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Corrosion behavior and morphological features of archeological bronze coins from ancient China
Microchemical Journal 99 (2011) 203–212
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Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Corrosion behavior and morphological features of archeological bronze coins from ancient China Ling He a, Junyan Liang a, Xiang Zhao a,⁎, Baolian Jiang b a b
Department of Chemistry, School of Science, Xi'an Jiaotong University, Xi'an 710049, China Xi'an Center for the Conservation and Restoration of Cultural Heritage, Xi'an, 710061, China
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 7 May 2011 Accepted 8 May 2011 Available online 14 May 2011 Keywords: Bronze coin Corrosion Patina Morphology SEM X-RD
a b s t r a c t The archeological round bronze coins, nominated as Wu Zhou and regarded as the first issued effective money in the Han Dynasty of China, have been systematically investigated to disclose their chemical composition, nature of the patina and corrosion features on the coin surface by optical microscopy (OM), X-ray diffraction (X-RD), and scanning electron microscopy (SEM) equipped with backscattered electron (BSE) detector and energy dispersive spectrometry (EDS) techniques. It is revealed morphologically that there are some rough surface cracks, pits, and multicolor patina on the surface of the coins. We prove that the coins are made from bronze material of Cu–Sn–Pb–Sb alloy with contents of 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6–2.9 wt.% Sb, and covered by two corrosion layers, 25–35 μm for the upper-layer and 20–25 μm for the sub-layer. High chloride content has been detected at the interface between the sub-layer and body of the coins. The lead-rich and tin-rich areas in the coin samples indicate the poor metal compatibility during minting in some locations of the coins. The main compositions of patina are ascertained to be Cu2(OH)3Cl, Cu3 (CO3)2(OH)2, Cu2(OH)2CO3, and Pb3O4, and the proposed corrosion mechanism is discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The study of chemical compositions and corrosion features on bronze art objects is greatly important to explore the corrosion behavior and to achieve the aim in the preservation to the of bronze art objects [1]. In the most cases, corrosion of archeological bronze is extensively studied through the analyses of corrosion products with two aims: (1) to clarify the morphological corrosion phenomena occurring on bronze surface when it is exposed to different corrosive media and (2) to prevent corrosion processes for preservation of the cultural heritage [2]. Our investigation on the archeological Wu Zhu coins used in the West Han Dynasty of ancient China is expected to be significant on both corrosion science and the preservation of cultural heritage. Wu Zhu coin is the first currency round coin issued by the central government in ancient China. It has been archeologically proven that the coin had greatly promoted the development of social economy and politics since the Western Han Dynasty from B.C. 206, and its casting technique stood for the highest minting level in ancient China. As one kind of old currency coin, Wu Zhu coin is one of the most typical round bronze coins which had been circulated more than 1000 years. The coin, with a square hole at the center for bunching up together, is named after two Chinese characters “Wu” (means five) and “Zhu” (means baht) on the surface of the coin, which represents the monetary unit in ancient
⁎ Corresponding author. Tel.: +86 29 8266 5671; fax: +86 29 8266 8559. E-mail address: [email protected] (X. Zhao). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.05.009
China (Fig. 1) and is similar to five pence of an English coin. From the observation of facade, the excavated coin has been buried under soil over a century before unearth and has been eroded heavily. To our limited knowledge, neither the chemical composition for Wu Zhu coins nor the patina characteristics on coin surface has been undertaken to scientific analyses so far. Therefore, a detailed analysis of Wu Zhu coins in order to gain intensive information of chemical composition, nature of the patina and corrosion features on the morphological coin surface is greatly important to explore the corrosion mechanism of bronze coin and to achieve a goal of the ultimate preservation, so our investigation on archeological coins is very important and clearly a challenge work on both corrosion science and cultural heritage. It is reported that common features of the passive corrosion layer on various ancient bronze objects, known as noble patina, has been analyzed and highlighted as presenting a double-structured layer consisting of an inner layer of copper (I) salts [3–5] and an external layer of copper (II) salts, which depended on the history and the elemental composition of ancient bronze object [6–9]. It is proved that the “passive corrosion layer” is created under a mild corrosion condition, but the “coarse” surface is generally assigned to severe corrosion conditions, often in the presence of chloride anion [10]. The so-called bronze disease corrosion is due to chloride anions from cuprous chloride in the patinas [11,12]. On the other hand, it is generally accepted that the corrosion surface layer is constituted of Cu (II) salts such as malachite (CuCl2·3Cu(OH)2, in soil), brochantite (CuSO4·3Cu(OH)2, in the atmosphere) or atacamite (CuCO3·Cu(OH)2, in seawater) [13,14]. Recent studies have reported the characterization of bronze corrosion
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Fig. 1. Sample WZ1 and WZ2, the right word is “Wu” and the left word is “Zhu”.
through the analyses of the corrosion products formed on the bronze surface by scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (SEM–EDS) [15,16], X-ray photoelectron spectroscopy (XPS) [17,18], energy dispersive X-ray fluorescence (EDXRF) spectrometry and X-ray diffraction (X-RD) [19]. It is also pointed out that the backscattered electron (BSE) imaging is an effective approach to characterize the corrosion section of bronze objects [20]. However, even though some reports have described the corrosion phenomena and natural patinas formed on archeological bronze artwork [21,22], few corrosion mechanism have been proposed to explain the formation of bronze patinas in the case of the buried artwork in soil so far. Additionally, although some studies on the archeological bronze samples have predicted that the chloride anion was one of the major causes for bronze corrosion in soil [23], the distribution of chloride anion in the corrosive section of bronze object is somehow neglected. In the present work, analysis is focused on two Wu Zhu coins selected from among about 30-coin-collection during the excavations performed at the national archeological site. Our aim is to understand the chemical compositions of two Wu Zhu coins, to characterize the corrosion features and to explore the nature of the patina on the coin surface by means of optical microscopy (OM), scanning electron microscopy (SEM) equipped with backscattered electron (BSE) detector and energy dispersive spectrometry (EDS), and X-ray diffraction (X-RD). The distribution of chloride content in the cross section of coin is determined for the first time. The proposed corrosion mechanism is discussed based on the intensive experimental data. 2. Experimental 2.1. Description of archeological site and samples The analyzing sample of Wu Zhu coins were excavated from 100 to 200 cm underground during the archeological investigation in 2006 on the famous Zhongguan minting site, 25 km west from Xi'an city (the ancient Chinese Capital) in the western area of China. The total area of the site covers about more than 900,000 m 2, measuring about 600 m from east to west and 1500 m from south to north, and is found as the biggest one in ancient China up to now. A river (named as Xin River) flows along the site from north to south. The site area has a semi-dry continental climate and the annual rainfall is 659 mm. The rain season is mainly from early July to late September, and the average annual temperature is between 13.5 °C and 15.4 °C with the maximum of 42 °C and the minimum of − 15 °C. The annual mean of relative humidity is circa 60%, increase to 75% in October. It has been found that the surface of Wu Zhu coin was covered by green and red (or yellowish) products after being unearthed from the
soil, and the corrosion layer was formed on the surface of coin with a submillimeter thickness. The coin is about 2.5–2.6 cm in diameter and around 3.32–3.81 g in weight. The square hole at the coin center is 0.9–1.0 cm in width. The slight difference in diameter and weight was due to its corrosion. Two typical coins (WZ1 and WZ2) from the archeological site were selected in this work and served as the analysis samples (Fig. 1). 2.2. Characterization approaches After mounting in epoxy resin, the coin sample was mechanically rubbed for the metallographic examination by metallographic microscope. An HI-Scope Advance KH-3000 stereo microscope with 700–1400 times magnification was used for the preliminary observation of the corrosion products on the coin surface. The aim of metallographic examination was to reveal the microstructures of the coin samples and provide the direct insight into the compatibility of different metals employed in Wu Zhu coins. SEM images were obtained on Cambridge S-360 Scanning Electron Microscope (Cambridge Technology, Cambridge, England) with an accelerating voltage of 25 kV. The morphology of the corrosion layers and the rubbed surfaces were analyzed by SEM measurements. Energy dispersive spectrometry (NSS-300 EDS) coupled to the SEM was acquired from various concerned points on the specimen in order to study the main elemental composition in the corrosion layers and the rubbed surfaces. The EDS analysis was performed with a lifetime of 100 s, a pulse-counting rate of about 4000 cps, a working distance of 23 mm and an accelerating voltage of 20 kV. Standard samples provided by NSS-300 EDS were used for quantifying the elements to be detected with about 2% relative error. The information about compositional distributions in the rubbed surfaces and in the cross section of coin was further provided by the BSE imaging measurements. XRD was used to characterize the nature of patina on the coin surface. The powder samples, taken mechanically by scraping the corroded coin surfaces gently with a very fine tungsten needle, was grounded finely in an agate mortar and pressed into the specimen holder, and then mounted in a Philips 1015 X-ray diffraction instrument. The measuring conditions were set as follows: Cu target, 40 kV accelerating voltage, 40 mA current. The scanning range of 2θ was from 0° to 80° with the scanning speed was 3°/min. The XRD data were analyzed using Difrac software (Bruker Advanced X-ray Solutions, Germany) and with the JCPDF database. 2.3. Investigation of the soil The pH value and concentration of chloride anion in soil surrounding the coin are measured by IQ 150 pH instrument in site and a LC-2010C
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high performance liquid chromatography (HPLC, SHIMADZU, Japan), respectively. It is revealed archeologically that the soil was a khaki calcareous sandy soil with pH value of 6.5–7.1, which was mixed with organic mineral components derived from domestic residues and metallurgical activities, such as ashes, charcoal, pottery and construction materials. The data of groundwater level and climate information obtained from the local weather bureau have indicated that the groundwater level of this area was 18–20 m, containing 35–55% chlorine ion and less sulfate ion.
