Tarnishing and Pitting Corrosion Mechanism Revealed by Nanoparticles on the Gold Foil of Western Han Dynasty in Taiyuan, Shanxi, China through a Multi-analytical Approach

Tarnishing and Pitting Corrosion Mechanism Revealed by Nanoparticles on the Gold Foil of Western Han Dynasty in Taiyuan, Shanxi, China through a Multi-analytical Approach | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Tarnishing and Pitting Corrosion Mechanism Revealed by Nanoparticles on the Gold Foil of Western Han Dynasty in Taiyuan, Shanxi, China through a Multi-analytical Approach Zisang Gong, Siyuan Sun, Pei Hu, Jingrong Pei, Gang Hu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5316041/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 27 Mar, 2025 Read the published version in npj Heritage Science → Version 1 posted 7 You are reading this latest preprint version Abstract To figure out optical and corrosion mechanism of tarnishing and pitting corrosion of the archaeological gold foil, ESEM-EDS, XRD, XPS were used to obtain chemical composition of different depths and observe surface topography. The results show Ag is enriched on the surface, mainly as Ag 2 O nanoparticles, and Au is enriched internally due to the selective corrosion of Ag. Optical mechanism of gold tarnishing is surface plasmon resonance (SPR) of Ag 2 O nanoparticles. The colour darkens as nanoparticle size increases. Flaming in the atmosphere results in accelerated oxidation of Ag and edge-off, leading to electrochemical crevice, pitting corrosion and perforation during burial. Gold foil tarnishing pitting corrosion ESEM-EDS XPS XRD SPR nanoparticle Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 10 Figure 11 Figure 12 Figure 13 1 Introduction Gold is almost the most corrosion-resistant metal in nature. Due to its stable chemical properties and shiny appearance, gold has been refined and fabricated into artefacts for a long time in different cultures. Since archaeological gold artefacts are usually not pure gold, tarnishing is a common phenomenon. However, the colours of tarnishing and corrosion products are not always the same, depending on the components of alloys and the environments[ 1 – 9 ]. Seven pieces of gold foils were unearthed on the surface of the lacquerware from Chamber 2 of the tomb 2018TJLSTM4, which is located at the burial area of Jin Yang (晋阳) ancient city site, Taiyuan (太原) city, Shanxi Province, central China and is thought to be from the late Western Han Dynasty according to the burial form. The lacquerware was crushed during burial, leading to the separation of the gold foils from the lacquerware. The lacquerware is a round lacquer trousseau with silver persimmon on top of the lid. All seven pieces of gold foils are drawn with black paint and some with red paint[ 10 ]. Although adhered with sandy soil, the surface is still bright gold, except for one piece depicting the tiger, which is completely tarnished except for the places where black lines cracking and peeling off on the front and partially tarnished near the holes on the back. There are grey build-ups near the holes on the back. The special tarnishing and pitting phenomena of only one piece of gold foils attract our attention, making us curious about the formation of such tarnishing and even pitting corrosion of normally non-corrosive gold and why the corrosion is limited to this piece. We also want to figure out why the colour of the tarnished gold foil ranges from black to brown. A multi-tech study of the extraordinary corrosion phenomena of the gold foil using ESEM-EDS, XPS, and XRD will cast a new light on understanding the optical mechanism of gold tarnishing and corrosion mechanism of tarnishing and pitting, offering guidance for the conservation of precious gold artefacts, especially fragile gold foils. 2 Materials and Methods 2.1 Materials The gold foil is about 2×1.5 cm and 26.02 ± 2.95 µm[ 10 ], depicting an image of a tiger, with partially peeling-off black lines on the front. The front surface is completely tarnished while still retaining metallic lustre and the places where black paints peeled off are especially shiny. There are several holes in the gold foil and grey build-ups near the holes on the back. The back is mostly gold, while the surface under grey build-ups is brown, fading as the distance from the holes increases. Gold foils for the simulation experiment were purchased from the Nanjing Gold Foil Sales Centre (Nanjing, Jiangsu, China). Au wt% is 98%. Thickness is 0.12 µm. The lacquer was purchased from Xiao Li Natural Lacquer Culture Limited (Mianyang, Sichuan, China). The bamboo slice was brushed with a layer of lacquer and then applied with a piece of gold foil. The simulation sample was placed in a shaded room (temperature: 20.7 ℃, humidity level: 89%) for 8 days to dry up before the flaming experiment. 2.2 Stereo Microscopy An optical stereo microscope (SX-5, Shanghai Yongheng Optical Instrument Manufacturing Company) with a video capture device (YH-500 1 USB2.0 Camera, Shanghai Yongheng Optical Instrument Manufacturing Company) was used to observe and photograph the details of the gold foil. 2.3 Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) Two kinds of Environmental Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (ESEM-EDS, Quanta 200F, FEI Company, Hillsboro, OR, USA; Quattro, Thermo Fisher, Carlsbad, CA, USA) were used to observe the morphology and elemental distribution of the gold foil under an accelerating voltage of 10 kV. The alloy composition of the cross-section of the gold foil was analysed by SEM-EDS (TM3030, HITACHI, Tokyo, Japan). The cross-section sample is a small unattributable fragment sample embedded in epoxy resin, polished with sandpapers (P = 600, 1500), and finished with water-soluble diamond paste (W0.5, Naibo, Shanghai, China). The frequency count of nanoparticle sizes was conducted by ImageJ. 2.4 X-ray Diffraction (XRD) An in-situ 2D Wide-angle X-ray Diffraction reflectometry (WAXD, D8-Discover, Bruker Company, Germany) was used to obtain the composition of the gold foil and corrosion products. The tube voltage is 50 kV and the current is 1000 µA. The wavelength of CuKα radiation is 0.1542 nm. The beam diameter is 0.5mm. The pixel size of the 2D detector is 1024×1024. The pixel size is 136 µm×136 µm. The distance from the sample to the detector is 199 mm. The exposure time is 10 min. XRD data were measured in the range of 10° ≤ 2θ ≤ 70°. Search/Match phase identification was performed using the MDI Jade 9 software against the ICCD PDF4 + 2009 database. Data were analysed by OriginPro 2022. 