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 matrix. The results were quantified by solution was verified, the solution was transferred to a volumetric flask to obtain 50 g of obtaining the average value of three analyses for various elements, namely copper (Cu), tin dissolved sample. The solution was then diluted with a 1000 ppm BDH Spectrosol solu- (Sn), lead (Pb), arsenic (As), silver (Ag), cobalt (Co), iron (Fe), nickel (Ni), antimony (Sb), tion, and 1 mL of aqua regia was added to match a sample matrix. The results were quan- and zinc (Zn). tified by obtaining the average value of three analyses for various elements, namely cop- 2.4. SEM and EDSper Analysis(Cu), tin (Sn), lead (Pb), arsenic (As), silver (Ag), cobalt (Co), iron (Fe), nickel (Ni), antimony (Sb), and zinc (Zn). A scanning electron microscope (SEM; TESCAN, MIRA LMH) and an energy disper- sive spectrometer (EDS; Perkin Elmer, Waltham, MA, USA, Optima 4300 DV) were used for cross-sectional observation. The samples were coated in platinum (Pt) to enhance their surface conductivity before being subjected to backscattered electron image (BSEI) analysis to obtain clearer, contrasting images. Appl. Sci. 2021, 11, 2441 3 of 13
2.5. Field Emission Electron Probe Microanalysis Field emission electron probe microanalyzer (EPMA) analysis (JEOL Ltd., Akishima, Japan, JXA-8530F) was used to map the images’ cross-sections. Once completed, line analysis was used to identify the elemental distribution differences at each level.
2.6. Micro-Raman Spectroscopic Analysis Micro-Raman spectroscopic analysis (Horiba Jobin Yvon, Edison, NJ, USA, LabRAM Aramis) was used to identify corrosion products on the bronze mirrors, and identification was performed using Raman reference values [5]. For Raman analysis conditions, the following settings were used: grid (600 gr/mm), resolution (3.0 cm−1), laser power (1 mW), and acquisition time (60 s). An argon-ion laser, set to 514 nm (wavelength), was used Appl. Sci. 2021, 11, 2441 during this step. 4 of 14
3. Results and Discussion 3.1. Bronze Mirror Excavated from Pit Tomb Ga-2 (No. 1) 3.1.1. Metal The mirror’s microstructure consisted of an δ phase, containing fine lines and nodular particles, and an α + δ eutectoid phase between the δ phase’s spaces. Here, the δ phase, found in the γ phase’s boundary lines, was created during early microstructure formation or in circular patterns around the Pb particles [6]. The microstructure’s presence suggested Appl. Sci. 2021, 11, 2441 that the mirror was cast using molten metal with an even chemical composition,4 of 14 and no
additional treatments were performed after casting (Figures2a and3a). Furthermore, the microstructure contains sporadic amounts of sulfides and Pb (Table1).
Figure 2. (a) Ga-2 (No. 1) optical micrograph showing secondary copper formed in cracks and (b) secondary copper formed in the corroded (α+δ) phase.
Secondary copper is a highly pure metallic Cu formed within a bronze artifact‘s mi- crostructure. These are mostly formed between layers of corrosion materials, particularly in fractures created by external forces, defects in the casting process, holes created by cor- roded Pb particles, or α+δ eutectoid sites lost through corrosion. Multiple processes, such as electrolysis, generation of residual Cu particles, and rapid changes in the burial envi- ronment, can explain secondary copper formation [7]. Here, secondary copper observed in the Ga-2 bronze artifact were small particles found in cracks (Figures 2b and 3b). These FigureareFigure common 2. ( 2.a) (Ga-2a) features Ga-2 (No. (No. 1) foundoptical 1) optical inmicr ancientograph micrograph bronze showing showing artifacts, secondary secondary which copper can copperformed be observed formedin cracks in whenand cracks (b the) and (b) secondary copper formed in the corroded (α+δ) phase. artifactssecondary corrode copper over formed extended in the periods corroded [8]. (α + δ) phase. Secondary copper is a highly pure metallic Cu formed within a bronze artifact‘s mi- crostructure. These are mostly formed between layers of corrosion materials, particularly in fractures created by external forces, defects in the casting process, holes created by cor- roded Pb particles, or α+δ eutectoid sites lost through corrosion. Multiple processes, such as electrolysis, generation of residual Cu particles, and rapid changes in the burial envi- ronment, can explain secondary copper formation [7]. Here, secondary copper observed in the Ga-2 bronze artifact were small particles found in cracks (Figures 2b and 3b). These are common features found in ancient bronze artifacts, which can be observed when the artifacts corrode over extended periods [8].