3. Results and discussion 3.1. Chemical compositions The chemical compositions of Wu Zhu coin are determined on the original samples by SEM–EDX and on the rubbed samples by BSE– EDX. The analysis results of original samples by SEM–EDX are listed in Table 1. The main elements in the coins are copper, tin and lead (Table 1). Pb is enriched in green crust and Sn in red or rubefacient crust. The elemental compositions are 83–88 wt.% Cu, 5–10 wt.% Pb, and less than 5 wt.% Sn, which indicated that the coins were shaped in the period of West Han Dynasty, compared to the historical investigation. In ancient China, the bronze coins in different shape were first come into use in the Shang dynasty (B.C. 1766–B.C. 1122) with about 39 wt.% Pb. During the Spring and Autumn (Chun-Qiu) and the Warring States (Zhan-Guo) periods (B.C. 770–B.C. 221), bronze coins (with some typical geometrical shapes) had contained more than 60 wt.% Cu, 20 wt.% Pb, and less than 10 wt.% Sn. The elemental compositions of coins circulated from Qin to Sui times (B.C. 221–A.D. 618) were detected about more than 80 wt.% Cu, 4–12 wt.% Pb, less than 10 wt.% Sn, a kind of typical Cu–Sn–Pb bronze. A great deal historical investigation had displayed that Wu Zhu coins were made of a traditional bronze material. Moreover, the elemental compositions are identified by BSE–EDS analyses on the rubbed Wu Zhu samples (Figs. 2 and 3) and are listed in Table 2. In the WZ1 sample (Fig. 2), point 1 and point 3, two whitish islands, are the high lead-rich area containing 87.75 wt.% Pb and 55.94 wt.% Pb, respectively. Point 2 and point 4, two gray islands, are the tin-rich area containing 21% Sn. Both of the whitish islands and the gray islands have exhibited that the excess lead and tin was not dissolved well in the metal solution during the minting of Wu Zhu coin, i.e. the lead and tin did not amalgamate with copper during minting of the coin. Furthermore, a definite amount of antimony (Sb, 4.41–4.86 wt.%) is detected at point 2 and point 4, but no antimony is found at point 1 and point 3. It is indicated that Sb is easy to co-exist with Sn but hard to exist together with Pb. This result might be explained by the difference of Pb– Table 1 Elemental composition of surface of Wu Zhu samples. Samples
Position of analyses
The elementary composition (wt.%) Cu
Sn
Pb
Si
Fe
Al
WZ1
Green crust 1 Green crust 2 Green crust 3 Green crust 4 Deep crust 1 Deep crust 2 Red crust 1 Red crust 2 Red crust 3 Green crust 1 Green crust 2 Deep crust 1 Deep crust 2 Deep crust 3
66.44 79.91 90.14 92.52 83.47 89.27 81.54 74.97 80.48 71.95 83.00 87.86 87.56 82.53
5.10 2.12 2.10 1.85 3.95 2.93 3.11 11.58 12.22 5.11 7.69 3.13 4.56 2.03
22.37 19.28 5.62 4.44 7.64 4.50 6.74 8.63 4.36 19.44 5.94 6.05 5.54 11.47
– – – – – – 0.55 2.15 0.80 0.93 1.15 0.93 0.84 1.43
1.83 1.45 1.70 – 0.86 0.93 7.97 2.13 1.84 1.90 1.91 1.20 1.11 1.87
– – – – – – 0.09 0.54 0.30 0.67 0.32 0.82 0.38 0.67
WZ2
205
Sb and Sn–Sb phase diagrams, which demonstrates that the possibility of co-existence for Sn/Sb phase is more prominent than Pb/Sb phase. Point 5 in Fig. 2 contains 93.35 wt.% Cu and indicates the elemental composition in the substrate of the bronze coin. Point 6, an opaque black one, represents a hole in the substrate. A similar phenomenon is observed in the WZ2 sample (Fig. 3, Table 2). Point 1 and point 6 are two whitish islands with 83–88 wt.% Pb and 10 wt.% Cu, which are the highest lead-rich area and is similar to point 1 and point 3 in the WZ1 sample (Fig. 2). Point 4 represents the bronze substrate and contains 92.02 Cu wt%, that is very similar to point 5 in the WZ1 sample (93.35 wt.%). A gray island at point 5 for the WZ2 sample is the highest tin-content area and contains 5.81 wt.% Sn together with 3.62 wt.% Sb. However, the black island with some white points at point 2 in Fig. 3 contains 10.83 wt.% Fe and 20.4 wt.% S, but very low in lead content (11.00 wt.%). Additionally, the opaque black island at point 3 contains much sulfur (17.07 wt.%), which has indicated that point 3 may be a heavily deteriorated area. It is also found about 10.83 wt.% Fe in the WZ2 sample, indicating this sample a rubefaction surface. The high amount of Fe present in the WZ2 sample is not certainly related with the alloy composition of coins, but may come from the environment (soil or objects buried in the soil). In fact, archeologists have found pieces of tools made of iron around the coins. On the other hand, the source of Fe may come from raw materials, because iron is a common element found in earth and co-exists with almost all other minerals, especially copper-containing minerals. As for minute amounts of Si and Al (less than 1 wt.% each) detected in some of areas investigated in the coins, the possible reason is that it is likely from the efflorescence of the coin surface because it is not detected in the substrate of the concerned coin. It is noteworthy that Sb is found in ancient Chinese round bronze coin for the first time in our work. But the content of Sb is less in substrates for both of coins than in the other areas. If Table 2 is compared with Table 1, it is not hard to see that the analysis results in Table 2 are 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6–2.9 wt.% Sb for the main elemental compositions of coins, whereas in Table 1, the elemental compositions of coins are 83–88 wt.% Cu, 5 wt.% Sn and 5– 10 wt.% Pb. The difference in the two tables is due to the coins studied being quite deteriorated, little amounts of corrosion products are removed from the surface and from the borders in Table 1. In this case, it is possible to obtain the composition of outer layers (Table 1). However, the composition of the inner layer of the coins is obtained in Table 2 and presents 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6– 2.9 wt.% Sb. Comparing the values of Cu in the substrate and in the corrosion products, it may be concluded that the amounts of copper salts in the latter are quite low related to the percentage of copper in the substrate [1]. The comparison result in both tables indicates that no antimony (Sb) is observed in Table 1 for the original samples. This result can be explained by the chemical inertness of antimony which is not liable to corrode in soil and thus it is not involved in the corrosion crust. But in comparing the values of Cu, Sn, Pb and Sb in two coins, the different chemical compositions are found and attributed to metal compatibility during casting of coins, to the large differences in melting points among Cu, Sn, Pb and Sb (Cu: 1083 °C, Sn: 231.9 °C, Pb: 327.5 °C, Sb: 763 °C) and to the limited minting technology used in the West Han Dynasty. Copper solidifies very fast due to its higher melting point. This makes it impossible for both tin and lead to solidify together with copper. As for Sb, it is detected as a constituent in coin substrates but with less content compared with other elements. It is found that antimony is not the main component in this kind of bronze coin but just an appendant from the raw materials of copper and lead, and the most interesting finding is that Sb is rather rich in the tin-rich area of the coins. On the other hand, the elemental compositions of the coin samples are greatly correlated to the compatibility of metals. In this work, the metal compatibility in Wu Zhu coins has been examined by metallographic analysis and SEM observation as shown in Fig. 4a and b. It is found that both WZ1 and WZ2 samples behave as dendrite
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Fig. 2. BSE image and EDS analyses of the WZ1 sample.
structures. The well-proportioned dark islands, an average diameter of 5–10 μm, are scattered on the bronze substrate in the WZ1 sample (Fig. 4a, point 1) and indicate that Pb is not compatible with the bronze substrate. Some corrosive pits with diameters of 1–5 μm and cracks with length of 10–15 μm have shown that the corrosion has already developed into the inner substrate (Fig. 4a). The metallographic analyses in the WZ2 sample reveal that area of the islands are much larger and length of the islands is varying from 28 μm for point 2 to 48 μm for point 4 (Fig. 4b). The SEM images of the WZ1 and WZ2 samples (Fig. 4c and d) have further exposed the metallurgical features of coin, and the lead-rich area is distinctly observed from point 5 of Fig. 4c and point 7 of Fig. 4d, and so does the tin-rich area from point 6 of Fig. 4c. It is clear that the WZ2 sample is much more corroded than the WZ1 sample.