2.5 X-ray Photoelectron Spectroscopy (XPS) X-ray Photoelectron Spectroscopy (XPS) was performed using an X-ray Photoelectron Spectrometer (AXIS Supra, Kratos Analytical Ltd., the United Kingdom) with an Al mono source. The pass energy is 160 eV and the energy step is 1.000 eV for the survey spectrum. The pass energy is 40 eV and the energy step is 0.100 eV for the high-resolution spectrum. Spectra were analysed using Thermo Avantage v5.9921 and OriginPro 2022. 3 Results and discussion 3.1 Stereo Microscopy Observation Figure 1 b shows the front side of the gold foil. The front side has completely darkened except for the places where black Chinese ink lines peeled off (Fig. 1 a). Though darkened, the surface still remains metallic lustre. Several holes can be observed from the front side. The largest hole is on the head of the tiger, with a width of 800 µm (Fig. 1 c). Figure 1 e shows the back side of the gold foil. Most of the back is gold in colour, while darkening at the edges. Several holes (Fig. 1 f) and grey build-ups (Fig. 1 d) can be observed on the back side, suggesting the existence of pitting corrosion. The surface under grey build-ups is brown, gradient to gold as the distance away from grey build-ups increases. The tarnishing of the front side and edges of the back side and the pitting corrosion phenomena of the gold foil observed under the stereo microscope make us curious about the optical mechanism of gold tarnishing and the chemical mechanism of the corrosion process. Thus, multi-techs were used to further analyse the corrosion phenomena of the gold foil. 3.2 Chemical composition To characterize the corrosion state of the gold foil, ESEM-EDS, XRD, and XPS were used to obtain the chemical composition of the gold foil and corrosion products. Table 1 SEM-EDS results of the gold foil. Element/Site Au wt% Ag wt% C wt% O wt% Au/(Au + Ag) wt% Count Front-gold 70.57 ± 3.75 17.98 ± 2.22 8.43 ± 2.11 3.03 ± 1.21 79.68 ± 2.54 5 Front-black 63.56 ± 2.41 25.29 ± 3.00 5.51 ± 1.41 5.64 ± 1.47 71.58 ± 2.84 8 Back-gold 76.20 ± 1.34 8.09 ± 1.98 9.09 ± 0.75 6.62 ± 1.13 90.43 ± 2.22 8 Back-brown to black 69.44 ± 5.35 13.87 ± 3.34 8.84 ± 1.02 7.85 ± 3.45 83.31 ± 4.02 14 Cross-section 71.94 ± 0.34 1.45 ± 0.43 21.37 ± 1.96 5.23 ± 1.24 98.03 ± 0.56 3 * Quanta 200F for surface, TM3030 for cross-section. Au, Ag, C, and O were detected on the gold foil (Table 1 ). C and O are mainly due to adventitious carbon, organic contamination or embedding material (epoxy resin). SEM-EDS results show the average Au/(Au + Ag) wt% of front-gold (79.68 ± 2.54) is lower than that of back-gold (90.43 ± 2.22). The average Au/(Au + Ag) wt% of front-black (71.58 ± 2.84) is lower than that of back-brown to black (83.31 ± 4.02). Au/(Au + Ag) wt% of all the locations on the surface is lower than that of the cross-section (98.03 ± 0.56)(Fig. 2 ). SEM-EDS results suggest tarnishing is related to low Au/(Au + Ag) wt%. On the same side, the lower the Au content, the darker the colour. However, Au/(Au + Ag) wt% is not the determining factor of the colour, since Au/(Au + Ag) wt% of some back-brown to black sites is higher than that of front-gold sites. Figure 5 XPS high-resolution spectra for (a) Ag 3d, (b) Au 4f, (c) C 1s, (d) O 1s, (e) S 2p, (f) Cl 2p. Figure 5c shows the XPS high-resolution spectrum for C 1s. C 1s peak for adventitious carbon (C-C/C-H) is centred at 284.80 eV, used to calibrate charge shift for all the XPS spectra[ 11 ]. C 1s peaks for C-O-C are 285.84 eV (front-black), 286.23 eV (back-brown), and 286.34 eV (back-gold). C 1s peaks for O-C = O are 288.79 eV (front-black), 288.66 eV (back-brown), 288.71 eV (back-gold), which may also be assigned to metal carbonate (288–290 eV, 288.93 eV for Ag 2 CO 3 )[ 12 ]. Figure 4 shows the XPS survey spectra of the black site on the front, brown and gold sites on the back of the gold foil. O, C, Ag, S, Cl, Ca, Mg, Al, and Si were detected on the black and brown sites, while S and Cl were not detected on the gold site. The intensity of Au peaks is too low to annotate in the survey spectra, while it’s illustrated in the high-resolution spectrum. Ag is the major metal element on the surface, different from the bulk composition results of ESEM-EDS and XRD, and the intensity increases as the colour darkens (black > brown > gold), suggesting that tarnishing may be related to the existence of silver or silver compounds on the surface. High intensity of O and C mainly associates with surface contaminants. C, O, S, and Cl may be components of silver corrosion products, which needs further analysis through XPS high-resolution spectra. Ca, Mg, Al, and Si mainly associates with surface-adhered sand and soil. Figure 5a shows the XPS high-resolution spectrum for Ag 3d. Ag 3d region of metal has well-separated spin-orbit components (Δ = 6.0 eV), which are 368.2 eV for Ag 3d 5/2 and 374.2 eV for Ag 3d 3/2 [ 1 ]. There exist small binding energy shifts for compounds, such as oxides or halides. Ag 3d peaks broaden relative to metal peaks. As Table 2 presents, Ag 3d 5/2 BEs of black, brown, and gold sites are 367.68 eV, 367.45 eV, 367.59 eV; Ag 3d 3/2 BEs of black, brown, and gold sites are 373.68 eV, 373.50 eV, 373.63 eV. The BEs of Ag (368.2 eV), Ag 2 S (368.1 eV), AgCl (368.1 eV) are similar and above 368.00 eV, a little higher than the BEs of the gold foil sample (Table 3 ). The BEs of AgO (367.6 eV), Ag 2 CO 3 (367.7 eV), Ag 2 O (367.9 eV), Ag 2 SO 4 (367.9 eV) are below 368.00 eV (Table 3 ), closer to the BEs of the gold foil sample. Specific components shall be determined by combining XPS high-resolution spectra for C 1s, O 1s, S 2p, Cl 2p. Table 2 XPS Ag 3d fitting parameters. Site Ag 3d 5/2 BE (eV) FWHM (eV) Ag 3d 3/2 BE (eV) FWHM (eV) Black 367.68 0.94 373.68 0.93 Brown 367.45 1.06 373.50 1.04 Gold 367.59 1.11 373.63 1.15 Table 3 Ag 3d 5/2 chemical state BEs[ 13 ]. Species Ag Ag 2 O AgO Ag 2 CO 3 Ag 2 SO 4 Ag 2 S AgCl BE (eV) 368.2 367.9 367.6 367.7 367.9 368.1 368.1 Figure 5b shows the XPS high-resolution spectrum for Au 4f. Au 4f region of metal has well-separated spin-orbit components, which are 83.96 eV for Au 4f 7/2 and 87.63 eV for Au 4f 5/2 [ 14 , 15 ]. Binding energy shifts may be observed with Au nanoparticles, leading to the BE for Au 4f 7/2 over 84 eV. Au 4f 7/2 BEs of black, brown, and gold sites are 83.53 eV, 83.27 