FigureFigure 3. 3.Ga-2Ga-2 (No. (No. 1) scanning 1) scanning electron electron microscope microscope-energy-energy dispersive dispersive spectrometer spectrometer (SEM-EDS) (SEM-EDS) image.image. (a) ( aOriginal) Original microstructure microstructure without without corrosion corrosion and ( andb) secondary (b) secondary copper copper grains grainsin corroded in corroded microstructuresmicrostructures and and cracks. cracks.
3.1.2. Patina Enlarging the corrosion layers of the bronze mirror in DF photography revealed thick, localized layers of red and green corrosion products (Figure 4a). Alternating layers of corrosion material indicated an irregular series of changes in the burial environment. According to the bronze artifact corrosion layer classification system proposed by Robbi- ola et al. [9], this mirror’s corrosion was classified under the type II corrosion category; Figure 3. Ga-2 (No. 1) scanning electron microscope-energy dispersive spectrometer (SEM-EDS) type II corrosion is identified when corrosion products, such as malachite and cuprite, are image. (a) Original microstructure without corrosion and (b) secondary copper grains in corroded microstructurespresent. Micro-Raman and cracks. spectroscopy was then used to identify these corrosion products. Here, red corrosion products (a) were identified as cuprite (Cu2O), dark green corrosion 3.1.2.products Patina (c) were malachite ((Cu2CO3(OH)2), and bright green corrosion product (b and d) were cassiterite (SnO2). Notably, cassiterite is the mineral form of SnO2, and the Raman Enlarging the corrosion layers of the bronze mirror in DF photography revealed thick, localized layers of red and green corrosion products (Figure 4a). Alternating layers of corrosion material indicated an irregular series of changes in the burial environment. According to the bronze artifact corrosion layer classification system proposed by Robbi- ola et al. [9], this mirror’s corrosion was classified under the type II corrosion category; type II corrosion is identified when corrosion products, such as malachite and cuprite, are present. Micro-Raman spectroscopy was then used to identify these corrosion products. Here, red corrosion products (a) were identified as cuprite (Cu2O), dark green corrosion products (c) were malachite ((Cu2CO3(OH)2), and bright green corrosion product (b and d) were cassiterite (SnO2). Notably, cassiterite is the mineral form of SnO2, and the Raman Appl. Sci. 2021, 11, 2441 4 of 13
Table 1. Ga-2 (No. 1) microstructure energy dispersive spectrometer (EDS) results.
Element (wt%) No. Position O S Cu Sn Pb Fe Cl 1 0.83 − 69.58 28.02 1.57 − − 2 − 21.97 74.71 − 3.32 − − 3 0.42 − 65.98 33.60 − − − 1 4 − − 73.15 26.85 − − − 5 − − 67.54 30.36 2.10 − − 6 − 1.41 86.89 2.84 5.78 0.65 2.44 7 11.35 − 33.07 51.34 3.44 − 0.81
Secondary copper is a highly pure metallic Cu formed within a bronze artifact‘s mi- crostructure. These are mostly formed between layers of corrosion materials, particularly in fractures created by external forces, defects in the casting process, holes created by corroded Pb particles, or α + δ eutectoid sites lost through corrosion. Multiple processes, such as electrolysis, generation of residual Cu particles, and rapid changes in the burial en- vironment, can explain secondary copper formation [7]. Here, secondary copper observed in the Ga-2 bronze artifact were small particles found in cracks (Figures2b and3b). These are common features found in ancient bronze artifacts, which can be observed when the artifacts corrode over extended periods [8].
