Microchemical Journal 99 (2011) 203–212

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Microchemical Journal

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Corrosion behavior and morphological features of archeological bronze coins fromancient China

Ling He a, Junyan Liang a, Xiang Zhao a,⁎, Baolian Jiang b

a Department of Chemistry, School of Science, Xi'an Jiaotong University, Xi'an 710049, Chinab Xi'an Center for the Conservation and Restoration of Cultural Heritage, Xi'an, 710061, China

⁎ Corresponding author. Tel.: +86 29 8266 5671; faxE-mail address: xzhao@mail.xjtu.edu.cn (X. Zhao).

0026-265X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.microc.2011.05.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 April 2011Received in revised form 7 May 2011Accepted 8 May 2011Available online 14 May 2011

Keywords:Bronze coinCorrosionPatinaMorphologySEMX-RD

The archeological round bronze coins, nominated asWu Zhou and regarded as the first issued effective moneyin 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 andenergy dispersive spectrometry (EDS) techniques. It is revealed morphologically that there are some roughsurface cracks, pits, and multicolor patina on the surface of the coins. We prove that the coins are made frombronze 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.% Pband 2.6–2.9 wt.% Sb, and covered by two corrosion layers, 25–35 μm for the upper-layer and 20–25 μm for thesub-layer. High chloride content has been detected at the interface between the sub-layer and body of thecoins. The lead-rich and tin-rich areas in the coin samples indicate the poor metal compatibility duringminting 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.

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1. Introduction

The study of chemical compositions and corrosion features onbronze art objects is greatly important to explore the corrosion behaviorand to achieve the aim in the preservation to the of bronze art objects[1]. In the most cases, corrosion of archeological bronze is extensivelystudied through the analyses of corrosion products with two aims:(1) to clarify the morphological corrosion phenomena occurring onbronze surfacewhen it is exposed todifferent corrosivemedia and (2) toprevent corrosion processes for preservation of the cultural heritage [2].Our investigation on the archeological Wu Zhu coins used in the WestHan Dynasty of ancient China is expected to be significant on bothcorrosion science and the preservation of cultural heritage.

Wu Zhu coin is the first currency round coin issued by the centralgovernment in ancient China. It has been archeologically proven that thecoin had greatly promoted the development of social economy andpolitics since the Western Han Dynasty from B.C. 206, and its castingtechnique stood for the highest minting level in ancient China. As onekind of old currency coin, Wu Zhu coin is one of the most typical roundbronze coinswhichhad been circulatedmore than 1000 years. The coin,with a square hole at the center for bunchingup together, is named aftertwo Chinese characters “Wu” (means five) and “Zhu” (means baht) onthe surface of the coin, which represents the monetary unit in ancient

China (Fig. 1) and is similar to five pence of an English coin. From theobservation of facade, the excavated coin has been buried under soilover a century before unearth and has been eroded heavily. To ourlimited knowledge, neither the chemical composition for Wu Zhu coinsnor the patina characteristics on coin surface has been undertaken toscientific analyses so far. Therefore, a detailed analysis of Wu Zhu coinsin order to gain intensive information of chemical composition, natureof the patina and corrosion features on themorphological coin surface isgreatly important to explore the corrosion mechanism of bronze coinand to achieve a goal of the ultimate preservation, so our investigationon archeological coins is very important and clearly a challengework onboth corrosion science and cultural heritage.

It is reported that common features of the passive corrosion layer onvarious ancient bronze objects, known as noble patina, has beenanalyzed and highlighted as presenting a double-structured layerconsisting of an inner layer of copper (I) salts [3–5] and an externallayer of copper (II) salts, which depended on the history and theelemental composition of ancient bronze object [6–9]. It is proved thatthe “passive corrosion layer” is createdunder amild corrosioncondition,but the “coarse” surface is generally assigned to severe corrosionconditions, often in the presence of chloride anion [10]. The so-calledbronze disease corrosion is due to chloride anions fromcuprous chloridein thepatinas [11,12]. On theother hand, it is generally accepted that thecorrosion surface layer is constituted of Cu (II) salts such as malachite(CuCl2·3Cu(OH)2, in soil), brochantite (CuSO4·3Cu(OH)2, in theatmosphere) or atacamite (CuCO3·Cu(OH)2, in seawater) [13,14].Recent studies have reported the characterization of bronze corrosion

Fig. 1. Sample WZ1 and WZ2, the right word is “Wu” and the left word is “Zhu”.