3.2. The corrosion characteristics 3.2.1. The corrosion features in the surface of coin The common corrosion features for both WZ1 and WZ2 samples are shown by stereoscopic microscopic observation that not only the original surface is covered generally by the complicated compounds, but also the coin surfaces are rough with cracks and pits which present multicolor areas of green, red, brown, blue and metallic gray under polarized light, indicating the preliminary chemical composition of the patina. It is basically proved that the Wu Zhu coin samples analyzed were severely corroded. In the WZ1 sample, the coin surface is distributed mainly with the green compact patina (Fig. 5a) and sometimes existed together with the blue patina and mahogany crust (Fig. 5b). However, the coin
Fig. 3. BSE image and EDS analyses of sample WZ2.
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Table 2 Elemental composition of rubbed samples. Samples
WZ1
WZ2
Points of analyses Total Point Point Point Point Point Point Total Point Point Point Point Point Point
Description of points
The elemental composition (wt.%)
1 2 3 4 5 6
Whitish island Gray island Whitish island Gray island Substrate Opaque black island
1 2 3 4 5 6
Whitish island Black island with white points Opaque black island Substrate Gray island Whitish island
Cu
Sn
Pb
Sb
85.39 10.52 70.48 38.45 70.45 93.05 87.73 84.76 10.16 57.77 78.65 92.02 86.15 10.34
6.41 1.72 21.25 2.06 21.61 3.02 7.76 3.25 – – – 1.89 5.81 2.71
4.74 87.76 3.86 55.94 3.00 3.26 2.48 6.39 88.64 11.00 4.28 3.20 3.61 83.42
2.89 – 4.41 – 4.86 0.85 2.12 2.61 – – – 1.24 3.62 –
surface of the WZ2 sample has been mineralized or heavily fossilized, and scattered into the green crystallized patina area (Fig. 5c, much smaller than that in WZ1 sample) interweaving with brown corrosion surface (Fig. 5d). In particular, an altered layer and some interconnected microcracks are observed in the surface for both samples of coin, which are attributed to internal stresses in the corrosion layers, or due to the long-time corrosion caused by the periodic hydration
a
Fe 0.57 – – 2.11 – – – 1.53 – 10.83 – 1.59 0.81 –
Al
Si
O
S
– – – – – – – 0.43 0.86 – – – – –
– – – – – – – – 0.34 – – – – –
– – – – – – – 1.03 – – – – – –
– – – 1.45 – – – – – 20.4 17.07 – – –
and dehydration in soil. In normal case, the extension of the internal layer was accompanied by the incorporation of corrosion species. Furthermore, the detailed corrosion features are characterized by the BSE–EDS analyses in Fig. 6. For the WZ1 sample, high amounts of C, O and Cl, and low amounts of S, Fe, Al, Si, Ca and Mg are found (Table 3). Point 1 in Fig. 6 is a whitish area containing 38.79 wt.% Cu, 30.85 wt.% O and 8.21 wt.% C, which is corresponded to the composition of copper
b 1 4
3 2
c
d
7
5
6
Fig. 4. Metallographic images and SEM images for the rubbed surfaces of the WZ1 and WZ2 samples. Metallographic images of WZ1 sample (a) and sample WZ2 (b); SEM images of sample WZ1 (c) and WZ2 sample (d).
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a
b
c
d
Fig. 5. Stereoscopic observation of patina for sample WZ1 (a and b) and sample WZ2 (c and d).
carbonate. Point 2 and point 3 in Fig. 6 are the gray porous areas and indicates the chloride-rich character with 10.25 wt.% Cl and 11.51 wt.% Cl, respectively. Therefore, it is possible to deduce that this coin is preserved in the chloride-rich environments before excavation (see Section 3.3). The corrosive crust on the surface should be made of the mixture of Cu2(OH)3Cl and others, which has been confirmed by XRD analyses in this paper. Point 4 in Fig. 6 contains 83.49 wt.% Cu, 11.68 wt.% O, 3.13 wt.% C and 1.04 wt.% Cl, which has revealed the presence of Cu2 (OH)3Cl. The WZ2 sample is not discussed again here due to its quite similar result to that of WZ1 sample. The X-RD analyzing results on the scraping powder of corrosive surface of coin are shown in Fig. 7. The light green patina represents
Cu2(OH)3Cl, together with a little of Cu2(OH)2CO3 (Fig. 7a). The main component of the compact green surface is Cu2(OH)2CO3 which coexists with Cu2(OH)3Cl and Pb3O4 (Fig. 7b). The hard and dark compact green patinas represent Cu2(OH)2CO3 (Fig. 7c) and the blue patina is composed of Cu3(CO3)2(OH)2 (azurite), CuSO4·5H2O, Cu2 (OH)2CO3, Cu2(OH)3Cl, CuCl and Cu2O. On the porous or crack surface, the main composition is Cu2(OH)3Cl and Cu3(CO3)2(OH)2. The khaki crust is detected as a mixture of Al2Si2O5(OH)4·H2O, Fe6(PO4)4 (OH)5·H2O, Cu2(OH)3Cl, Cu2(OH)2CO3, Pb3O4, PbCO3, CuS and SiO2. The X-RD analysis results are not only in a good conformity with the EDS analysis results, but also fit the normal patinas in buried archeological objects made of copper alloys, composed of copper, lead
Fig. 6. BSE image and EDS diagrams for the surface of the WZ1 sample.
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Table 3 Elemental composition of the corrosion surface of the WZ1 sample. Points of analyses
Description of points
The elemental composition (wt.%) Cu
Fe
Al
Si
O
S
C
Ca
Mg
Cl
Point Point Point Point
Light whitish area Gray porous area Black porous area Compact area
38.79 60.93 53.90 83.49
1.05 – – –
2.91 1.64 – 0.26
4.82 2.19 1.45 –
30.85 16.05 25.73 11.68
0.91 0.75 1.14 –
8.21 7.42 5.47 3.13
2.18 – 0.80 0.40
1.38 0.77 – –
– 10.25 11.51 1.04
1 2 3 4
and tin oxides, carbonates, silicates and phosphates [1,3,5,24]. Actually, hydroxides and hydrated compounds of Cu(II) could be formed after the long-term (for more than 1000 years) interaction between the coins and corrosion environment of soil. Cu2(OH)3Cl, a powdered patina that is acknowledged to be harmful to the preservation of bronze coins, is presented in nearly all corrosion products or crack surfaces. Cu2(OH)2CO3 (malachite) and Cu3(CO3)2 (OH)2 (azurite) together with Cu2(OH)3Cl are the main corrosive components of green corrosion products on the surface of the coins. Although it had been reported that uncommon compounds among the corrosion products of bronze coins are chloride and phosphate of lead, Pb8Cu(Si2O7)3, Pb4Al4Si3O16 and Pb5(PO4)3Cl [1], what we have found is that both Pb3O4 and PbCO3 are in the coin surface.