3.1.2. Patina Enlarging the corrosion layers of the bronze mirror in DF photography revealed thick, localized layers of red and green corrosion products (Figure4a). Alternating layers of corro- sion material indicated an irregular series of changes in the burial environment. According to the bronze artifact corrosion layer classification system proposed by Robbiola et al. [9], this mirror’s corrosion was classified under the type II corrosion category; type II corro- sion is identified when corrosion products, such as malachite and cuprite, are present. Micro-Raman spectroscopy was then used to identify these corrosion products. Here, red corrosion products (a) were identified as cuprite (Cu2O), dark green corrosion products (c) were malachite ((Cu2CO3(OH)2), and bright green corrosion product (b and d) were cassi- terite (SnO2). Notably, cassiterite is the mineral form of SnO2, and the Raman shift observed in the bronze mirror had significant differences from the reference value (Figure4b) [5] . In this study, the major Raman shift of cassiterite was moderately observed at a range of 545 cm−1 to 568 cm−1. Meanwhile, the Raman shift of natural tin oxide cassiterite, the reference, generally shows a Raman shift ranging from 633 cm−1 to 775 cm−1 (Figure4c). Raman researchers reported that in nanocrystalline SnO2, a wide band that appears below −1 580 cm was observe (Figure4c) [ 10]. Based on existing research, nanocrystalline SnO2 Raman shifts were observed at 486 cm−1, 568 cm−1, and 706 cm−1 [11–13]. Therefore, the cassiterite found in the bronze mirror is nanocrystalline SnO2. Since cassiterite is generated during long-term soil burial, the possibility of uniform detection is low, and a Raman shift error occurred. The possibility of substitution due to various environmental changes dur- ing soil burial also cannot be excluded. More importantly, the presence of nanocrystalline SnO2 can be attributed to long-term bronze corrosion in a burial environment. EPMA analysis found that Sn content increased significantly toward the edges’ corro- sion layers, which contained SnO2 (Figure5). Excluding highly reducing environments with low pH, SnO2 could be found in a wide zone between the PO2 = 1 and PH2 = 1 bars in a stable state, as seen in the Cu-Sn-Cl-H2O Eh-pH diagram. Moreover, the Gibbs energies −1 −1 of SnO2 and Cu2O are −519 kJ/mol and −416 kJ/mol , respectively, at a temperature ◦ of 24.85 C. These findings, along with the insolubility of SnO2, explain the high levels of Sn in the artifact’s corrosion layers [7]. Appl. Sci. 2021, 11, 2441 5 of 14
shift observed in the bronze mirror had significant differences from the reference value (Figure 4b) [5]. In this study, the major Raman shift of cassiterite was moderately observed at a range of 545 cm−1 to 568 cm−1. Meanwhile, the Raman shift of natural tin oxide cassit- erite, the reference, generally shows a Raman shift ranging from 633 cm−1 to 775 cm−1 (Fig- ure 4c). Raman researchers reported that in nanocrystalline SnO2, a wide band that ap- pears below 580 cm−1 was observe (Figure 4c) [10]. Based on existing research, nanocrys- talline SnO2 Raman shifts were observed at 486 cm−1, 568 cm−1, and 706 cm−1 [11–13]. There- fore, the cassiterite found in the bronze mirror is nanocrystalline SnO2. Since cassiterite is generated during long-term soil burial, the possibility of uniform detection is low, and a Raman shift error occurred. The possibility of substitution due to various environmental changes during soil burial also cannot be excluded. More importantly, the presence of nanocrystalline SnO2 can be attributed to long-term bronze corrosion in a burial environ- Appl. Sci. 2021, 11, 2441 ment. 5 of 13
Appl. Sci. 2021, 11, 2441 6 of 14
with low pH, SnO2 could be found in a wide zone between the PO2 = 1 and PH2 = 1 bars in
a stable state, as seen in the Cu-Sn-Cl-H2O Eh-pH diagram. Moreover, the Gibbs energies −1 −1 FigureFigureof SnO 4. 4.(a2 )( andaGa-2) Ga-2 Cu(No.2 (No.O 1) arecorrosion 1) -519 corrosion kJ/mollayer darkfield layer and darkfield (DF)-416 images, kJ/mol (DF) (b images,), Ramanrespectively, (spectrum,b) Raman at and a spectrum, temperature (c) Ra- and of (c) Raman man24.85°C. spectrum These of nanocrystalline findings, along SnO2 .with the insolubility of SnO2, explain the high levels of Sn spectrum of nanocrystalline SnO2. in the artifact’s corrosion layers [7]. EPMA analysis found that Sn content increased significantly toward the edges’ cor- rosion layers, which contained SnO2 (Figure 5). Excluding highly reducing environments