204 L. He et al. / Microchemical Journal 99 (2011) 203–212

through the analyses of the corrosion products formed on the bronzesurface by scanning electron microscopy (SEM) coupled with energydispersive spectrometry (SEM–EDS) [15,16], X-ray photoelectronspectroscopy (XPS) [17,18], energy dispersive X-ray fluorescence(EDXRF) spectrometry and X-ray diffraction (X-RD) [19]. It is alsopointed out that the backscattered electron (BSE) imaging is an effectiveapproach to characterize the corrosion section of bronze objects [20].However, even though some reports have described the corrosionphenomena and natural patinas formed on archeological bronzeartwork [21,22], few corrosion mechanism have been proposed toexplain the formation of bronzepatinas in the caseof theburied artworkin soil so far. Additionally, although some studies on the archeologicalbronze samples have predicted that the chloride anion was one of themajor causes for bronze corrosion in soil [23], the distribution ofchloride anion in the corrosive section of bronze object is somehowneglected.

In the present work, analysis is focused on two Wu Zhu coinsselected from among about 30-coin-collection during the excavationsperformed at the national archeological site. Our aim is to understandthe chemical compositions of two Wu Zhu coins, to characterize thecorrosion features and to explore the nature of the patina on the coinsurface by means of optical microscopy (OM), scanning electronmicroscopy (SEM) equipped with backscattered electron (BSE)detector and energy dispersive spectrometry (EDS), and X-raydiffraction (X-RD). The distribution of chloride content in the crosssection of coin is determined for the first time. The proposed corrosionmechanism is discussed based on the intensive experimental data.

2. Experimental

2.1. Description of archeological site and samples

The analyzing sample ofWu Zhu coins were excavated from 100 to200 cm underground during the archeological investigation in 2006on the famous Zhongguan minting site, 25 km west from Xi'an city(the ancient Chinese Capital) in the western area of China. The totalarea of the site covers about more than 900,000 m2, measuring about600 m from east to west and 1500 m from south to north, and is foundas the biggest one in ancient China up to now. A river (named as XinRiver) flows along the site from north to south. The site area has asemi-dry continental climate and the annual rainfall is 659 mm. Therain season is mainly from early July to late September, and theaverage annual temperature is between 13.5 °C and 15.4 °C with themaximum of 42 °C and the minimum of −15 °C. The annual mean ofrelative humidity is circa 60%, increase to 75% in October.

It has been found that the surface of Wu Zhu coin was covered bygreen and red (or yellowish) products after being unearthed from the

soil, and the corrosion layer was formed on the surface of coin with asubmillimeter thickness. The coin is about 2.5–2.6 cm in diameter andaround 3.32–3.81 g in weight. The square hole at the coin center is0.9–1.0 cm in width. The slight difference in diameter and weight wasdue to its corrosion. Two typical coins (WZ1 and WZ2) from thearcheological site were selected in this work and served as theanalysis samples (Fig. 1).

2.2. Characterization approaches

After mounting in epoxy resin, the coin sample was mechanicallyrubbed for the metallographic examination by metallographicmicroscope. An HI-Scope Advance KH-3000 stereo microscope with700–1400 times magnification was used for the preliminary obser-vation of the corrosion products on the coin surface. The aim ofmetallographic examination was to reveal the microstructures of thecoin samples and provide the direct insight into the compatibility ofdifferent metals employed in Wu Zhu coins.