3.2.2. The corrosion behavior in cross section of the coin BSE and SEM images have indicated that thickness of patina on the surfaces of coin varies between 40 μm and 55 μm (Fig. 8). The corrosion layer is well separated from the bronze substrate in Fig. 8a–d. In most cases, the corrosion surface for both of the coin samples shows double layers. The upper-layer is about 25–35 μm in thickness and has been separated from the sub-layer. The sub-layer is about 20–25 μm in thickness and is adhered to the substrate of the coin body. In addition, several perpendicular and parallel cracks are observed in Fig. 8a–d. These cracks caused probably by hydration and dehydration of the corrosion components are served as a starting point for a localized corrosion. Meanwhile, SEM images of the upperlayer indicate a compact surface in the WZ1 sample (Fig. 8a and b) and a porous surface in the WZ2 sample (Fig. 8c and d). In this work, the WZ1 sample is chosen as the example to carry out the detailed BSE–EDS analyses for the corrosion in cross section, and results are shown in Fig. 9 and Table 4. Points 1–4 in Fig. 9 are located in the upper corrosion layer and the amount of copper increases with the corrosion depth from point 1 to point 4, but the content of oxygen and carbon decreases correspondingly. It is illustrated by the fact that the surface of coin contained corrosive products of copper carbonate. From data in Table 4, the main component in point 4 (much more copper, less oxygen and a little chlorine) was Cu2(OH)3Cl, accompanied by Cu inclusion. Point 5 in Fig. 9 is located at the interface of the sub-layer and upper-layer. Compared with point 4, point 5 contains much more sulfur, which indicates that it is perhaps a mixture of CuS (or CuSO4) and Cu2(OH)3Cl. The most important phenomenon is that high amount of chloride anion is found at the interface between the sub-layer and the coin substrate (point 6 in Fig. 9). Actually, the content of chloride anion increase dramatically from the top surface to the inner corrosion layer. The stereoscopic microscopic observation give a clear indication of Cu (II) salts at the interface of the upper-layer and sub-layer, probably paratacamite (Cu2(OH)3Cl, green). This indicates that the formation of the internal corroded layer is linked to an enrichment of the corrosion products of chloride anion from the soil. The presence of high chlorine content at the archeological site (3.98 × 10 − 4 mol L − 1 by HPLC analyses) suggests the occurrence of mass conversion of chlorine from its state in the soil to the metallic phase. 3.3. The corrosion mechanism
Fig. 7. XRD diagram of green corrosion products. (a) The light green corrosion products showing Cu2(OH)3Cl and Cu2(OH)2CO3; (b) the compact green corrosion products showing Cu2(OH)3Cl, Cu2(OH)2CO3 and Pb3O4; (c) the dark green crust showing Cu2 (OH)2CO3 and SiO2.
Evidence of different corrosion behavior of the bronze coins is supported by the surface analysis data and the gathered environmental information. Therefore, a corrosion mechanism of Wu Zhu coin is presented correspondingly according to the analytical result of patina and the buried environment discussed in this paper. The investigation on the soil surrounded coins has shown that the soil is a khaki calcareous sandy soil mixed with organic mineral components derived from domestic and metallurgical residues, such as ashes, charcoal, pottery and construction materials. The density, humidity, pH value and chloride concentration of buried soil were 1.6 g cm− 3, 17.2%, 7.76 and 3.98 × 10− 4 mol L− 1, respectively. The proposed corrosion mechanism for the Wu Zhu coin was elucidated as follows.
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Fig. 8. BSE and SEM images of the corrosion in the cross section in WZ1 sample (a and b) and WZ2 sample (c and d).
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Fig. 9. BSE image and EDS analysis of a cross section of the WZ1 sample.
After the Wu Zhu coins are minted, Cu2O and CuO are formed on the surface of coin due to the oxidization. 4Cu + O2 = 2Cu2 O; 2Cu + O2 = 2CuO When the coin is buried under soil for more than a thousand years, water and chloride ions (3.98 × 10 − 4 mol L − 1 in the soil) are two key factors for its corrosion. The porosity of oxidized compound formed on the surface of Wu Zhu coin (Cu2O, CuO) has provided the possibility for water movement and chloride ions activity, especially in the archeological soil for pH value around 7.76. Cu2 O + 2Cl− + 2Hþ = 2CuCl + H2 O Cu2 O + H2 O + CO2 + O2 = Cu2 ðOHÞ2 CO3 ðmalachiteÞ 2Cu2 O + 2H2 O + 2CO2 + O2 = Cu3 ðCO3 Þ2 ðOHÞ2 ðazuriteÞ 2Cu2 O + H2 O + HCl + O2 = CuCl2 ⋅3CuðOHÞ2 ðdisease corrosionÞ It should be pointed out that soil is of most importance in the formation of the final patina on the surface of coins. During the corrosion process, the chloride ion is enriched in the interface of corrosion layer and the coin body, as discussed in the section of corrosion behavior in the cross section of coin. In addition, Xin River flows along the site from north to south and the archeological site is located at the area between the maximum temperature 42 °C and the minimum temperature −15 °C, where this environmental condition accelerates the corrosion action by the movement of water and chloride ion. It is just by reason of corrosion that both of the original coin surfaces are distributed with nub and pits. This nub and pits should be attributed not only to the discoloration of the original surface, but also to the formation of patina on the surfaces of coins. The pits occur when soluble corrosion products are washed away, but the nub is formed by the sedimentation of other corrosion products on the surface of coins. Both pits and nubs lead to a loss in esthetic quality of the coin.
4. Conclusions The intensive analysis results in this work have shown that Wu Zhu coins are made of bronze materials. The elemental compositions are 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6–2.9 wt.% Sb. The element of Sb is observed for the first time in ancient Chinese round bronze coins. The detected amounts of Cu, Sn and Pb are in good conformity with the information gathered through the historical survey. The lead-rich and tin-rich areas in coin samples are discovered and have demonstrated the poor metal compatibility during minting. The analyses of corrosion surface have indicated that the corrosion surface is composed of two layers, the sub-layer (20–25 μm) is adhered to the substrate of the coin and the upper-layer (25–35 μm) is separated from the sub-layer. It has been proved in this work that the high content of chloride ion is measured at the interface of the sub-corrosion layer and the body of the coin, that the amount of Cl in the cross section of coin increases dramatically from the outer layer to the inner layer, and that the thickness of the patina varies from 40 μm to 55 μm depending on the position of coin. Cracks are also observed on the surface of coin samples and some of them have developed into the substrate of the coin. It is shown from the experimental data that the patina is mainly consisted of Cu2(OH)3Cl, Cu2(OH)2CO3, Cu3(CO3)2 (OH)2 and Pb3O4. In some part on the surface of coin, the crust is analyzed as a mixture of the corroded component above. The appearance of the bronze coin is consistent with the analyzed corrosion patina. Water and chloride ion are regarded as two of the most powerful corrosion agents for Wu Zhu coins due to the chloride ions enriched in the interface of the coin body and corrosion layer.
Acknowledgments This work was supported in part by the National Natural Science Foundation of China (NSFC Grants No. 50872143, and No. 20673081),
Table 4 Elemental composition of a cross section of the WZ1 sample. Points of analyses
The elemental composition (wt.%) Cu
Sn
Pb
Sb
Fe
O
S
C
Ca
Mg
Cl
Point Point Point Point Point Point
33.07 33.88 43.85 82.20 66.23 10.21
– – – – – 3.01
– – – – – 30.70
– – – – – 14.42
– – – – 0.55 0.62
51.40 50.88 47.58 15.44 9.02 22.51
– – – – 22.46 –
12.80 10.24 8.57 – – –
– – – – – –
1.76 – – – – –
– – – 2.36 1.74 18.53
1 2 3 4 5 6
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and partially supported by National Key Project on Basic Research for Science & Technology of China (Grant No. 2010BAK67B12). References [1] D. Satovic, L. Valek Zulj, V. Desnica, S. Fazinic, S. Martinez, Corrosion evaluation and surface characterization of the corrosion product layer formed on Cu–6Sn bronze in aqueous Na2SO4 solution, Corrosion Science 51 (2009) 1596–1603. [2] K. Leyssens, A. Adriaens, C. Degrigny, E. Pantos, Evaluation of corrosion potential measurements as a means to monitor the storage and stabilization processes of archaeological copper-based artifacts, Analytical Chemistry 78 (2006) 2794–2801. [3] A.M. Alfantazi, T.M. Ahmed, D. Tromans, Corrosion behavior of copper alloys in chloride media, Materials and Design 30 (2009) 2425–2430. [4] M.C. Bernard, S. Joiret, Understanding corrosion of ancient metals for the conservation of cultural heritage, Electrochimica Acta 54 (2009) 5199–5205. [5] K. Marusic, H. Otmacic-Curkovic, S. Horvat-Kurbegovic, H. Takenouti, E. Stupnisek-Lisac, Comparative studies of chemical and electrochemical preparation of artificial bronze patinas and their protection by corrosion inhibitor, Electrochimica Acta 54 (2009) 7106–7113. [6] F. Gallese, G. Laguzzi, L. Luvidi, V. Ferrari, Comparative investigation into the corrosion of divergent bronze alloys suitable for outdoor sculptures, Corrosion Science 50 (2008) 954–961. [7] J. Dodson, X.Q. Li, M. Ji, K.L. Zhao, X.Y. Zhou, V. Levchenko, Early bronze in two Holocene archaeological sites in Gansu, NW China, Quaternary Research 72 (2009) 309–314. [8] I. Schoukens, E. Tourwe, J. Guillaume, Viet Nguyen, Vereecken Jean, H. Terryn, Study of corrosion products on Vietnamese archaeological bronzes dating from the Dong Son period, Surface and Interface Analysis 40 (2008) 474–477. [9] E. Figueiredoa, P. Valerioa, M.F. Araujoa, J.C. Senna-Martinezc, Micro-EDXRF surface analyses of a bronze spear head: lead content in metal and corrosion layers, Nuclear Instruments and Methods in Physics Research A 580 (2007) 725–727. [10] F.J.R. de Oliveira, D.C.B. Lago, L.F. Senna, L.R.M. de Miranda, E.D. Elia, Study of patina formation on bronze specimens, Materials Chemistry and Physics 115 (2009) 761–770. [11] A.S.M.A. Haseeb, H.H. Masjuki, L.J. Ann, M.A. Fazal, Corrosion characteristics of copper and leaded bronze in palm biodiesel, Fuel Processing Technology 91 (2010) 329–334.