eV, 83.55 eV; Au 4f 5/2 BEs of black, brown, gold sites are 87.26 eV, 87.44 eV, 87.58 eV, suggesting the chemical states of Au are metal. Figure 5d shows the XPS high-resolution spectrum for O 1s. The O 1s BEs of many compounds and species fall within a narrow range, such as metal carbonates (531 eV), metal hydroxides (531.5 eV) and organic C-O (531.5–532 eV)[ 16 ]. The O 1s BE of organic C = O is 533 eV. The O 1s BEs of Ag 2 O, AgO, Ag 2 CO 3 , Ag 2 SO 4 are 531.27 eV[ 17 ], 528.63 eV[ 18 ], 531.39 eV[ 19 ], 531.41 eV[ 20 ]. Two components can be identified according to the fitting curves of the sample. The O 1s BEs of black, brown, and gold sites are 531.83 eV, 531.83 eV, 531.88 eV and 533.39 eV, 533.26 eV, 533.32 eV, which may probably be due to Ag 2 O, Ag 2 CO 3 , Ag 2 SO 4 or organic contamination. Figure 5e shows the XPS high-resolution spectrum for S 2p. S is not detected on the gold site. S 2p peak has closely spaced spin-orbit components (Δ = 1.16 eV). The S 2p 3/2 BEs of Ag 2 S and Ag 2 SO 4 are 160.7 eV and 168.2 eV[ 21 ]. S 2p 3/2 BEs of black and brown sites are 160.77 eV and 160.55 eV; S 2p 1/2 BEs of black, and brown sites are 161.96 eV and 161.76 eV, which coincide with the BE of Ag 2 S. Ag 2 SO 4 is ruled out. Thus, it’s confirmed that there exists Ag 2 S on the black and brown sites, but not on the gold site. Figure 5f shows the XPS high-resolution spectrum for Cl 2p. Cl is not detected on the gold site. Cl 2p peak has spin-orbit components (Δ = 1.6 eV). Each chemical state has two spin-orbit split peaks. Cl 2p 3/2 and 2p 1/2 BEs of AgCl are 197.7 eV and 199.3 eV[ 22 ]. Cl 2p 3/2 BEs of black and brown sites are 197.49 eV and 197.22 eV; Cl 2p 1/2 BEs of black, and brown sites are 199.15 eV and 199.15 eV, suggesting there exists AgCl on the black and brown sites, while not on the gold site. Combining the XPS high-resolution spectra for Ag 3d, Au 4f, C 1s, O 1s, S 2p, Cl 2p, surface components of the back-gold site are mainly Ag 2 O or Ag 2 CO 3 (interfered by organic contamination). Ag 2 CO 3 easily blackens when exposed to light, so it’s less likely to be the main component on the surface of gold sites. Ag 2 S and AgCl also exist on the surface of front-black and back-brown sites, which may contribute to the tarnishing phenomenon of the gold foil. Comparing the results of SEM-EDS, XRD, and XPS, it is worth pointing out the differences in depth and lateral resolution of the 3 methods (Table 4 ). The depth resolution of EDS is about 0.3 ~ 5 µm, which means the chemical composition obtained by EDS is information of bulk. The lateral resolution of EDS is about 10 nm, far smaller than the other two, which means EDS is the most precise to obtain regional heterogeneous information. The lateral resolution of XPS is about 10 nm, so that XPS is used to detect surface information. However, the lateral resolution of XPS is about 15 µm, making it hard to analyse the chemical composition of a single crystal interested. The lateral resolution of XRD is 0.5 mm, and the depth resolution of XRD is from ~ 2 nm to ~ 30 µm, depending on material properties and X-ray incidence angle, which means XRD also provides information of bulk. Table 4 Depth and lateral resolution of EDS, XRD and XPS. Technique Signal Detected Depth Resolution Lateral Resolution EDS Characteristic X-rays 0.3 ~ 5 µm[ 23 ] ~ 10 nm[ 24 ] XRD Diffracted X-rays ~ 2 nm to ~ 30 µm[ 25 ] 0.5 mm[ 26 ] XPS Photoelectrons from near-surface atoms 10 nm[ 27 ] 15 µm[ 27 ] Based on the comparison of depth and lateral resolution of SEM-EDS, XRD, and XPS, differences in results can be explained reasonably. Since XRD detects the information of the bulk, the peaks of black, brown, and grey sites are the same. Only the intensity of the grey site is lower than the others, which may be due to the amorphous pattern of the grey build-ups (discussed later). SEM-EDS results reveal a relevance between Au/(Au + Ag) wt% and the tarnishing degree, which is the lower Au/(Au + Ag) wt%, the more severe the tarnishing degree generally. Au/(Au + Ag) wt% of the cross-section is much higher than that of the surface, suggesting a possibility of selective corrosion of the Au-Ag alloy, which is further supported by the high XPS signal intensity of silver compounds rather than Au on the very surface (< 10 nm). Combining the results of SEM-EDS, XRD, and XPS, the gold foil is an Au-Ag alloy, with a cross-section Au/(Au + Ag) wt% of 98%. Au/(Au + Ag) wt% on the front is generally lower than that on the back. On the same side, Au/(Au + Ag) wt% decreases as the colour darkens. The dominant element on the very surface (< 10 nm) is Ag, mainly existing as Ag 2 O, while Au is nearly undetectable. Ag 2 S and AgCl also exist on the surface of front-black and back-brown sites, which may contribute to the tarnishing phenomenon of the gold foil. However, there still remains a problem unresolved: why Au/(Au + Ag) wt% of back-brown is higher than that of front-gold, while displaying a darker colour? According to band theory [ 28 ], the colour of bulk metal is determined by the width of the energy band, which means the colour is closely related to the chemical composition of the metal. The paradox here indicates that chemical composition merely is insufficient to explain the problem. So we turned our attention to structural colours, looked into surface nanostructures, and tried to figure out the optical mechanism of gold tarnishing. 3.3 Optical mechanism of gold tarnishing ESEM-EDS was conducted to observe the surface nanostructures of the gold foil. Figure 6 shows an ESEM-EDS map scan of the front-gold site. Nanoparticles of about 50 nm are visible on the surface. In the map scan area, Au (74.6 ± 0.2 wt%) almost disperses homogeneously, except for a particle marked in the red circle, where Ag (16.3 ± 0.1 wt%) concentrates, suggesting the enrichment of silver compounds. Table 5 ESEM-EDS (Quattro) results of the gold foil. Site/ Element Front-gold wt% Back-gold wt% Back-brown* wt% Front-black wt% (at%) Spectrum 41 Spectrum 38 Spectrum 27 Au 74.6 ± 0.2 77.3 ± 0.3 73.69 ± 0.60 69.11 ± 0.66 (21.92) 30.88 ± 0.62 (7.55) 23.81 ± 0.55 (6.26) Ag 16.3 ± 0.1 11.7 ± 0.2 9.82 ± 0.42 16.77 ± 0.46 (9.72) 44.74 ± 0.62 (19.97) 53.60 ± 0.65 (25.71) C 7.9 ± 0.2 8.5 ± 0.3 9.24 ± 0.52 10.20 ± 0.56 (53.05) 10.91 ± 0.62 (43.75) 6.17 ± 0.56 (26.56) O 1.0 ± 0.1 2.5 ± 0.1 7.26 ± 0.26 3.92 ± 0.36 (15.31) 5.64 ± 0.45 (16.98) 9.50 ± 0.52 (30.74) S 7.82 ± 0.24 (11.75) Cl 5.46 ± 0.16 (7.98) Fe 0.2 ± 0.1 Al 0.87 ± 0.10 (1.67) Si 0.59 ± 0.09 (1.08) * Data collected by Quanta 200F. Figure 7 shows ESEM-EDS of front-black sites. The size of nanoparticles is about 140 nm (Fig. 7 a). There are two kinds of larger particles (grains and polygons) above nanoparticles. The diameter of grains is about 1200 nm (Fig. 7 d). Ag (19.97 at%) to S (11.75 at%) is approximately 2:1 (Table 5 ), suggesting the chemical composition of grains is Ag 2 S, which coincides with the result of XPS (Fig. 5). The diameter of grains inside polygons is about 274 nm (Fig. 7 g). Ag (25.71 at%) and Cl (7.98 at%) suggest the existence of AgCl (Table 5 ), which coincides with the result of XPS (Fig. 5). The diameters of nanoparticles of Fig. 7 d & g are about 130 nm, similar to those of Fig. 7 a. Since only Au, Ag, C, and O are detected on the sites of nanoparticles, nanoparticles are probably Ag 2 O combined with XPS results (Fig. 5). In summary, there exist Ag 2 O nanoparticles (~ 140 nm) on the surface of front-black sites. Above nanoparticles, Ag 2 S (~ 1200 nm) grains and AgCl (~ 274 nm) grains inside polygons are scattered on the surface layer. Figure 8 shows ESEM comparison and frequency counts of nanoparticle sizes of front-gold, back-gold, front-black, and back-brown sites. Average nanoparticle sizes of front-gold, back-gold, front-black, and back-brown sites are 36.44 ± 6.22 nm, 44.46 ± 9.47 nm, 123.10 ± 32.63 nm, 82.02 ± 11.97 nm, suggesting a strict correlation between colour depth and nanoparticle sizes, which is the larger nanoparticle sizes, the deeper colours. The phenomenon can be explained by localized surface plasmon resonance (LSPR) of metal nanoparticles. Localized surface plasmon resonance is the collective electron oscillation phenomenon between metal nanoparticles at the time when the frequency of the incident light is equal to the vibration frequency of the conduction electrons[ 29 ]. The frequency of LSPR is dependent on the shape, size, spacing, composition of the nanometric features and the dielectric constants of the surrounding medium[ 30 ]. LSPR of nanoparticles causes strong absorption at specific wavelengths, which can be detected by UV-vis spectrum. The wavelength of the maximum absorbance (𝜆 max ) of the surface plasmon (SP) band increases with the increase of nanoparticle size, which coincides with the simulation of Mie theory[ 30 , 31 ]. The red shift is also accompanied by a small broadening of the SP band in the long wavelength region[ 30 ]. For Au nanoparticle sizes larger than 25 nm, the red shift of 𝜆 max is about 0.7 nm per 1 nm increase in particle size[ 30 ]. When the size of Ag is above 53 nm, a new peak starts to develop at shorter wavelengths, which is characteristic of the quadrupole component of the plasmon resonance[ 31 ]. LSPR also exists in Ag-Ag 2 O nanoparticles. As the oxidation of Ag, the size of nanoparticles and the dielectric of the surrounding medium increase, leading to the red shift of 𝜆 max [ 29 ]. Based on the linear fitting of 𝜆 max -size ( d ) data of Ag nanoparticles in water[ 31 ] (𝜆 max = 0.9591 d + 384.94, R 2 = 0.9951, 415 nm< 𝜆 max < 510 nm), calculated 𝜆 max of front-gold ( d = 36.44 nm), back-gold ( d = 44.46 nm), back-brown ( d = 82.02 nm), front-black ( d = 123.10 nm) are 419.89 nm (yellow), 427.58 nm (yellow), 463.61 nm (orange), 503.01 nm (brown), coincided with measured 𝜆 max of 430 nm for Ag 2 O ( d = 46.6 nm)[ 32 , 33 ]. It’s a pity that we were not able to obtain the UV-vis spectrum of the gold foil sample before returning the artefact. Hence, as shown in Fig. 9 , we used Mieplot software[ 31 ] to calculate the extinction efficiency (Q ext = C ext / (π r 2 ), C ext : extinction cross-section), scattering efficiency (Q sca = C sca / (π r 2 ), C sca : scattering cross-section), absorption efficiency (Q abs = C abs / (π r 2 ), C abs : absorption cross-section) of Ag 2 O spheres[ 34 ] with the same mean size and size deviation as measured in Fig. 8 in the air and water. As Ag 2 O nanoparticle size increases, Q sca decreases, Q abs increases except for the black site, Q ext = Q sca + Q abs increases, the wavelength of the maximum extinction (𝜆 max ) increases, and the range of extinction broadens. 𝜆 max of Ag 2 O nanoparticles in water is about 100 nm larger than that in air. CIE x - y chromaticity diagrams of black, brown, back-gold, and front-gold sites were converted from Q ext -wavelength spectra. The colours in CIE x - y chromaticity diagrams of black, brown, back-gold, and front-gold sites are orange, yellowish, whitish, whitish in air and red-purple, orange, yellowish, yellowish in water. Simulated colours in the air are kind of different from real colours, which may be due to Ag 2 O nanoparticles being surrounded half by air and half by metal, not completely by air. However, the trend towards darker colours with the increase of nanoparticle size coincides with the tarnishing degrees observed. In summary, optical mechanism of gold tarnishing is localized surface plasmon resonance (LSPR) of Ag 2 O nanoparticles on the surface. With the increase of nanoparticle size, the wavelength of the maximum extinction (𝜆 max ) of the surface plasmon band increases, leading to the change of hues; the range of absorbance broadens, leading to the decrease of saturation, i.e. tarnishing. Results of other similar tarnishing phenomena and corrosion simulation samples will be discussed in more detail in our coming work. 3.4 Grey build-ups on the back Table 6 ESEM-EDS (Quanta 200) of grey build-ups (at%). Element/Site 28 Grey 32 Brown 44 Grey polyhedra 47 Grey layers Au 29.71 1.02 Ag 33.37 6.43 78.10 54.10 Cl 26.05 45.90 C 31.60 47.09 20.88 O 8.98 16.77 There are 15 irregular holes in the gold foil sample, mainly distributing in the lower body part of the gold foil tiger, with the largest lengths ranging from 41 µm to 732 µm (Fig. 1 ). Near the holes on the back, there exist grey build-ups, which also exist on brown tarnishing areas. According to ESEM photos (Fig. 11 b-f), grey build-ups consist of multiple amorphous layers, in which embedded some polyhedra. Clear gaps exist between polyhedra and layers. EDS results show grey polyhedra are Ag, and grey layers are AgCl (Table 6 ). The surface under grey build-ups is brown, suggesting the formation of grey build-ups is related to corrosion. Ag polyhedra are well encapsulated in the layers, suggesting Ag polyhedra are converted from AgCl layers, which may probably be due to the photodecomposition of AgCl[ 35 ]. 