SEM images were obtained on Cambridge S-360 Scanning ElectronMicroscope (Cambridge Technology, Cambridge, England) with anaccelerating voltage of 25 kV. The morphology of the corrosion layersand the rubbed surfaces were analyzed by SEM measurements. Energydispersive spectrometry (NSS-300 EDS) coupled to the SEM wasacquired from various concerned points on the specimen in order tostudy the main elemental composition in the corrosion layers and therubbed surfaces. The EDS analysis was performed with a lifetime of100 s, a pulse-counting rate of about 4000 cps, a working distance of23 mmand an accelerating voltage of 20 kV. Standard samples providedby NSS-300 EDS were used for quantifying the elements to be detectedwith about 2% relative error. The information about compositionaldistributions in the rubbed surfaces and in the cross section of coin wasfurther provided by the BSE imaging measurements.

XRD was used to characterize the nature of patina on the coinsurface. The powder samples, taken mechanically by scraping thecorroded coin surfaces gently with a very fine tungsten needle, wasgrounded finely in an agate mortar and pressed into the specimenholder, and thenmounted inaPhilips 1015X-raydiffraction instrument.The measuring conditions were set as follows: Cu target, 40 kVaccelerating voltage, 40 mA current. The scanning range of 2θ wasfrom 0° to 80° with the scanning speed was 3°/min. The XRD data wereanalyzed using Difrac software (Bruker Advanced X-ray Solutions,Germany) and with the JCPDF database.

2.3. Investigation of the soil

The pHvalue and concentration of chloride anion in soil surroundingthe coin are measured by IQ 150 pH instrument in site and a LC-2010C

205L. He et al. / Microchemical Journal 99 (2011) 203–212

high performance liquid chromatography (HPLC, SHIMADZU, Japan),respectively. It is revealed archeologically that the soil was a khakicalcareous sandy soil with pH value of 6.5–7.1, which was mixed withorganic mineral components derived from domestic residues andmetallurgical activities, suchasashes, charcoal, potteryandconstructionmaterials. The data of groundwater level and climate informationobtained from the local weather bureau have indicated that thegroundwater 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 theoriginal samples by SEM–EDX and on the rubbed samples by BSE–EDX. The analysis results of original samples by SEM–EDX are listed inTable 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. Theelemental compositions are 83–88 wt.% Cu, 5–10 wt.% Pb, and lessthan 5 wt.% Sn, which indicated that the coins were shaped in theperiod of West Han Dynasty, compared to the historical investigation.In ancient China, the bronze coins in different shape were first comeinto use in the Shangdynasty (B.C. 1766–B.C. 1122)with about 39 wt.%Pb. During the Spring and Autumn (Chun-Qiu) and theWarring States(Zhan-Guo) periods (B.C. 770–B.C. 221), bronze coins (with sometypical geometrical shapes) had contained more than 60 wt.% Cu,20 wt.% Pb, and less than 10 wt.% Sn. The elemental compositions ofcoins circulated from Qin to Sui times (B.C. 221–A.D. 618) weredetected 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 historicalinvestigation had displayed that Wu Zhu coins were made of atraditional bronze material.

Moreover, the elemental compositions are identified by BSE–EDSanalyses on the rubbedWu Zhu samples (Figs. 2 and 3) and are listed inTable 2. In the WZ1 sample (Fig. 2), point 1 and point 3, two whitishislands, are the high lead-rich area containing 87.75 wt.% Pb and55.94 wt.% Pb, respectively. Point 2 and point 4, two gray islands, are thetin-rich area containing 21% Sn. Both of the whitish islands and the grayislands have exhibited that the excess lead and tin was not dissolvedwell in themetal solutionduring themintingofWuZhucoin, i.e. the leadand tin did not amalgamate with copper during minting of the coin.Furthermore, a definite amount of antimony (Sb, 4.41–4.86 wt.%) isdetected at point 2 and point 4, but no antimony is found at point 1 andpoint 3. It is indicated that Sb is easy to co-existwith Sn but hard to existtogetherwith Pb. This resultmight be explained by the difference of Pb–

Table 1Elemental composition of surface of Wu Zhu samples.