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Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Corrosion behavior and morphological features of archeological bronze coins from ancient China Ling He a, Junyan Liang a, Xiang Zhao a,⁎, Baolian Jiang b a b
Department of Chemistry, School of Science, Xi'an Jiaotong University, Xi'an 710049, China Xi'an Center for the Conservation and Restoration of Cultural Heritage, Xi'an, 710061, China
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 7 May 2011 Accepted 8 May 2011 Available online 14 May 2011 Keywords: Bronze coin Corrosion Patina Morphology SEM X-RD
a b s t r a c t The archeological round bronze coins, nominated as Wu Zhou and regarded as the first issued effective money in the Han Dynasty of China, have been systematically investigated to disclose their chemical composition, nature of the patina and corrosion features on the coin surface by optical microscopy (OM), X-ray diffraction (X-RD), and scanning electron microscopy (SEM) equipped with backscattered electron (BSE) detector and energy dispersive spectrometry (EDS) techniques. It is revealed morphologically that there are some rough surface cracks, pits, and multicolor patina on the surface of the coins. We prove that the coins are made from bronze material of Cu–Sn–Pb–Sb alloy with contents of 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6–2.9 wt.% Sb, and covered by two corrosion layers, 25–35 μm for the upper-layer and 20–25 μm for the sub-layer. High chloride content has been detected at the interface between the sub-layer and body of the coins. The lead-rich and tin-rich areas in the coin samples indicate the poor metal compatibility during minting in some locations of the coins. The main compositions of patina are ascertained to be Cu2(OH)3Cl, Cu3 (CO3)2(OH)2, Cu2(OH)2CO3, and Pb3O4, and the proposed corrosion mechanism is discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The study of chemical compositions and corrosion features on bronze art objects is greatly important to explore the corrosion behavior and to achieve the aim in the preservation to the of bronze art objects [1]. In the most cases, corrosion of archeological bronze is extensively studied through the analyses of corrosion products with two aims: (1) to clarify the morphological corrosion phenomena occurring on bronze surface when it is exposed to different corrosive media and (2) to prevent corrosion processes for preservation of the cultural heritage [2]. Our investigation on the archeological Wu Zhu coins used in the West Han Dynasty of ancient China is expected to be significant on both corrosion science and the preservation of cultural heritage. Wu Zhu coin is the first currency round coin issued by the central government in ancient China. It has been archeologically proven that the coin had greatly promoted the development of social economy and politics since the Western Han Dynasty from B.C. 206, and its casting technique stood for the highest minting level in ancient China. As one kind of old currency coin, Wu Zhu coin is one of the most typical round bronze coins which had been circulated more than 1000 years. The coin, with a square hole at the center for bunching up together, is named after two Chinese characters “Wu” (means five) and “Zhu” (means baht) on the surface of the coin, which represents the monetary unit in ancient
⁎ Corresponding author. Tel.: +86 29 8266 5671; fax: +86 29 8266 8559. E-mail address: [email protected] (X. Zhao). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.05.009
China (Fig. 1) and is similar to five pence of an English coin. From the observation of facade, the excavated coin has been buried under soil over a century before unearth and has been eroded heavily. To our limited knowledge, neither the chemical composition for Wu Zhu coins nor the patina characteristics on coin surface has been undertaken to scientific analyses so far. Therefore, a detailed analysis of Wu Zhu coins in order to gain intensive information of chemical composition, nature of the patina and corrosion features on the morphological coin surface is greatly important to explore the corrosion mechanism of bronze coin and to achieve a goal of the ultimate preservation, so our investigation on archeological coins is very important and clearly a challenge work on both corrosion science and cultural heritage. It is reported that common features of the passive corrosion layer on various ancient bronze objects, known as noble patina, has been analyzed and highlighted as presenting a double-structured layer consisting of an inner layer of copper (I) salts [3–5] and an external layer of copper (II) salts, which depended on the history and the elemental composition of ancient bronze object [6–9]. It is proved that the “passive corrosion layer” is created under a mild corrosion condition, but the “coarse” surface is generally assigned to severe corrosion conditions, often in the presence of chloride anion [10]. The so-called bronze disease corrosion is due to chloride anions from cuprous chloride in the patinas [11,12]. On the other hand, it is generally accepted that the corrosion surface layer is constituted of Cu (II) salts such as malachite (CuCl2·3Cu(OH)2, in soil), brochantite (CuSO4·3Cu(OH)2, in the atmosphere) or atacamite (CuCO3·Cu(OH)2, in seawater) [13,14]. Recent studies have reported the characterization of bronze corrosion
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Fig. 1. Sample WZ1 and WZ2, the right word is “Wu” and the left word is “Zhu”.