3.5 Corrosion mechanism of the gold foil According to analyses of chemical composition and ESEM-EDS observation of the gold foil, it has already been confirmed that tarnishing is due to the corrosion of the less noble metal Ag in the Au-Ag alloy. The darkening degree is associated with the size of the corrosion product Ag 2 O. Optical mechanism of tarnishing is localized surface plasmon resonance (LSPR) of Ag 2 O nanoparticles on the surface. The corrosion mechanism of tarnishing is probably surface segregation, or rather selective corrosion of Ag[ 2 ]. In binary alloys, one element tends to move to the surface driven by thermodynamics, leading to the lowest surface free energy. The Enthalpy ( H ) and Gibbs Free Energy ( G ) of Au and Ag are 0, so surface segregation of Au-Ag alloy will not happen in the vacuum. However, O 2 , H 2 S, carboxylic sulphide (COS), chloride etc. in the air promote the formation of silver compounds ( E < 0, G < 0), such as Ag 2 O, Ag 2 S, AgCl, facilitating silver separation to the surface (Table 7 ). As shown in Table 1 , Au/(Au + Ag) wt% of surface ranges from 66.72–93.59%, all much lower than that of cross-section (98.03%). On the very surface (< 10 nm), Au is even nearly undetectable (Fig. 4 ). As Ag moves to the surface, Au is enriched internally. Table 7 Enthalpy and Gibbs free energy values for Au, Ag and silver compounds[ 2 ]. Species Au Ag AgCl Ag 2 S Ag 2 O Enthalpy ( H , kJ/mol) 0 0 −127.07 −29.41 −31.05 Gibbs Free Energy ( G , kJ/mol) 0 0 −109.80 −39.46 −11.21 Nevertheless, since H of AgCl, Ag 2 S and G of AgCl are lower than Ag 2 O, why the component of nanoparticles on the surface is Ag 2 O not the more stable silver compounds? According to the analysis of the former work[ 10 ], the gold foil was attached to the surface (probably the lateral side) of the lacquerware, which was usually placed in the atmosphere, not in direct contact with the human body as much as gold jewellery. O 2 concentration is much higher than chloride or sulphide in the atmosphere environment, resulting in a full exposure and a higher reaction rate. O 2 seizes the initiative to occupy reaction sites, forming a uniform layer of Ag 2 O nanoparticles, above which Ag 2 S and AgCl form afterwards as shown in Fig. 7 d&g. It’s worth noticing that this gold foil is the only one piece with the severe tarnishing phenomenon, grey build-ups on the back and irregular holes of all the seven pieces. Since the burial environment is similar, the difference is more likely due to experience before burial. A possible speculation is the gold foil may undergo flaming by an oil lamp or other flaming devices (Fig. 12 ). Gold foil applied on the bamboo slice was close to the flaming side for 20 s. After being flamed, the surface turned from gold to reddish, similar to the brown site of the archaeological gold foil sample. Heating accelerates the oxidation of Ag[ 29 , 34 , 36 ] and leads to the deformation of the substrate wood, resulting in the separation of the gold foil edges and the lacquerware. As shown in Fig. 1 , the gold foil edges are brown on the back, suggesting a fuller exposure to O 2 and more severe corrosion than the centre area. Pitting corrosion is also only observed on this gold foil, which is probably related to the edge-off phenomenon due to the flaming. Crevice exists between the gold foil and the lacquerware surface. During burial, rainwater infiltration and groundwater circulation provide a liquid microenvironment. Due to the restricted mass diffusion, dissolved O 2 and pH decrease, while Cl − accumulates inside the crevice[ 37 ], offering an electrolyte environment. Enrichment of Cl − also facilitates Ag dissolution by reducing reduction potential ( E 0 ): Ag + + e − → Ag ( E 0 = 0.799 eV), AgCl + e − → Ag + Cl − ( E 0 = 0.222 eV)[ 37 ]. The gold foil surface inside the crevice has a low O 2 concentration, becoming the anode, while the surface outside the crevice is well-oxygenated, becoming the cathode (Fig. 13 a). Gold foil with Ag 2 O passive film outside crevice acts as an inert electrode, where the cathodic reaction occurs: O 2 + 2H 2 O + 4e − → 4OH − ( E 0 = 0.401 eV) [ 4 ]. AgCl forms and builds up inside the crevice. Pit grows under AgCl build-ups. The AgCl layer passivates the sidewall of the pit, while Ag continues to dissolve at the tip of the pit, eventually resulting in the perforation (Fig. 13 b). As shown in Fig. 1 , some sites of AgCl build-ups are in the stage of Fig. 13 a or b, with AgCl build-ups while not perforated. Holes with AgCl build-ups on the back are in the next stage of Fig. 13 b, where pitting corrosion has been accomplished. Gold foil surface besides AgCl build-ups is brown for full exposure to O 2 . Ag 2 O nanoparticle size of the back-brown site is not as large as that on the front (exposed to O 2 while being heated), but larger than that closely attached to the lacquerware (back-gold site). 4 Conclusion Combining the chemical composition results of SEM-EDS, XRD, and XPS, the gold foil is an Au-Ag alloy, with a cross-section Au/(Au + Ag) wt% of 98.03%. Au/(Au + Ag) wt% on the front (66.72%-82.94%) is generally lower than that on the back (75.92%-93.56%). On the same side, Au/(Au + Ag) wt% decreases as the colour darkens. The dominant element on the very surface (< 10 nm) is Ag, mainly existing as Ag 2 O nanoparticles, while Au is nearly undetectable. Selective corrosion results in Ag moving to the surface, while Au is enriched internally. Ag 2 S grains (~ 1200 nm) and AgCl grains (~ 300 nm) inside polygons (~ 1500 nm) also exist on the surface of front-black and back-brown sites, above the Ag 2 O nanoparticle layer. Average nanoparticle sizes of front-gold, back-gold, front-black, and back-brown sites are 36.44 ± 6.22 nm, 44.46 ± 9.47 nm, 123.10 ± 32.63 nm, 82.02 ± 11.97 nm, suggesting a strict correlation between colour depth and nanoparticle sizes, which is the larger nanoparticle sizes, the deeper colours. Optical mechanism of gold tarnishing is localized surface plasmon resonance (LSPR) of Ag 2 O nanoparticles on the surface. With the increase of nanoparticle size, the wavelength of the maximum extinction (𝜆 max ) of the surface plasmon band increases, leading to the change of hues; the range of absorbance broadens, leading to the decrease of saturation, i.e. tarnishing. Tarnishing and pitting corrosion only happened in this gold foil, which is probably due to the experience of being flamed