Figure 5. Ga-2 (No. 1) electron probe microanalyzer (EPMA) mapping image. Figure 5. Ga-2 (No. 1) electron probe microanalyzer (EPMA) mapping image. 3.2. Bronze Mirror Excavated from Pit Tomb Ga-31 (No. 2) Metal Given the selective corrosion in the α+δ eutectoid phases along the sample’s outline and the lack of malachite and cuprite corrosion layers, this mirror’s corrosion was classi- fied under type I corrosion [9]. This type of corrosion grows toward the original metallic layer without outward changes in volume and shows high corrosion resistance. Here, this type resulted from generalized corrosion related to the formation of passive layers. Be- cause the α+δ eutectoid phase is a combination of two distinct phases, it has a higher elec- tric potential difference compared to the single solid phases at the α or δ phases. Namely, it is a priority target for corrosion because of its structural flaws. Corrosion on the α+δ eutectoid phase gradually changed to a dark-blue corrosion product, which is unclear and occluded under microscopic examination and etched during the macroscopic examina- tion. EDS and EPMA mapping analyses revealed that severe corrosion lowered its Cu content and increased oxygen (O) and Sn levels, indicating partial tin oxide mineralization (Figure 6). This oxide formed cryptocrystalline cassiterite structures (SnO2) and created unclear boundaries [14]. Appl. Sci. 2021, 11, 2441 6 of 13
3.2. Bronze Mirror Excavated from Pit Tomb Ga-31 (No. 2) Metal Given the selective corrosion in the α + δ eutectoid phases along the sample’s outline and the lack of malachite and cuprite corrosion layers, this mirror’s corrosion was classified under type I corrosion [9]. This type of corrosion grows toward the original metallic layer without outward changes in volume and shows high corrosion resistance. Here, this type resulted from generalized corrosion related to the formation of passive layers. Because the α + δ eutectoid phase is a combination of two distinct phases, it has a higher electric potential difference compared to the single solid phases at the α or δ phases. Namely, it is a priority target for corrosion because of its structural flaws. Corrosion on the α + δ eutectoid phase gradually changed to a dark-blue corrosion product, which is unclear and occluded under microscopic examination and etched during the macroscopic examination. EDS and EPMA mapping analyses revealed that severe corrosion lowered its Cu content and increased Appl. Sci. 2021, 11, 2441 oxygen (O) and Sn levels, indicating partial tin oxide mineralization (Figure6) . This oxide7 of 14
formed cryptocrystalline cassiterite structures (SnO2) and created unclear boundaries [14].
Figure 6. Ga-31 (No. 2) electron probe microanalyzer (EPMA) mapping image. Figure 6. Ga-31 (No. 2) electron probe microanalyzer (EPMA) mapping image.
TheThe microstructure microstructure consisted consisted of ofα α++δδeutectoid eutectoidand andδ δphases phases with with sporadic sporadic sulfide sulfide and and leadlead particles particles (Figure (Figure7a). 7a). Its Its secondary secondary copper copper likely likely formed formed as clumps as clumps in the in corroded the corroded Pb particlesPb particles or cracks, or cracks, and and twin twin crystals crystals were were observed observed in some in some cases cases (Figure (Figure8a). Secondary8a). Second- copperary copper with with twin twin crystals crystals is a productis a product of the of alloy’sthe alloy’s constituent constituent components components undergoing undergo- extendeding extended mineralization, mineralization, production production technique, techni andque, burialand burial environment environment [14]. [14]. In addition, In addi- blacktion, corrosionblack corrosion products products observed observed on the on mirror’s the mirror’s surface surface during during excavation excavation appeared ap- becausepeared mineralizedbecause mineralized cassiterite cassiterite formed on formed the surface. on the surface.
Figure 7. (a) Ga-31 (No. 2) scanning electron microscope-energy dispersive spectrometer (SEM- EDS) images that show the original microstructure without corrosion and (b) secondary copper present in cracks formed from corrosion compounds. Appl. Sci. 2021, 11, 2441 7 of 14
Figure 6. Ga-31 (No. 2) electron probe microanalyzer (EPMA) mapping image.
The microstructure consisted of α+δ eutectoid and δ phases with sporadic sulfide and lead particles (Figure 7a). Its secondary copper likely formed as clumps in the corroded Pb particles or cracks, and twin crystals were observed in some cases (Figure 8a). Second- ary copper with twin crystals is a product of the alloy’s constituent components undergo- ing extended mineralization, production technique, and burial environment [14]. In addi- Appl. Sci. 2021, 11, 2441 tion, black corrosion products observed on the mirror’s surface during excavation7 ofap- 13 peared because mineralized cassiterite formed on the surface.