Samples Position ofanalyses

The elementary composition (wt.%)

Cu Sn Pb Si Fe Al

WZ1 Green crust 1 66.44 5.10 22.37 – 1.83 –

Green crust 2 79.91 2.12 19.28 – 1.45 –

Green crust 3 90.14 2.10 5.62 – 1.70 –

Green crust 4 92.52 1.85 4.44 – – –

Deep crust 1 83.47 3.95 7.64 – 0.86 –

Deep crust 2 89.27 2.93 4.50 – 0.93 –

WZ2 Red crust 1 81.54 3.11 6.74 0.55 7.97 0.09Red crust 2 74.97 11.58 8.63 2.15 2.13 0.54Red crust 3 80.48 12.22 4.36 0.80 1.84 0.30Green crust 1 71.95 5.11 19.44 0.93 1.90 0.67Green crust 2 83.00 7.69 5.94 1.15 1.91 0.32Deep crust 1 87.86 3.13 6.05 0.93 1.20 0.82Deep crust 2 87.56 4.56 5.54 0.84 1.11 0.38Deep crust 3 82.53 2.03 11.47 1.43 1.87 0.67

Sb and Sn–Sb phase diagrams, which demonstrates that the possibilityof 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 elementalcomposition in the substrate of the bronze coin. Point 6, an opaqueblackone, 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 similarto point 1 and point 3 in the WZ1 sample (Fig. 2). Point 4 representsthe bronze substrate and contains 92.02 Cu wt%, that is very similar topoint 5 in the WZ1 sample (93.35 wt.%). A gray island at point 5 fortheWZ2 sample is the highest tin-content area and contains 5.81 wt.%Sn together with 3.62 wt.% Sb. However, the black island with somewhite 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 opaqueblack island at point 3 contains much sulfur (17.07 wt.%), which hasindicated that point 3 may be a heavily deteriorated area.

It is also found about 10.83 wt.% Fe in the WZ2 sample, indicatingthis sample a rubefaction surface. The high amount of Fe present inthe WZ2 sample is not certainly related with the alloy composition ofcoins, but may come from the environment (soil or objects buried inthe soil). In fact, archeologists have found pieces of tools made of ironaround the coins. On the other hand, the source of Fe may come fromraw materials, because iron is a common element found in earth andco-exists with almost all other minerals, especially copper-containingminerals. As for minute amounts of Si and Al (less than 1 wt.% each)detected in some of areas investigated in the coins, the possible reasonis that it is likely from the efflorescence of the coin surface because it isnot detected in the substrate of the concerned coin.

It is noteworthy that Sb is found in ancient Chinese round bronzecoin for the first time in our work. But the content of Sb is less insubstrates for both of coins than in theother areas. If Table 2 is comparedwith Table 1, it is not hard to see that the analysis results in Table 2 are84.8–85.4 wt.% Cu, 3.3–6.1 wt.% Sn, 4.7–6.4 wt.% Pb and 2.6–2.9 wt.% Sbfor the main elemental compositions of coins, whereas in Table 1, theelemental 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 studiedbeing quite deteriorated, little amounts of corrosion products areremoved from the surface and from the borders in Table 1. In this case, itis 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 andpresents 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 thecorrosionproducts, itmaybe concluded that theamounts of copper saltsin the latter are quite low related to the percentage of copper in thesubstrate [1]. The comparison result in both tables indicates that noantimony (Sb) is observed in Table 1 for the original samples. This resultcan be explained by the chemical inertness of antimony which is notliable 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, thedifferent chemical compositions are found and attributed to metalcompatibility during casting of coins, to the large differences in meltingpoints among Cu, Sn, Pb and Sb (Cu: 1083 °C, Sn: 231.9 °C, Pb: 327.5 °C,Sb: 763 °C) and to the limitedminting technology used in theWest HanDynasty. Copper solidifies very fast due to its higher melting point. Thismakes it impossible for both tin and lead to solidify together withcopper. As for Sb, it is detected as a constituent in coin substrates butwith less content compared with other elements. It is found thatantimony is not themain component in this kind of bronze coin but justan appendant from the rawmaterials of copper and lead, and the mostinterestingfinding is that Sb is rather rich in the tin-richareaof the coins.