in the atmosphere. Heating accelerates the oxidation of Ag[ 29 , 34 , 36 ] and leads to the deformation of the substrate wood, resulting in the separation of the gold foil edges and the lacquerware. The edge-off phenomenon leads to crevice corrosion during burial, which develops into pitting corrosion and eventually results in the perforation of the gold foil. AgCl build-ups near holes are electrochemical corrosion products. Ag polyhedra are well encapsulated in the AgCl layers, which may probably be due to the photodecomposition of AgCl. A comprehensive analysis of the extraordinary corrosion phenomena of the gold foil puts forward the understanding of optical and chemical mechanisms of gold corrosion and offers guidance for gold foil conservation. Declarations Availability of data and materials The data presented in this study are available in the article. Competing interests No competing interests. Funding This research received no external funding. Author’s contributions Zisang Gong: Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Siyuan Sun: Methodology, Investigation, Writing – review & editing. Pei Hu: Methodology, Conceptualization, Project administration, Supervision, Writing – review & editing. Gang Hu: Methodology, Supervision, Conceptualization, Project administration, Writing – review & editing, Funding acquisition. Jingrong Pei: Resources. Acknowledgements Thanks to Li Chen in School of Physics, Peking University, for the support of ESEM-EDS. Thanks to Jianfeng Cui in School of Archaeology and Museology, Peking University, for the support of SEM-EDS. Thanks to Minqiao Ren in SINOPEC Beijing Research Institute of Chemical Industry, for the support of XRD. Thanks to Jinglin Xie in Analytical Instrumentation Center, Peking University, for the support of XPS. Thanks to Taiyuan Institute of Cultural Relics and Archaeology for providing the samples in this work. Thanks to Xin Wang in School of Archaeology and Museology, Peking University, for the preparation of lacquer. Thanks to Xiaolin Yang in Nation Museum of China, Jian Kang in Feifanshangshi Studio, Xinrui Zhang in College of Chemistry and Molecular Engineering, Shengyu Liu, Kaiwen Zhao, Minghao Jia, and Bo Tang in School of Archaeology and Museology of Peking University, for the discussion of corrosion phenomena. 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Cite Share Download PDF Status: Published Journal Publication published 27 Mar, 2025 Read the published version in npj Heritage Science → Version 1 posted Editorial decision: Revision requested 20 Nov, 2024 Reviews received at journal 08 Nov, 2024 Reviewers agreed at journal 08 Nov, 2024 Reviewers invited by journal 07 Nov, 2024 Editor assigned by journal 05 Nov, 2024 Submission checks completed at journal 05 Nov, 2024 First submitted to journal 23 Oct, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5316041","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":375489205,"identity":"f9a9139f-d53b-4064-8b43-527866ebf961","order_by":0,"name":"Zisang Gong","email":"","orcid":"","institution":"The International Center for Chinese Heritage and Archaeology, School of Archaeology and Museology, Peking 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06:08:47","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5316041/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5316041/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s40494-025-01616-y","type":"published","date":"2025-03-27T15:57:03+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":68506890,"identity":"f653f320-271c-4ab9-b273-2e83417e94d1","added_by":"auto","created_at":"2024-11-08 04:41:12","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":243313,"visible":true,"origin":"","legend":"\u003cp\u003eStereo microscope photographs of the gold foil: (b) front, (e) back, (a) black phenomenon of the front side with Chinese ink peeling off, (c) a hole, (d) a grey build-up, (f) a hole.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/eeff20f96bd8a00260ef6785.jpg"},{"id":68506950,"identity":"297717f5-6ec2-4d01-bbc7-0302b11e9d1f","added_by":"auto","created_at":"2024-11-08 04:49:12","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":21730,"visible":true,"origin":"","legend":"\u003cp\u003eSEM-EDS results of the gold foil (Ag-Au).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/6d8f09e28d41cc5605c66bda.png"},{"id":68507663,"identity":"388b4031-916d-42af-90ef-0bb61b28c67a","added_by":"auto","created_at":"2024-11-08 04:57:12","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":20220,"visible":true,"origin":"","legend":"\u003cp\u003eXRD results of the gold foil.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/6eea67f36243a6e623557798.png"},{"id":68506888,"identity":"6459c788-586f-4036-bdd9-ea338dcbc25f","added_by":"auto","created_at":"2024-11-08 04:41:12","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":45395,"visible":true,"origin":"","legend":"\u003cp\u003eXPS survey spectra of the gold foil.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/325c3a86b9e25ee780c5739c.png"},{"id":68506952,"identity":"2d969e65-2dab-4baf-93d1-19df25cfed8a","added_by":"auto","created_at":"2024-11-08 04:49:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":140876,"visible":true,"origin":"","legend":"\u003cp\u003eXPS high-resolution spectra for (a) Ag 3d, (b) Au 4f, (c) C 1s, (d) O 1s, (e) S 2p, (f) Cl 2p.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/6d00d26a7007c45f6948dba6.png"},{"id":68506892,"identity":"14de9bd8-0615-4f37-b0ad-eb785cfb1182","added_by":"auto","created_at":"2024-11-08 04:41:12","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1758087,"visible":true,"origin":"","legend":"\u003cp\u003eESEM-EDS (Quattro)map scan of the front-gold site: (a) ESEM photograph of nanoparticles (white: nanoparticle size, green: interparticle spacing), (b) scan site with black paints peeling off, (c) ESEM-EDS map scan, (d) elemental distribution, (e) map scan spectrum.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/61e1c1dbb911b8bb25236006.png"},{"id":68507924,"identity":"31f8fc3a-349c-429c-9cba-9c11f1f71f72","added_by":"auto","created_at":"2024-11-08 05:05:12","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1541520,"visible":true,"origin":"","legend":"\u003cp\u003eESEM-EDS (Quattro S)of front-black sites: (a) ESEM photograph of nanoparticles (white: nanoparticle size, green: interparticle spacing), (b) test sites (red: a, c-f; orange: g-h), (c-d) ESEM photograph of grains above nanoparticles, (e) map scan spectrum of nanoparticles (spectrum 41 in d), (f) point scan spectrum of grains (spectrum 38 in d), (g) ESEM photograph of polygons with grains inside above nanoparticles, (h) point scan spectrum of grains inside the polygon (spectrum 27 in g).