Figure 7. (a) Ga-31 (No. 2) scanning electron microscope-energy dispersive spectrometer (SEM-EDS) Appl. Sci. 2021, 11, 2441 8 of 14
Figureimages 7. that (a) showGa-31 the (No. original 2) scanning microstructure electron microsco without corrosionpe-energy and dispersive (b) secondary spectrometer copper (SEM- present in EDS)cracks images formed that from show corrosion the original compounds. microstructure without corrosion and (b) secondary copper present in cracks formed from corrosion compounds.
FigureFigure 8. Ga-31 8. Ga-31 (No. (No. 2) optical2) optical micrograph micrograph images images showing showing (a ()a twinned) twinned secondary secondary coppercopper andand ((b)) secondarysecondary copper copper in in corrosion.corrosion.
InIn the the secondary secondary copper, copper, Cu Cu and and O O differed differed according according toto thethe degree of of corrosion, corrosion, and and greatergreater corrosion corrosion levels levels increased increased O andO and reduced reduced Cu Cu content. content. Because Because the the Cu Cu and and O contentO con- standardstent standards between between secondary secondary copper copper and and copper copper oxides oxides remain remain unclear,unclear, determining secondarysecondary copper copper and and copper copper oxide oxide can can be be difficult. difficult. Here, Here, a a sample sample was was observed observed under under a metallurgicala metallurgical microscope. microscope. Pure Pure secondary secondary copper copper has has an an orange orange huehue similar to to metallic metallic copper,copper, while while copper copper oxides oxides have have grey, grey, cloudy cloudy hues. hues. DespiteDespite losinglosing its hue, copper copper oxide oxide undergoingundergoing early early stages stages of of corrosion corrosion is nearlyis nearly as as pure pure as as secondary secondary copper. copper. Thus, Thus, secondary second- copperary copper and copper and copper oxide oxide were were determined determined using using EDS EDS analysis analysis and and visuallyvisually confirmed confirmed using a metallurgical microscope. EDS positions 6 to 8, based on Cu and O content (6: metallic copper, 7: copper oxide undergoing corrosion, 8: copper oxide mineralized through corrosion), showed that metallic copper undergoing corrosion created cracks within the microstructure and exited to form corrosion compounds on the bronze artifact. (Figure 7b and Table 2). Appl. Sci. 2021, 11, 2441 8 of 13
using a metallurgical microscope. EDS positions 6 to 8, based on Cu and O content (6: metallic copper, 7: copper oxide undergoing corrosion, 8: copper oxide mineralized through Appl. Sci. 2021, 11, 2441 corrosion), showed that metallic copper undergoing corrosion created cracks within9 of 14 the
microstructure and exited to form corrosion compounds on the bronze artifact. (Figure7b and Table2). Table 2. Ga-31 (No. 2) microstructure energy dispersive spectrometer (EDS) results. Table 2. Ga-31 (No. 2) microstructure energy dispersive spectrometer (EDS) results. Element (wt%) No. Position O S Cu ElementSn (wt%)Pb Fe Cl No. Position 1 0.37 O− S68.24 Cu27.90 Sn3.48 Pb− Fe− Cl 2 10.49 0.37 16.12 − 78.26 68.242.58 27.90− 3.48 2.55 −− − 3 2− 0.49− 16.1268.29 78.2631.71 2.58 − − − 2.55 − − 4 3 − −− − 72.17 68.2927.83 31.71 − −− −− − − − − − − 2 5 4 5.67 − 6.98 72.17− 27.8387.35 − − 2 5 5.67 − 6.98 − 87.35 − − 6 6 − −− − 100.00100.00 − −− −− −− − 7 74.33 4.33 − − 90.07 90.074.79 4.790.34 0.34 − − 0.47 0.47 8 817.86 17.86 − − 79.94 79.94 − − 2.20 2.20 − −− − 9 93.87 3.87 − − 55.67 55.6735.28 35.285.18 5.18 − −− −
3.3.3.3. Bronze Bronze Mirror Mirror Excavated Excavated from from Pit PitTomb Tomb Ga-35 Ga-35 (No. (No. 3) 3) 3.3.1.3.3.1. Metal Metal TheThe sample sample showed showed selective selective corrosion corrosion on onthe the α+δα eutectoid+ δ eutectoid phase phase along along the bound- the bound- ariesaries and and cracks cracks as as thick thick layers layers of ofred red and and green green corrosion corrosion materials. materials. According According to the to the bronzebronze artifact artifact corrosion corrosion laye layerr classification classification system, system, this thismirror mirror was wasclassified classified under under the the typetype II II corrosion corrosion category category [9]. [9 The]. The microstructure microstructure consisted consisted of α of+δα eutectoid+ δ eutectoid and δ and phases,δ phases, withwith scattered scattered sulfide sulfide and and lead lead particles particles (Figure (Figure 9a 9anda and Table Table 3).3 ).