through the analyses of the corrosion products formed on the bronze surface by scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (SEM–EDS) [15,16], X-ray photoelectron spectroscopy (XPS) [17,18], energy dispersive X-ray fluorescence (EDXRF) spectrometry and X-ray diffraction (X-RD) [19]. It is also pointed out that the backscattered electron (BSE) imaging is an effective approach to characterize the corrosion section of bronze objects [20]. However, even though some reports have described the corrosion phenomena and natural patinas formed on archeological bronze artwork [21,22], few corrosion mechanism have been proposed to explain the formation of bronze patinas in the case of the buried artwork in soil so far. Additionally, although some studies on the archeological bronze samples have predicted that the chloride anion was one of the major causes for bronze corrosion in soil [23], the distribution of chloride anion in the corrosive section of bronze object is somehow neglected. In the present work, analysis is focused on two Wu Zhu coins selected from among about 30-coin-collection during the excavations performed at the national archeological site. Our aim is to understand the chemical compositions of two Wu Zhu coins, to characterize the corrosion features and to explore the nature of the patina on the coin surface by means of optical microscopy (OM), scanning electron microscopy (SEM) equipped with backscattered electron (BSE) detector and energy dispersive spectrometry (EDS), and X-ray diffraction (X-RD). The distribution of chloride content in the cross section of coin is determined for the first time. The proposed corrosion mechanism is discussed based on the intensive experimental data. 2. Experimental 2.1. Description of archeological site and samples The analyzing sample of Wu Zhu coins were excavated from 100 to 200 cm underground during the archeological investigation in 2006 on the famous Zhongguan minting site, 25 km west from Xi'an city (the ancient Chinese Capital) in the western area of China. The total area of the site covers about more than 900,000 m 2, measuring about 600 m from east to west and 1500 m from south to north, and is found as the biggest one in ancient China up to now. A river (named as Xin River) flows along the site from north to south. The site area has a semi-dry continental climate and the annual rainfall is 659 mm. The rain season is mainly from early July to late September, and the average annual temperature is between 13.5 °C and 15.4 °C with the maximum of 42 °C and the minimum of − 15 °C. The annual mean of relative humidity is circa 60%, increase to 75% in October. It has been found that the surface of Wu Zhu coin was covered by green and red (or yellowish) products after being unearthed from the
soil, and the corrosion layer was formed on the surface of coin with a submillimeter thickness. The coin is about 2.5–2.6 cm in diameter and around 3.32–3.81 g in weight. The square hole at the coin center is 0.9–1.0 cm in width. The slight difference in diameter and weight was due to its corrosion. Two typical coins (WZ1 and WZ2) from the archeological site were selected in this work and served as the analysis samples (Fig. 1). 2.2. Characterization approaches After mounting in epoxy resin, the coin sample was mechanically rubbed for the metallographic examination by metallographic microscope. An HI-Scope Advance KH-3000 stereo microscope with 700–1400 times magnification was used for the preliminary observation of the corrosion products on the coin surface. The aim of metallographic examination was to reveal the microstructures of the coin samples and provide the direct insight into the compatibility of different metals employed in Wu Zhu coins. SEM images were obtained on Cambridge S-360 Scanning Electron Microscope (Cambridge Technology, Cambridge, England) with an accelerating voltage of 25 kV. The morphology of the corrosion layers and the rubbed surfaces were analyzed by SEM measurements. Energy dispersive spectrometry (NSS-300 EDS) coupled to the SEM was acquired from various concerned points on the specimen in order to study the main elemental composition in the corrosion layers and the rubbed surfaces. The EDS analysis was performed with a lifetime of 100 s, a pulse-counting rate of about 4000 cps, a working distance of 23 mm and an accelerating voltage of 20 kV. Standard samples provided by NSS-300 EDS were used for quantifying the elements to be detected with about 2% relative error. The information about compositional distributions in the rubbed surfaces and in the cross section of coin was further provided by the BSE imaging measurements. XRD was used to characterize the nature of patina on the coin surface. The powder samples, taken mechanically by scraping the corroded coin surfaces gently with a very fine tungsten needle, was grounded finely in an agate mortar and pressed into the specimen holder, and then mounted in a Philips 1015 X-ray diffraction instrument. The measuring conditions were set as follows: Cu target, 40 kV accelerating voltage, 40 mA current. The scanning range of 2θ was from 0° to 80° with the scanning speed was 3°/min. The XRD data were analyzed using Difrac software (Bruker Advanced X-ray Solutions, Germany) and with the JCPDF database. 2.3. Investigation of the soil The pH value and concentration of chloride anion in soil surrounding the coin are measured by IQ 150 pH instrument in site and a LC-2010C
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high performance liquid chromatography (HPLC, SHIMADZU, Japan), respectively. It is revealed archeologically that the soil was a khaki calcareous sandy soil with pH value of 6.5–7.1, which was mixed with organic mineral components derived from domestic residues and metallurgical activities, such as ashes, charcoal, pottery and construction materials. The data of groundwater level and climate information obtained from the local weather bureau have indicated that the groundwater level of this area was 18–20 m, containing 35–55% chlorine ion and less sulfate ion.
3. Results and discussion 3.1. Chemical compositions The chemical compositions of Wu Zhu coin are determined on the original samples by SEM–EDX and on the rubbed samples by BSE– EDX. The analysis results of original samples by SEM–EDX are listed in Table 1. The main elements in the coins are copper, tin and lead (Table 1). Pb is enriched in green crust and Sn in red or rubefacient crust. The elemental compositions are 83–88 wt.% Cu, 5–10 wt.% Pb, and less than 5 wt.% Sn, which indicated that the coins were shaped in the period of West Han Dynasty, compared to the historical investigation. In ancient China, the bronze coins in different shape were first come into use in the Shang dynasty (B.C. 1766–B.C. 1122) with about 39 wt.% Pb. During the Spring and Autumn (Chun-Qiu) and the Warring States (Zhan-Guo) periods (B.C. 770–B.C. 221), bronze coins (with some typical geometrical shapes) had contained more than 60 wt.% Cu, 20 wt.% Pb, and less than 10 wt.% Sn. The elemental compositions of coins circulated from Qin to Sui times (B.C. 221–A.D. 618) were detected about more than 80 wt.% Cu, 4–12 wt.% Pb, less than 10 wt.% Sn, a kind of typical Cu–Sn–Pb bronze. A great deal historical investigation had displayed that Wu Zhu coins were made of a traditional bronze material. Moreover, the elemental compositions are identified by BSE–EDS analyses on the rubbed Wu Zhu samples (Figs. 2 and 3) and are listed in Table 2. In the WZ1 sample (Fig. 2), point 1 and point 3, two whitish islands, are the high lead-rich area containing 87.75 wt.% Pb and 55.94 wt.% Pb, respectively. Point 2 and point 4, two gray islands, are the tin-rich area containing 21% Sn. Both of the whitish islands and the gray islands have exhibited that the excess lead and tin was not dissolved well in the metal solution during the minting of Wu Zhu coin, i.e. the lead and tin did not amalgamate with copper during minting of the coin. Furthermore, a definite amount of antimony (Sb, 4.41–4.86 wt.%) is detected at point 2 and point 4, but no antimony is found at point 1 and point 3. It is indicated that Sb is easy to co-exist with Sn but hard to exist together with Pb. This result might be explained by the difference of Pb– Table 1 Elemental composition of surface of Wu Zhu samples. Samples
Position of analyses
The elementary composition (wt.%) Cu
Sn
Pb
Si
Fe
Al
WZ1
Green crust 1 Green crust 2 Green crust 3 Green crust 4 Deep crust 1 Deep crust 2 Red crust 1 Red crust 2 Red crust 3 Green crust 1 Green crust 2 Deep crust 1 Deep crust 2 Deep crust 3
66.44 79.91 90.14 92.52 83.47 89.27 81.54 74.97 80.48 71.95 83.00 87.86 87.56 82.53
5.10 2.12 2.10 1.85 3.95 2.93 3.11 11.58 12.22 5.11 7.69 3.13 4.56 2.03
22.37 19.28 5.62 4.44 7.64 4.50 6.74 8.63 4.36 19.44 5.94 6.05 5.54 11.47
– – – – – – 0.55 2.15 0.80 0.93 1.15 0.93 0.84 1.43
1.83 1.45 1.70 – 0.86 0.93 7.97 2.13 1.84 1.90 1.91 1.20 1.11 1.87
– – – – – – 0.09 0.54 0.30 0.67 0.32 0.82 0.38 0.67
WZ2
205
Sb and Sn–Sb phase diagrams, which demonstrates that the possibility of co-existence for Sn/Sb phase is more prominent than Pb/Sb phase. Point 5 in Fig. 2 contains 93.35 wt.% Cu and indicates the elemental composition in the substrate of the bronze coin. Point 6, an opaque black one, represents a hole in the substrate. A similar phenomenon is observed in the WZ2 sample (Fig. 3, Table 2). Point 1 and point 6 are two whitish islands with 83–88 wt.% Pb and 10 wt.% Cu, which are the highest lead-rich area and is similar to point 1 and point 3 in the WZ1 sample (Fig. 2). Point 4 represents the bronze substrate and contains 92.02 Cu wt%, that is very similar to point 5 in the WZ1 sample (93.35 wt.%). A gray island at point 5 for the WZ2 sample is the highest tin-content area and contains 5.81 wt.% Sn together with 3.62 wt.% Sb. However, the black island with some white points at point 2 in Fig. 3 contains 10.83 wt.% Fe and 20.4 wt.% S, but very low in lead content (11.00 wt.%). Additionally, the opaque black island at point 3 contains much sulfur (17.07 wt.%), which has indicated that point 3 may be a heavily deteriorated area. It is also found about 10.83 wt.% Fe in the WZ2 sample, indicating this sample a rubefaction surface. The high amount of Fe present in the WZ2 sample is not certainly related with the alloy composition of coins, but may come from the environment (soil or objects buried in the soil). In fact, archeologists have found pieces of tools made of iron around the coins. On the other hand, the source of Fe may come from raw materials, because iron is a common element found in earth and co-exists with almost all other minerals, especially copper-containing minerals. As for minute amounts of Si and Al (less than 1 wt.% each) detected in some of areas investigated in the coins, the possible reason is that it is likely from the efflorescence of the coin surface because it is not detected in the substrate of the concerned coin. It is noteworthy that Sb is found in ancient Chinese round bronze coin for the first time in our work. But the content of Sb is less in substrates for both of coins than in the other areas. If Table 2 is compared with Table 1, it is not hard to see that the analysis results in Table 2 are 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6–2.9 wt.% Sb for the main elemental compositions of coins, whereas in Table 1, the elemental compositions of coins are 83–88 wt.% Cu, 5 wt.% Sn and 5– 10 wt.% Pb. The difference in the two tables is due to the coins studied being quite deteriorated, little amounts of corrosion products are removed from the surface and from the borders in Table 1. In this case, it is possible to obtain the composition of outer layers (Table 1). However, the composition of the inner layer of the coins is obtained in Table 2 and presents 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6– 2.9 wt.% Sb. Comparing the values of Cu in the substrate and in the corrosion products, it may be concluded that the amounts of copper salts in the latter are quite low related to the percentage of copper in the substrate [1]. The comparison result in both tables indicates that no antimony (Sb) is observed in Table 1 for the original samples. This result can be explained by the chemical inertness of antimony which is not liable to corrode in soil and thus it is not involved in the corrosion crust. But in comparing the values of Cu, Sn, Pb and Sb in two coins, the different chemical compositions are found and attributed to metal compatibility during casting of coins, to the large differences in melting points among Cu, Sn, Pb and Sb (Cu: 1083 °C, Sn: 231.9 °C, Pb: 327.5 °C, Sb: 763 °C) and to the limited minting technology used in the West Han Dynasty. Copper solidifies very fast due to its higher melting point. This makes it impossible for both tin and lead to solidify together with copper. As for Sb, it is detected as a constituent in coin substrates but with less content compared with other elements. It is found that antimony is not the main component in this kind of bronze coin but just an appendant from the raw materials of copper and lead, and the most interesting finding is that Sb is rather rich in the tin-rich area of the coins. On the other hand, the elemental compositions of the coin samples are greatly correlated to the compatibility of metals. In this work, the metal compatibility in Wu Zhu coins has been examined by metallographic analysis and SEM observation as shown in Fig. 4a and b. It is found that both WZ1 and WZ2 samples behave as dendrite
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Fig. 2. BSE image and EDS analyses of the WZ1 sample.