On the other hand, the elemental compositions of the coin samplesare greatly correlated to the compatibility of metals. In this work, themetal compatibility in Wu Zhu coins has been examined bymetallographic analysis and SEM observation as shown in Fig. 4aand b. It is found that both WZ1 andWZ2 samples behave as dendrite

Fig. 2. BSE image and EDS analyses of the WZ1 sample.

206 L. He et al. / Microchemical Journal 99 (2011) 203–212

structures. The well-proportioned dark islands, an average diameterof 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 thebronze substrate. Some corrosive pits with diameters of 1–5 μm andcracks with length of 10–15 μm have shown that the corrosion hasalready developed into the inner substrate (Fig. 4a). The metallo-graphic analyses in the WZ2 sample reveal that area of the islands aremuch larger and length of the islands is varying from 28 μm for point 2to 48 μm for point 4 (Fig. 4b). The SEM images of the WZ1 and WZ2samples (Fig. 4c and d) have further exposed the metallurgicalfeatures of coin, and the lead-rich area is distinctly observed frompoint 5 of Fig. 4c and point 7 of Fig. 4d, and so does the tin-rich areafrom point 6 of Fig. 4c. It is clear that the WZ2 sample is much morecorroded than the WZ1 sample.

Fig. 3. BSE image and EDS a

3.2. The corrosion characteristics

3.2.1. The corrosion features in the surface of coinThe common corrosion features for both WZ1 and WZ2 samples

are shown by stereoscopic microscopic observation that not only theoriginal surface is covered generally by the complicated compounds,but also the coin surfaces are rough with cracks and pits whichpresent multicolor areas of green, red, brown, blue and metallic grayunder polarized light, indicating the preliminary chemical composi-tion of the patina. It is basically proved that the Wu Zhu coin samplesanalyzed were severely corroded.

In the WZ1 sample, the coin surface is distributed mainly with thegreen compact patina (Fig. 5a) and sometimes existed together withthe blue patina and mahogany crust (Fig. 5b). However, the coin

nalyses of sample WZ2.

Table 2Elemental composition of rubbed samples.

Samples Points ofanalyses

Description of points The elemental composition (wt.%)

Cu Sn Pb Sb Fe Al Si O S

WZ1 Total 85.39 6.41 4.74 2.89 0.57 – – – –

Point 1 Whitish island 10.52 1.72 87.76 – – – – – –

Point 2 Gray island 70.48 21.25 3.86 4.41 – – – – –

Point 3 Whitish island 38.45 2.06 55.94 – 2.11 – – – 1.45Point 4 Gray island 70.45 21.61 3.00 4.86 – – – – –

Point 5 Substrate 93.05 3.02 3.26 0.85 – – – – –

Point 6 Opaque black island 87.73 7.76 2.48 2.12 – – – – –

WZ2 Total 84.76 3.25 6.39 2.61 1.53 0.43 – 1.03 –

Point 1 Whitish island 10.16 – 88.64 – – 0.86 0.34 – –

Point 2 Black island with white points 57.77 – 11.00 – 10.83 – – – 20.4Point 3 Opaque black island 78.65 – 4.28 – – – – – 17.07Point 4 Substrate 92.02 1.89 3.20 1.24 1.59 – – – –