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/34fe104197947f1a5ebb90ec.png"},{"id":68506899,"identity":"29cd0f9d-9bbd-4dee-bb46-80f582db0cc4","added_by":"auto","created_at":"2024-11-08 04:41:12","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1884169,"visible":true,"origin":"","legend":"\u003cp\u003eESEM comparison of nanoparticle sizes of front-gold (c), back-gold (d), front-black (e), and back-brown (f) sites (white: nanoparticle size, green: nanoparticle spacing). Position of ESEM: front (a) orange: front-gold, white: front-black; back (b) orange: back-gold, red: back-brown. Frequency counts of nanoparticle sizes of front-gold (g), back-gold (h), front-black (i), and back-brown (j) sites.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/fd6150f1b58d93a7e8877386.png"},{"id":68507664,"identity":"516519a8-4547-4134-b970-09bc749316db","added_by":"auto","created_at":"2024-11-08 04:57:12","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":124990,"visible":true,"origin":"","legend":"\u003cp\u003eCIE \u003cem\u003ex\u003c/em\u003e - \u003cem\u003ey\u003c/em\u003e chromaticity diagrams of black, brown, back-gold, and front-gold sites in air (a) and water (b) converted from calculated extinction spectra in Figure 9.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/1b9dd2fb7ca0fd143403c6fa.png"},{"id":68506956,"identity":"338a3ad3-3a37-4ce7-a088-9525d104abaa","added_by":"auto","created_at":"2024-11-08 04:49:12","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":2113548,"visible":true,"origin":"","legend":"\u003cp\u003eESEM-EDS (Quanta 200) of grey build-ups. (a) Test locations (orange: b-c, red: d-f); (g) map scan of (d); (h-i) point scans of 28, 32 in (d) and 44, 47 in (c).\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/922da486c3b293c8c314070f.png"},{"id":68506898,"identity":"e59e1789-73a8-46cd-a796-6ac0464813fa","added_by":"auto","created_at":"2024-11-08 04:41:12","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":468703,"visible":true,"origin":"","legend":"\u003cp\u003eImagery recovery of the flaming scene (a). Flaming simulation experiment of gold foil applied on a bamboo slice by lacquer (b).\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/836d0750949f5ab62da38fe8.png"},{"id":68506895,"identity":"c3917a71-2479-44a7-a5db-57b80e48b5d2","added_by":"auto","created_at":"2024-11-08 04:41:12","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":91800,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of crevice corrosion (a) and pitting corrosion (b) mechanisms of the gold foil applied on the lacquerware.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/d77ff8e19c8d9948ed699f7f.png"},{"id":79604878,"identity":"717b07fb-3ee5-461d-b0d6-a53052fa208e","added_by":"auto","created_at":"2025-03-31 16:08:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8699927,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5316041/v1/af33b3ae-3be7-4e21-888c-a6fc4e69e049.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Tarnishing and Pitting Corrosion Mechanism Revealed by Nanoparticles on the Gold Foil of Western Han Dynasty in Taiyuan, Shanxi, China through a Multi-analytical Approach","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eGold is almost the most corrosion-resistant metal in nature. Due to its stable chemical properties and shiny appearance, gold has been refined and fabricated into artefacts for a long time in different cultures. Since archaeological gold artefacts are usually not pure gold, tarnishing is a common phenomenon. However, the colours of tarnishing and corrosion products are not always the same, depending on the components of alloys and the environments[\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6 CR7 CR8\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSeven pieces of gold foils were unearthed on the surface of the lacquerware from Chamber 2 of the tomb 2018TJLSTM4, which is located at the burial area of Jin Yang (晋阳) ancient city site, Taiyuan (太原) city, Shanxi Province, central China and is thought to be from the late Western Han Dynasty according to the burial form. The lacquerware was crushed during burial, leading to the separation of the gold foils from the lacquerware. The lacquerware is a round lacquer trousseau with silver persimmon on top of the lid. All seven pieces of gold foils are drawn with black paint and some with red paint[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Although adhered with sandy soil, the surface is still bright gold, except for one piece depicting the tiger, which is completely tarnished except for the places where black lines cracking and peeling off on the front and partially tarnished near the holes on the back. There are grey build-ups near the holes on the back. The special tarnishing and pitting phenomena of only one piece of gold foils attract our attention, making us curious about the formation of such tarnishing and even pitting corrosion of normally non-corrosive gold and why the corrosion is limited to this piece. We also want to figure out why the colour of the tarnished gold foil ranges from black to brown.\u003c/p\u003e \u003cp\u003eA multi-tech study of the extraordinary corrosion phenomena of the gold foil using ESEM-EDS, XPS, and XRD will cast a new light on understanding the optical mechanism of gold tarnishing and corrosion mechanism of tarnishing and pitting, offering guidance for the conservation of precious gold artefacts, especially fragile gold foils.\u003c/p\u003e"},{"header":"2 Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eThe gold foil is about 2\u0026times;1.5 cm and 26.02\u0026thinsp;\u0026plusmn;\u0026thinsp;2.95 \u0026micro;m[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], depicting an image of a tiger, with partially peeling-off black lines on the front. The front surface is completely tarnished while still retaining metallic lustre and the places where black paints peeled off are especially shiny.\u003c/p\u003e \u003cp\u003eThere are several holes in the gold foil and grey build-ups near the holes on the back. The back is mostly gold, while the surface under grey build-ups is brown, fading as the distance from the holes increases.