structures. The well-proportioned dark islands, an average diameter of 5–10 μm, are scattered on the bronze substrate in the WZ1 sample (Fig. 4a, point 1) and indicate that Pb is not compatible with the bronze substrate. Some corrosive pits with diameters of 1–5 μm and cracks with length of 10–15 μm have shown that the corrosion has already developed into the inner substrate (Fig. 4a). The metallographic analyses in the WZ2 sample reveal that area of the islands are much larger and length of the islands is varying from 28 μm for point 2 to 48 μm for point 4 (Fig. 4b). The SEM images of the WZ1 and WZ2 samples (Fig. 4c and d) have further exposed the metallurgical features of coin, and the lead-rich area is distinctly observed from point 5 of Fig. 4c and point 7 of Fig. 4d, and so does the tin-rich area from point 6 of Fig. 4c. It is clear that the WZ2 sample is much more corroded than the WZ1 sample.
3.2. The corrosion characteristics 3.2.1. The corrosion features in the surface of coin The common corrosion features for both WZ1 and WZ2 samples are shown by stereoscopic microscopic observation that not only the original surface is covered generally by the complicated compounds, but also the coin surfaces are rough with cracks and pits which present multicolor areas of green, red, brown, blue and metallic gray under polarized light, indicating the preliminary chemical composition of the patina. It is basically proved that the Wu Zhu coin samples analyzed were severely corroded. In the WZ1 sample, the coin surface is distributed mainly with the green compact patina (Fig. 5a) and sometimes existed together with the blue patina and mahogany crust (Fig. 5b). However, the coin
Fig. 3. BSE image and EDS analyses of sample WZ2.
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Table 2 Elemental composition of rubbed samples. Samples
WZ1
WZ2
Points of analyses Total Point Point Point Point Point Point Total Point Point Point Point Point Point
Description of points
The elemental composition (wt.%)
1 2 3 4 5 6
Whitish island Gray island Whitish island Gray island Substrate Opaque black island
1 2 3 4 5 6
Whitish island Black island with white points Opaque black island Substrate Gray island Whitish island
Cu
Sn
Pb
Sb
85.39 10.52 70.48 38.45 70.45 93.05 87.73 84.76 10.16 57.77 78.65 92.02 86.15 10.34
6.41 1.72 21.25 2.06 21.61 3.02 7.76 3.25 – – – 1.89 5.81 2.71
4.74 87.76 3.86 55.94 3.00 3.26 2.48 6.39 88.64 11.00 4.28 3.20 3.61 83.42
2.89 – 4.41 – 4.86 0.85 2.12 2.61 – – – 1.24 3.62 –
surface of the WZ2 sample has been mineralized or heavily fossilized, and scattered into the green crystallized patina area (Fig. 5c, much smaller than that in WZ1 sample) interweaving with brown corrosion surface (Fig. 5d). In particular, an altered layer and some interconnected microcracks are observed in the surface for both samples of coin, which are attributed to internal stresses in the corrosion layers, or due to the long-time corrosion caused by the periodic hydration
a
Fe 0.57 – – 2.11 – – – 1.53 – 10.83 – 1.59 0.81 –
Al
Si
O
S
– – – – – – – 0.43 0.86 – – – – –
– – – – – – – – 0.34 – – – – –
– – – – – – – 1.03 – – – – – –
– – – 1.45 – – – – – 20.4 17.07 – – –
and dehydration in soil. In normal case, the extension of the internal layer was accompanied by the incorporation of corrosion species. Furthermore, the detailed corrosion features are characterized by the BSE–EDS analyses in Fig. 6. For the WZ1 sample, high amounts of C, O and Cl, and low amounts of S, Fe, Al, Si, Ca and Mg are found (Table 3). Point 1 in Fig. 6 is a whitish area containing 38.79 wt.% Cu, 30.85 wt.% O and 8.21 wt.% C, which is corresponded to the composition of copper
b 1 4
3 2
c
d
7
5
6
Fig. 4. Metallographic images and SEM images for the rubbed surfaces of the WZ1 and WZ2 samples. Metallographic images of WZ1 sample (a) and sample WZ2 (b); SEM images of sample WZ1 (c) and WZ2 sample (d).
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a
b
c
d
Fig. 5. Stereoscopic observation of patina for sample WZ1 (a and b) and sample WZ2 (c and d).
carbonate. Point 2 and point 3 in Fig. 6 are the gray porous areas and indicates the chloride-rich character with 10.25 wt.% Cl and 11.51 wt.% Cl, respectively. Therefore, it is possible to deduce that this coin is preserved in the chloride-rich environments before excavation (see Section 3.3). The corrosive crust on the surface should be made of the mixture of Cu2(OH)3Cl and others, which has been confirmed by XRD analyses in this paper. Point 4 in Fig. 6 contains 83.49 wt.% Cu, 11.68 wt.% O, 3.13 wt.% C and 1.04 wt.% Cl, which has revealed the presence of Cu2 (OH)3Cl. The WZ2 sample is not discussed again here due to its quite similar result to that of WZ1 sample. The X-RD analyzing results on the scraping powder of corrosive surface of coin are shown in Fig. 7. The light green patina represents
Cu2(OH)3Cl, together with a little of Cu2(OH)2CO3 (Fig. 7a). The main component of the compact green surface is Cu2(OH)2CO3 which coexists with Cu2(OH)3Cl and Pb3O4 (Fig. 7b). The hard and dark compact green patinas represent Cu2(OH)2CO3 (Fig. 7c) and the blue patina is composed of Cu3(CO3)2(OH)2 (azurite), CuSO4·5H2O, Cu2 (OH)2CO3, Cu2(OH)3Cl, CuCl and Cu2O. On the porous or crack surface, the main composition is Cu2(OH)3Cl and Cu3(CO3)2(OH)2. The khaki crust is detected as a mixture of Al2Si2O5(OH)4·H2O, Fe6(PO4)4 (OH)5·H2O, Cu2(OH)3Cl, Cu2(OH)2CO3, Pb3O4, PbCO3, CuS and SiO2. The X-RD analysis results are not only in a good conformity with the EDS analysis results, but also fit the normal patinas in buried archeological objects made of copper alloys, composed of copper, lead
Fig. 6. BSE image and EDS diagrams for the surface of the WZ1 sample.