Point 5 Gray island 86.15 5.81 3.61 3.62 0.81 – – – –

Point 6 Whitish island 10.34 2.71 83.42 – – – – – –

207L. He et al. / Microchemical Journal 99 (2011) 203–212

surface of the WZ2 sample has been mineralized or heavily fossilized,and scattered into the green crystallized patina area (Fig. 5c, muchsmaller than that inWZ1 sample) interweaving with brown corrosionsurface (Fig. 5d). In particular, an altered layer and some inter-connectedmicrocracks are observed in the surface for both samples ofcoin, which are attributed to internal stresses in the corrosion layers,or due to the long-time corrosion caused by the periodic hydration

1

5

6

a b

c d

Fig. 4.Metallographic images and SEM images for the rubbed surfaces of the WZ1 andWZ2 ssample WZ1 (c) and WZ2 sample (d).

and dehydration in soil. In normal case, the extension of the internallayer was accompanied by the incorporation of corrosion species.

Furthermore, thedetailed corrosion features are characterizedby theBSE–EDS analyses in Fig. 6. For the WZ1 sample, high amounts of C, Oand 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.% Oand 8.21 wt.%C, which is corresponded to the composition of copper

3

2

7

4

amples. Metallographic images of WZ1 sample (a) and sample WZ2 (b); SEM images of

a

d

b

c

Fig. 5. Stereoscopic observation of patina for sample WZ1 (a and b) and sample WZ2 (c and d).

208 L. He et al. / Microchemical Journal 99 (2011) 203–212

carbonate. Point 2 and point 3 in Fig. 6 are the gray porous areas andindicates the chloride-rich character with 10.25 wt.% Cl and 11.51 wt.%Cl, respectively. Therefore, it is possible to deduce that this coin ispreserved in the chloride-rich environments before excavation (seeSection 3.3). The corrosive crust on the surface should be made of themixture of Cu2(OH)3Cl and others, which has been confirmed by XRDanalyses in this paper. Point4 in Fig. 6 contains83.49 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 quitesimilar result to that of WZ1 sample.

The X-RD analyzing results on the scraping powder of corrosivesurface of coin are shown in Fig. 7. The light green patina represents

Fig. 6. BSE image and EDS diagrams fo

Cu2(OH)3Cl, together with a little of Cu2(OH)2CO3 (Fig. 7a). The maincomponent of the compact green surface is Cu2(OH)2CO3 whichcoexists with Cu2(OH)3Cl and Pb3O4 (Fig. 7b). The hard and darkcompact green patinas represent Cu2(OH)2CO3 (Fig. 7c) and the bluepatina 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 khakicrust 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 withthe EDS analysis results, but also fit the normal patinas in buriedarcheological objects made of copper alloys, composed of copper, lead

r the surface of the WZ1 sample.

Table 3Elemental composition of the corrosion surface of the WZ1 sample.

Points ofanalyses

Description ofpoints

The elemental composition (wt.%)

Cu Fe Al Si O S C Ca Mg Cl

Point 1 Light whitish area 38.79 1.05 2.91 4.82 30.85 0.91 8.21 2.18 1.38 –

Point 2 Gray porous area 60.93 – 1.64 2.19 16.05 0.75 7.42 – 0.77 10.25Point 3 Black porous area 53.90 – – 1.45 25.73 1.14 5.47 0.80 – 11.51Point 4 Compact area 83.49 – 0.26 – 11.68 – 3.13 0.40 – 1.04

209L. He et al. / Microchemical Journal 99 (2011) 203–212

and tin oxides, carbonates, silicates and phosphates [1,3,5,24].Actually, hydroxides and hydrated compounds of Cu(II) could beformed after the long-term (for more than 1000 years) interactionbetween the coins and corrosion environment of soil. Cu2(OH)3Cl, apowdered patina that is acknowledged to be harmful to thepreservation of bronze coins, is presented in nearly all corrosionproducts or crack surfaces. Cu2(OH)2CO3 (malachite) and Cu3(CO3)2(OH)2 (azurite) together with Cu2(OH)3Cl are the main corrosivecomponents of green corrosion products on the surface of the coins.Although it had been reported that uncommon compounds amongthe corrosion products of bronze coins are chloride and phosphate oflead, Pb8Cu(Si2O7)3, Pb4Al4Si3O16 and Pb5(PO4)3Cl [1], what we havefound is that both Pb3O4 and PbCO3 are in the coin surface.