FigureFigure 9. 9.Ga-35Ga-35 (No. (No. 3) scanning 3) scanning electron electron microscope-energy microscope-energy dispersive dispersive spectrometer spectrometer (SEM-EDS) (SEM-EDS) imagesimages that that show show an an un uncorrodedcorroded microstructure microstructure (a) (anda) and a line a line composed composed of secondary of secondary copper copper in in the thecorroded corroded microstructure microstructure (b ().b).
TableTable 3. 3. Ga-35Ga-35 (No. (No. 3) 3)microstructure microstructure energy energy dispersive dispersive spectromet spectrometerer (EDS) (EDS) results. results. Element (wt%) No. Position Element (wt%) No. Position O S Cu Sn Pb Fe 1 1.34 O0.15 S64.87 Cu29.10 Sn4.55 Pb− Fe 2 11.24 1.34− 0.1568.37 64.8727.73 29.102.66 4.55 − − 2 1.24 − 68.37 27.73 2.66 − 3 − − 66.51 33.49 − − 3 − − 66.51 33.49 − − 4 40.73 0.7323.91 23.91 60.13 60.130.60 0.60 − − 14.63 14.63 3 3 5 57.13 7.13 − − 8.50 8.501.61 1.6182.76 82.76 − − 6 613.22 13.22 6.81 6.812.06 2.06 − − 77.91 77.91 − − − − − − 7 75.12 5.12 − 94.8894.88 − − − 8 17.51 − 8.59 66.95 6.96 − 8 17.51 − 8.59 66.95 6.96 − Because the bronze mirrors showed similar microstructures and distributions, they were most likely produced with the same techniques in a similar period. According to the Appl. Sci. 2021, 11, 2441 9 of 13
Appl. Sci. 2021, 11, 2441 10 of 14
Because the bronze mirrors showed similar microstructures and distributions, they were most likely produced with the same techniques in a similar period. According to the ICP-AES analysis, a ICP-AESrelatively analysis, small amount a relatively of Pb small (0.85% amount to 1.10%) of Pb was (0.85% detected, to 1.10%) and was all detected, and all of them are ternary alloys of Cu-Sn-Pb (Table4). of them are ternary alloys of Cu-Sn-Pb (Table 4).
Table 4. Chemical composition of bronze mirrors obtained from the Sinpung site using inductively coupled plasma atomic Table 4. Chemical composition of bronze mirrors obtained from the Sinpung site using inductively coupled plasma atomic emission spectroscopyemission (ICP-AES) spectroscopy analysis. (ICP-AES) analysis.
Composition (%) Composition (%) No. Name No. Name Cu Sn Pb CuAs SnAg PbCo AsFe AgNi Co Sb FeZn Ni Sb Zn 1 Ga-2 64.31 28.2 Ga-2 0.85 64.30.1684 28.2 0.0182 0.85 0.1204 0.1684 0.0194 0.0182 0.1555 0.1204 0.0670 0.0194 0.000273 0.1555 0.0670 0.000273 2 Ga-31 63.92 28.3 Ga-31 1.10 63.90.1858 28.3 0.0292 1.10 0.1520 0.1858 0.0745 0.0292 0.1605 0.1520 0.0867 0.0745 0.000450 0.1605 0.0867 0.000450 3 Ga-35 60.83 25.6 Ga-35 1.21 60.80.3567 25.6 0.0156 1.21 0.0410 0.3567 0.0816 0.0156 0.0791 0.0410 0.1827 0.0816 0.000941 0.0791 0.1827 0.000941
In the Ga-35 (No. 3)In bronze the Ga-35 mirror, (No. Pb 3) and bronze S appear mirror, to Pbbe andpresent S appear as lead to sulfide, be present as lead sulfide, caused by the ore beingcaused added by theduring ore beingthe manufa addedcturing during process. the manufacturing Regarding the process. lead ore, Regarding the lead ore, previous studies onprevious bronze production studies on by bronze-products production excavated by-products from the excavatedWanju Sinpung from the Wanju Sinpung ruins suggested thatruins galena suggested (PbS) obtained that galena from (PbS) southern obtained Korean from mines southern may have Korean been mines may have been used for smelting [15].used Sulfi fordes smelting were present [15]. Sulfides in the werebronze present mirror’s in themicrostructures, bronze mirror’s but microstructures, but EDS detected only SEDS or a detected combination only Sof or S a and combination Fe. Secondary of S and copper Fe. Secondary was found copper in clus- was found in clustered tered particles or lines,particles and twins or lines, were and observed twins were in some observed secondary in some copper secondary (Figure copper 10). (Figure 10).