L. He et al. / Microchemical Journal 99 (2011) 203–212
209
Table 3 Elemental composition of the corrosion surface of the WZ1 sample. Points of analyses
Description of points
The elemental composition (wt.%) Cu
Fe
Al
Si
O
S
C
Ca
Mg
Cl
Point Point Point Point
Light whitish area Gray porous area Black porous area Compact area
38.79 60.93 53.90 83.49
1.05 – – –
2.91 1.64 – 0.26
4.82 2.19 1.45 –
30.85 16.05 25.73 11.68
0.91 0.75 1.14 –
8.21 7.42 5.47 3.13
2.18 – 0.80 0.40
1.38 0.77 – –
– 10.25 11.51 1.04
1 2 3 4
and tin oxides, carbonates, silicates and phosphates [1,3,5,24]. Actually, hydroxides and hydrated compounds of Cu(II) could be formed after the long-term (for more than 1000 years) interaction between the coins and corrosion environment of soil. Cu2(OH)3Cl, a powdered patina that is acknowledged to be harmful to the preservation of bronze coins, is presented in nearly all corrosion products or crack surfaces. Cu2(OH)2CO3 (malachite) and Cu3(CO3)2 (OH)2 (azurite) together with Cu2(OH)3Cl are the main corrosive components of green corrosion products on the surface of the coins. Although it had been reported that uncommon compounds among the corrosion products of bronze coins are chloride and phosphate of lead, Pb8Cu(Si2O7)3, Pb4Al4Si3O16 and Pb5(PO4)3Cl [1], what we have found is that both Pb3O4 and PbCO3 are in the coin surface.
3.2.2. The corrosion behavior in cross section of the coin BSE and SEM images have indicated that thickness of patina on the surfaces of coin varies between 40 μm and 55 μm (Fig. 8). The corrosion layer is well separated from the bronze substrate in Fig. 8a–d. In most cases, the corrosion surface for both of the coin samples shows double layers. The upper-layer is about 25–35 μm in thickness and has been separated from the sub-layer. The sub-layer is about 20–25 μm in thickness and is adhered to the substrate of the coin body. In addition, several perpendicular and parallel cracks are observed in Fig. 8a–d. These cracks caused probably by hydration and dehydration of the corrosion components are served as a starting point for a localized corrosion. Meanwhile, SEM images of the upperlayer indicate a compact surface in the WZ1 sample (Fig. 8a and b) and a porous surface in the WZ2 sample (Fig. 8c and d). In this work, the WZ1 sample is chosen as the example to carry out the detailed BSE–EDS analyses for the corrosion in cross section, and results are shown in Fig. 9 and Table 4. Points 1–4 in Fig. 9 are located in the upper corrosion layer and the amount of copper increases with the corrosion depth from point 1 to point 4, but the content of oxygen and carbon decreases correspondingly. It is illustrated by the fact that the surface of coin contained corrosive products of copper carbonate. From data in Table 4, the main component in point 4 (much more copper, less oxygen and a little chlorine) was Cu2(OH)3Cl, accompanied by Cu inclusion. Point 5 in Fig. 9 is located at the interface of the sub-layer and upper-layer. Compared with point 4, point 5 contains much more sulfur, which indicates that it is perhaps a mixture of CuS (or CuSO4) and Cu2(OH)3Cl. The most important phenomenon is that high amount of chloride anion is found at the interface between the sub-layer and the coin substrate (point 6 in Fig. 9). Actually, the content of chloride anion increase dramatically from the top surface to the inner corrosion layer. The stereoscopic microscopic observation give a clear indication of Cu (II) salts at the interface of the upper-layer and sub-layer, probably paratacamite (Cu2(OH)3Cl, green). This indicates that the formation of the internal corroded layer is linked to an enrichment of the corrosion products of chloride anion from the soil. The presence of high chlorine content at the archeological site (3.98 × 10 − 4 mol L − 1 by HPLC analyses) suggests the occurrence of mass conversion of chlorine from its state in the soil to the metallic phase. 3.3. The corrosion mechanism
Fig. 7. XRD diagram of green corrosion products. (a) The light green corrosion products showing Cu2(OH)3Cl and Cu2(OH)2CO3; (b) the compact green corrosion products showing Cu2(OH)3Cl, Cu2(OH)2CO3 and Pb3O4; (c) the dark green crust showing Cu2 (OH)2CO3 and SiO2.
Evidence of different corrosion behavior of the bronze coins is supported by the surface analysis data and the gathered environmental information. Therefore, a corrosion mechanism of Wu Zhu coin is presented correspondingly according to the analytical result of patina and the buried environment discussed in this paper. The investigation on the soil surrounded coins has shown that the soil is a khaki calcareous sandy soil mixed with organic mineral components derived from domestic and metallurgical residues, such as ashes, charcoal, pottery and construction materials. The density, humidity, pH value and chloride concentration of buried soil were 1.6 g cm− 3, 17.2%, 7.76 and 3.98 × 10− 4 mol L− 1, respectively. The proposed corrosion mechanism for the Wu Zhu coin was elucidated as follows.
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Fig. 8. BSE and SEM images of the corrosion in the cross section in WZ1 sample (a and b) and WZ2 sample (c and d).
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211
Fig. 9. BSE image and EDS analysis of a cross section of the WZ1 sample.
After the Wu Zhu coins are minted, Cu2O and CuO are formed on the surface of coin due to the oxidization. 4Cu + O2 = 2Cu2 O; 2Cu + O2 = 2CuO When the coin is buried under soil for more than a thousand years, water and chloride ions (3.98 × 10 − 4 mol L − 1 in the soil) are two key factors for its corrosion. The porosity of oxidized compound formed on the surface of Wu Zhu coin (Cu2O, CuO) has provided the possibility for water movement and chloride ions activity, especially in the archeological soil for pH value around 7.76. Cu2 O + 2Cl− + 2Hþ = 2CuCl + H2 O Cu2 O + H2 O + CO2 + O2 = Cu2 ðOHÞ2 CO3 ðmalachiteÞ 2Cu2 O + 2H2 O + 2CO2 + O2 = Cu3 ðCO3 Þ2 ðOHÞ2 ðazuriteÞ 2Cu2 O + H2 O + HCl + O2 = CuCl2 ⋅3CuðOHÞ2 ðdisease corrosionÞ It should be pointed out that soil is of most importance in the formation of the final patina on the surface of coins. During the corrosion process, the chloride ion is enriched in the interface of corrosion layer and the coin body, as discussed in the section of corrosion behavior in the cross section of coin. In addition, Xin River flows along the site from north to south and the archeological site is located at the area between the maximum temperature 42 °C and the minimum temperature −15 °C, where this environmental condition accelerates the corrosion action by the movement of water and chloride ion. It is just by reason of corrosion that both of the original coin surfaces are distributed with nub and pits. This nub and pits should be attributed not only to the discoloration of the original surface, but also to the formation of patina on the surfaces of coins. The pits occur when soluble corrosion products are washed away, but the nub is formed by the sedimentation of other corrosion products on the surface of coins. Both pits and nubs lead to a loss in esthetic quality of the coin.
4. Conclusions The intensive analysis results in this work have shown that Wu Zhu coins are made of bronze materials. The elemental compositions are 84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6–2.9 wt.% Sb. The element of Sb is observed for the first time in ancient Chinese round bronze coins. The detected amounts of Cu, Sn and Pb are in good conformity with the information gathered through the historical survey. The lead-rich and tin-rich areas in coin samples are discovered and have demonstrated the poor metal compatibility during minting. The analyses of corrosion surface have indicated that the corrosion surface is composed of two layers, the sub-layer (20–25 μm) is adhered to the substrate of the coin and the upper-layer (25–35 μm) is separated from the sub-layer. It has been proved in this work that the high content of chloride ion is measured at the interface of the sub-corrosion layer and the body of the coin, that the amount of Cl in the cross section of coin increases dramatically from the outer layer to the inner layer, and that the thickness of the patina varies from 40 μm to 55 μm depending on the position of coin. Cracks are also observed on the surface of coin samples and some of them have developed into the substrate of the coin. It is shown from the experimental data that the patina is mainly consisted of Cu2(OH)3Cl, Cu2(OH)2CO3, Cu3(CO3)2 (OH)2 and Pb3O4. In some part on the surface of coin, the crust is analyzed as a mixture of the corroded component above. The appearance of the bronze coin is consistent with the analyzed corrosion patina. Water and chloride ion are regarded as two of the most powerful corrosion agents for Wu Zhu coins due to the chloride ions enriched in the interface of the coin body and corrosion layer.
Acknowledgments This work was supported in part by the National Natural Science Foundation of China (NSFC Grants No. 50872143, and No. 20673081),
Table 4 Elemental composition of a cross section of the WZ1 sample. Points of analyses
The elemental composition (wt.%) Cu
Sn
Pb
Sb
Fe
O
S
C
Ca
Mg
Cl
Point Point Point Point Point Point
33.07 33.88 43.85 82.20 66.23 10.21
– – – – – 3.01
– – – – – 30.70
– – – – – 14.42
– – – – 0.55 0.62
51.40 50.88 47.58 15.44 9.02 22.51
– – – – 22.46 –
12.80 10.24 8.57 – – –
– – – – – –
1.76 – – – – –
– – – 2.36 1.74 18.53
1 2 3 4 5 6
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