Fig. 7. XRD diagram of green corrosion products. (a) The light green corrosion productsshowing Cu2(OH)3Cl and Cu2(OH)2CO3; (b) the compact green corrosion productsshowing Cu2(OH)3Cl, Cu2(OH)2CO3 and Pb3O4; (c) the dark green crust showing Cu2

(OH)2CO3 and SiO2.

3.2.2. The corrosion behavior in cross section of the coinBSE and SEM images have indicated that thickness of patina on the

surfaces of coin varies between 40 μm and 55 μm (Fig. 8). The corrosionlayer is well separated from the bronze substrate in Fig. 8a–d. In mostcases, the corrosion surface for both of the coin samples shows doublelayers. The upper-layer is about 25–35 μm in thickness and has beenseparated from the sub-layer. The sub-layer is about 20–25 μm inthickness and is adhered to the substrate of the coin body.

In addition, several perpendicular and parallel cracks are observedin Fig. 8a–d. These cracks caused probably by hydration anddehydration of the corrosion components are served as a startingpoint for a localized corrosion. Meanwhile, SEM images of the upper-layer indicate a compact surface in theWZ1 sample (Fig. 8a and b) anda porous surface in the WZ2 sample (Fig. 8c and d).

In this work, theWZ1 sample is chosen as the example to carry outthe detailed BSE–EDS analyses for the corrosion in cross section, andresults are shown in Fig. 9 and Table 4. Points 1–4 in Fig. 9 are locatedin the upper corrosion layer and the amount of copper increases withthe corrosion depth from point 1 to point 4, but the content of oxygenand carbon decreases correspondingly. It is illustrated by the fact thatthe surface of coin contained corrosive products of copper carbonate.From data in Table 4, the main component in point 4 (much morecopper, less oxygen and a little chlorine) was Cu2(OH)3Cl, accompa-nied by Cu inclusion. Point 5 in Fig. 9 is located at the interface of thesub-layer and upper-layer. Compared with point 4, point 5 containsmuch 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 chlorideanion is found at the interface between the sub-layer and the coinsubstrate (point 6 in Fig. 9). Actually, the content of chloride anionincrease dramatically from the top surface to the inner corrosionlayer. The stereoscopic microscopic observation give a clear indicationof Cu (II) salts at the interface of the upper-layer and sub-layer,probably paratacamite (Cu2(OH)3Cl, green). This indicates that theformation of the internal corroded layer is linked to an enrichment ofthe corrosion products of chloride anion from the soil. The presence ofhigh chlorine content at the archeological site (3.98×10−4 mol L−1

by HPLC analyses) suggests the occurrence of mass conversion ofchlorine from its state in the soil to the metallic phase.

3.3. The corrosion mechanism

Evidence of different corrosion behavior of the bronze coins issupported by the surface analysis data and the gathered environ-mental information. Therefore, a corrosion mechanism of Wu Zhucoin is presented correspondingly according to the analytical result ofpatina and the buried environment discussed in this paper.

The investigation on the soil surrounded coins has shown that thesoil is a khaki calcareous sandy soil mixed with organic mineralcomponents derived from domestic and metallurgical residues, such asashes, charcoal, pottery and construction materials. The density,humidity, pH value and chloride concentration of buried soil were1.6 g cm−3, 17.2%, 7.76 and 3.98×10−4 mol L−1, respectively. Theproposed corrosion mechanism for the Wu Zhu coin was elucidated asfollows.

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).

210 L. He et al. / Microchemical Journal 99 (2011) 203–212

Fig. 9. BSE image and EDS analysis of a cross section of the WZ1 sample.

