceDM4 M). corrosion A brightfield compounds (BF) setting were observed was used using to observe the darkfield the microstructure (DF) setting. and secondary copper, while surface corrosion compounds were observed using the darkfield (DF) set- 2.3. Inductively Coupled Plasma Atomic Emission Spectroscopy Analysis ting. Inductively coupled plasma atomic emission spectroscopy (ICP-AES; Perkin Elmer, Optima 4300 DV)2.3. was Inductively used for Coupled composition Plasma analysis. Atomic TheEmission samples Spectroscopy were initially Analysis placed in a 25 mL conical flask and dissolved with 2 mL of aqua regia while heated on a heating plate. Inductively coupled plasma atomic emission spectroscopy (ICP-AES; Perkin Elmer, After the samples were gradually dissolved at room temperature and complete dissolution Optima 4300 DV) was used for composition analysis. The samples were initially placed in was verified, the solution was transferred to a volumetric flask to obtain 50 g of dissolved a 25 mL conical flask and dissolved with 2 mL of aqua regia while heated on a heating sample. The solution was then diluted with a 1000 ppm BDH Spectrosol solution, and plate. After the samples were gradually dissolved at room temperature and complete dis- 1 mL of aqua regia was added to match a sample