Figure 10. Ga-35 (No. 3)Figure optical 10. micrographGa-35 (No. with 3) secondary optical micrograph copper particles with secondary (a) and twinned copper sec- particles (a) and twinned ondary copper (b). secondary copper (b).
3.3.2. Patina 3.3.2. Patina Enlarging the corrosionEnlarging layers the of corrosionthe bronze layers mirror of in the DF bronze photography mirror in revealed DF photography revealed thick, localized layersthick, of red localized and green layers corrosi of redon andproducts. green corrosionAccording products. to the classification According to the classification system proposed bysystem Robbiola proposed et al. [10], by this Robbiola falls under et al. [type10], thisII corrosion. falls under Through type II micro- corrosion. Through micro- Raman spectroscopy,Raman the dark spectroscopy, green corrosion the dark product green (a) corrosion was determined product (a)as wasmalachite determined as malachite ((Cu2CO3(OH)2), the red corrosion product (b) was cuprite (Cu2O), and the bright green corrosion product (c) was cassiterite (SnO2) (Figure 11a). Appl. Sci. 2021, 11, 2441 10 of 13 Appl. Sci. 2021, 11, 2441 11 of 14 Appl. Sci. 2021, 11, 2441 11 of 14
((Cu2CO3(OH)2), the red corrosion product (b) was cuprite (Cu2O), and the bright green corrosion product (c) was cassiterite (SnO2) (Figure 11a).
FigureFigure 11. 11.( a()a) Ga-35 Ga-35 (No. (No. 3) 3) corrosion corrosion layer layer darkfield darkfield (DF) (DF) images images and and (b ()b Raman) Raman spectrum. spectrum. Figure 11. (a) Ga-35 (No. 3) corrosion layer darkfield (DF) images and (b) Raman spectrum. Based on the EPMA mapping analysis results (Figure 12), corrosion layers with large BasedBased on on the the EPMA EPMA mapping mapping analysis analysis results results (Figure (Figure 12), 12), corrosio corrosion layersn layers with with large large amountsamounts of of SnSn formedformed inin the corroded microstructure microstructure because because O O and and Sn Sn content content was was re- amountsremarkably of Sn high formed in the in sections the corroded under microstructure the corrosion layer. because From O theand EPMASn content line analysis,was re- markablymarkably high high in inthe the sections sections under under the the corrosion corrosion layer. layer. From From the the EPMA EPMA line line analysis, analysis, Cu Cu Cuand and Sn Snexhibit exhibit inversely inversely proportional proportional correlations correlations relative relative to their to their overall overall positions positions (Fig- and Sn exhibit inversely proportional correlations relative to2− their overall2− positions (Fig- (Figure 13). In an external environment, anions such as CO3 and O move to the bronze ure 13). In an external environment, anions such as CO32− and O2− move to the bronze urealloy’s 13). interior,In an external while cationsenvironment, of metals anions such such as Cu as and CO Sn32− moveand O outward2− move andto the bond bronze with alloy’s interior, while cations of metals such as Cu and Sn move outward and bond with alloy’seach other interior, to form while and cations maintain of metals a corrosion such as layer Cu [and16]. Sn move outward and bond with eacheach other other to toform form and and maintain maintain a corrosiona corrosion layer layer [16]. [16].
Figure 12. Ga-35 (No. 3) electron probe microanalyzer (EPMA) mapping image. FigureFigure 12. 12. Ga-35Ga-35 (No. (No. 3) 3) electron electron probe probe micr microanalyzeroanalyzer (EPMA) (EPMA) mapping mapping image. image. Appl. Sci. 2021, 11, 2441 12 of 14
Appl. Sci. 2021, 11, 2441 11 of 13
FigureFigure 13. ( 13.a) Ga-35(a) Ga-35 (No. (No. 3) electron 3) electron probe probe microanalyzer microanalyzer (EPMA) (EPMA) image image and and (b) (lineb) line analysis. analysis.