211L. He et al. / Microchemical Journal 99 (2011) 203–212

After the Wu Zhu coins are minted, Cu2O and CuO are formed onthe surface of coin due to the oxidization.

4Cu + O2 = 2Cu2O; 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 keyfactors for its corrosion. The porosity of oxidized compound formedon the surface of Wu Zhu coin (Cu2O, CuO) has provided thepossibility for water movement and chloride ions activity, especiallyin the archeological soil for pH value around 7.76.

Cu2O + 2Cl− + 2Hþ = 2CuCl + H2OCu2O + H2O + CO2 + O2 = Cu2 OHð Þ2CO3 malachiteð Þ2Cu2O + 2H2O + 2CO2 + O2 = Cu3 CO3ð Þ2 OHð Þ2 azuriteð Þ2Cu2O + H2O + HCl + O2 = CuCl2⋅3Cu OHð Þ2 disease corrosionð Þ

It should be pointed out that soil is of most importance in theformationof thefinal patinaon the surfaceof coins. During the corrosionprocess, the chloride ion is enriched in the interface of corrosion layerand the coin body, as discussed in the section of corrosion behavior inthe cross section of coin.

In addition,XinRiverflows along the site fromnorth to southand thearcheological site is located at the area between the maximumtemperature 42 °C and the minimum temperature −15 °C, where thisenvironmental condition accelerates the corrosion action by themovement of water and chloride ion. It is just by reason of corrosionthat both of the original coin surfaces are distributed with nub and pits.This nub and pits should be attributed not only to the discoloration ofthe original surface, but also to the formation of patina on the surfaces ofcoins. The pits occurwhen soluble corrosion products arewashed away,but the nub is formed by the sedimentation of other corrosion productson the surface of coins. Both pits and nubs lead to a loss in estheticquality of the coin.

Table 4Elemental composition of a cross section of the WZ1 sample.

Points ofanalyses

The elemental composition (wt.%)

Cu Sn Pb Sb Fe

Point 1 33.07 – – – –

Point 2 33.88 – – – –

Point 3 43.85 – – – –

Point 4 82.20 – – – –

Point 5 66.23 – – – 0.55Point 6 10.21 3.01 30.70 14.42 0.62

4. Conclusions

The intensive analysis results in this work have shown thatWu Zhucoins are made of bronze materials. The elemental compositions are84.8–85.4 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 roundbronze coins. The detected amounts of Cu, Sn and Pb are in goodconformitywith the information gathered through the historical survey.The lead-rich and tin-rich areas in coin samples are discovered and havedemonstrated the poor metal compatibility during minting.

The analyses of corrosion surface have indicated that the corrosionsurface is composed of two layers, the sub-layer (20–25 μm) isadhered 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 thatthe high content of chloride ion is measured at the interface of thesub-corrosion layer and the body of the coin, that the amount of Cl inthe cross section of coin increases dramatically from the outer layer tothe inner layer, and that the thickness of the patina varies from 40 μmto 55 μm depending on the position of coin. Cracks are also observedon the surface of coin samples and some of them have developed intothe substrate of the coin. It is shown from the experimental data thatthe 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 isanalyzed as a mixture of the corroded component above. Theappearance of the bronze coin is consistent with the analyzedcorrosion patina. Water and chloride ion are regarded as two of themost powerful corrosion agents for Wu Zhu coins due to the chlorideions enriched in the interface of the coin body and corrosion layer.

Acknowledgments

This work was supported in part by the National Natural ScienceFoundation of China (NSFC Grants No. 50872143, and No. 20673081),

O S C Ca Mg Cl

51.40 – 12.80 – 1.76 –

50.88 – 10.24 – – –

47.58 – 8.57 – – –

15.44 – – – – 2.369.02 22.46 – – – 1.74

22.51 – – – – 18.53

212 L. He et al. / Microchemical Journal 99 (2011) 203–212

and partially supported by National Key Project on Basic Research forScience & Technology of China (Grant No. 2010BAK67B12).

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