4. Conclusions4. Conclusions ThisThis study study analyzed analyzed the themetallurgical metallurgical and and corrosion corrosion characteristics characteristics of Early of Early Iron Iron Age Age bronze mirrors excavated in the Sinpung site in Wanju. According to the ICP-AES results, bronze mirrors excavated in the Sinpung site in Wanju. According to the ICP-AES results, the bronze mirrors were cast with a triple bronze Cu-Sn-Pb alloy (60% to 65% Cu, 25% to the bronze mirrors were cast with a triple bronze Cu-Sn-Pb alloy (60% to 65% Cu, 25% to 28% Sn, and 0.85% to 1.21% Pb). Their metallic microstructures consisted of an δ phase and 28% Sn, and 0.85% to 1.21% Pb). Their metallic microstructures consisted of an δ phase an α + δ eutectoid phase, and there was no evidence of further treatment after casting. Lead and an α+δ eutectoid phase, and there was no evidence of further treatment after casting. sulfide was observed in the microstructure of the Ga-35 (No. 3) bronze mirror, suggesting Lead sulfide was observed in the microstructure of the Ga-35 (No. 3) bronze mirror, sug- that its lead content originated from lead ores used to produce the artifacts. Sulfides containing Fe from copper sulfide ore or Cu-Fe-S residual matte were also added to the bronze mirrors while being manufactured. Appl. Sci. 2021, 11, 2441 12 of 13
For corrosion characteristics, selective corrosion in the α + δ eutectoid phase was observed at the sample’s surface, where selective corrosion in the α phase was correlated with the oxidation environment. In contrast, selective corrosion in the α + δ eutectoid phase was correlated with a reducing burial environment [8]. According to Robbiola et al.’s classification system [9], Ga-2 and Ga-35 contained corrosion materials classified as type I corrosion. Meanwhile, nanocrystalline SnO2 observed in bronze mirrors buried for long periods in soil was observed in type I and type II corrosion. High-purity secondary copper in the artifacts was mainly located in cracks, holes left by Pb corrosion, and the corroded α + δ eutectoid phase. The secondary copper was classified according to Wang and Merkel’s [8] proposal as it followed similar patterns. They distinguished the formation mechanism of secondary copper into two types: redeposition and destannification. In metallic copper formed through the former, Cu ions could not move freely and form metallic structures. In the samples, metallic copper was found in cracks, holes, and between cuprite formations formed through redeposition. In comparison, metallic copper formations found as pseudomorphs on the α + δ eutectoid phase formed through destannification because copper moved inward and replaced the existing metallic structures following Sn’s selective corrosion. Thus, the secondary copper observed in holes left by corroded Pb particles and cracks on the excavated mirrors formed through redeposition. In contrast, the secondary copper formations observed on the α + δ eutectoid phase were created through destannification. Various analytical techniques were used on bronze mirrors excavated from the Sin- pung site to identify their production techniques and corrosion characteristics. Compared to other artifacts excavated from the site, weapons (swords) and tools (plows) contained Pb oxides, such as cerussite (PbCO3), anglesite (PbSO4), and litharge (PbO), as corrosion products, which were absent in bronze mirrors [17]. This contrast shows that the mirrors exhibited different corrosion characteristics caused by differences in alloy compositions or production techniques. The presence of malachite, which does not form in an under- water burial environment, indicated that corrosion characteristics could only be found in bronze mirrors excavated from the ground [18]. The corrosive properties of pure bronze artifacts also showed a heterogeneity that cannot be simulated in an artificial environment. Therefore, this study’s results are of great importance for preserving artifacts and eval- uating an artifact’s authenticity. Moreover, future studies must develop a multifaceted analysis of various bronze artifacts and develop a comprehensive understanding of burial environments to establish long-term corrosion mechanisms in ancient bronze artifacts.
Author Contributions: Conceptualization, N.C.C.; methodology, I.K.H.; software, I.K.H.; valida- tion, N.C.C.; formal analysis, M.K.J.; investigation, I.K.H.; resources, M.K.J.; data curation, M.K.J.; writing—original draft preparation, M.K.J.; writing—review and editing, I.K.H.; visualization, M.K.J.; supervision, N.C.C.; project administration, M.K.J.; funding acquisition, N.C.C. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the research grant of the Kongju National University in 2018. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are openly available in Mendeley Data at DOI: 10.17632/8h2k4j9xw6.1 Conflicts of Interest: The authors declare no conflict of interest.
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