\u003c/p\u003e \u003cp\u003eGold foils for the simulation experiment were purchased from the Nanjing Gold Foil Sales Centre (Nanjing, Jiangsu, China). Au wt% is 98%. Thickness is 0.12 \u0026micro;m. The lacquer was purchased from Xiao Li Natural Lacquer Culture Limited (Mianyang, Sichuan, China). The bamboo slice was brushed with a layer of lacquer and then applied with a piece of gold foil. The simulation sample was placed in a shaded room (temperature: 20.7 ℃, humidity level: 89%) for 8 days to dry up before the flaming experiment.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Stereo Microscopy\u003c/h2\u003e \u003cp\u003eAn optical stereo microscope (SX-5, Shanghai Yongheng Optical Instrument Manufacturing Company) with a video capture device (YH-500 1 USB2.0 Camera, Shanghai Yongheng Optical Instrument Manufacturing Company) was used to observe and photograph the details of the gold foil.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS)\u003c/h2\u003e \u003cp\u003eTwo kinds of Environmental Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (ESEM-EDS, Quanta 200F, FEI Company, Hillsboro, OR, USA; Quattro, Thermo Fisher, Carlsbad, CA, USA) were used to observe the morphology and elemental distribution of the gold foil under an accelerating voltage of 10 kV.\u003c/p\u003e \u003cp\u003eThe alloy composition of the cross-section of the gold foil was analysed by SEM-EDS (TM3030, HITACHI, Tokyo, Japan). The cross-section sample is a small unattributable fragment sample embedded in epoxy resin, polished with sandpapers (P\u0026thinsp;=\u0026thinsp;600, 1500), and finished with water-soluble diamond paste (W0.5, Naibo, Shanghai, China).\u003c/p\u003e \u003cp\u003eThe frequency count of nanoparticle sizes was conducted by ImageJ.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 X-ray Diffraction (XRD)\u003c/h2\u003e \u003cp\u003eAn in-situ 2D Wide-angle X-ray Diffraction reflectometry (WAXD, D8-Discover, Bruker Company, Germany) was used to obtain the composition of the gold foil and corrosion products. The tube voltage is 50 kV and the current is 1000 \u0026micro;A. The wavelength of CuKα radiation is 0.1542 nm. The beam diameter is 0.5mm. The pixel size of the 2D detector is 1024\u0026times;1024. The pixel size is 136 \u0026micro;m\u0026times;136 \u0026micro;m. The distance from the sample to the detector is 199 mm. The exposure time is 10 min. XRD data were measured in the range of 10\u0026deg; \u0026le; 2θ\u0026thinsp;\u0026le;\u0026thinsp;70\u0026deg;. Search/Match phase identification was performed using the MDI Jade 9 software against the ICCD PDF4\u0026thinsp;+\u0026thinsp;2009 database. Data were analysed by OriginPro 2022.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 X-ray Photoelectron Spectroscopy (XPS)\u003c/h2\u003e \u003cp\u003eX-ray Photoelectron Spectroscopy (XPS) was performed using an X-ray Photoelectron Spectrometer (AXIS Supra, Kratos Analytical Ltd., the United Kingdom) with an Al mono source. The pass energy is 160 eV and the energy step is 1.000 eV for the survey spectrum. The pass energy is 40 eV and the energy step is 0.100 eV for the high-resolution spectrum. Spectra were analysed using Thermo Avantage v5.9921 and OriginPro 2022.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Stereo Microscopy Observation\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb shows the front side of the gold foil. The front side has completely darkened except for the places where black Chinese ink lines peeled off (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Though darkened, the surface still remains metallic lustre. Several holes can be observed from the front side. The largest hole is on the head of the tiger, with a width of 800 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee shows the back side of the gold foil. Most of the back is gold in colour, while darkening at the edges. Several holes (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef) and grey build-ups (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed) can be observed on the back side, suggesting the existence of pitting corrosion. The surface under grey build-ups is brown, gradient to gold as the distance away from grey build-ups increases.\u003c/p\u003e \u003cp\u003eThe tarnishing of the front side and edges of the back side and the pitting corrosion phenomena of the gold foil observed under the stereo microscope make us curious about the optical mechanism of gold tarnishing and the chemical mechanism of the corrosion process. Thus, multi-techs were used to further analyse the corrosion phenomena of the gold foil.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Chemical composition\u003c/h2\u003e \u003cp\u003eTo characterize the corrosion state of the gold foil, ESEM-EDS, XRD, and XPS were used to obtain the chemical composition of the gold foil and corrosion products.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSEM-EDS results of the gold foil.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElement/Site\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAu wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAg wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eO wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAu/(Au\u0026thinsp;+\u0026thinsp;Ag) wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eCount\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFront-gold\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e70.57\u0026thinsp;\u0026plusmn;\u0026thinsp;3.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e17.98\u0026thinsp;\u0026plusmn;\u0026thinsp;2.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e8.43\u0026thinsp;\u0026plusmn;\u0026thinsp;2.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e3.03\u0026thinsp;\u0026plusmn;\u0026thinsp;1.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e79.68\u0026thinsp;\u0026plusmn;\u0026thinsp;2.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFront-black\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e63.56\u0026thinsp;\u0026plusmn;\u0026thinsp;2.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e25.29\u0026thinsp;\u0026plusmn;\u0026thinsp;3.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e5.51\u0026thinsp;\u0026plusmn;\u0026thinsp;1.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e5.64\u0026thinsp;\u0026plusmn;\u0026thinsp;1.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e71.58\u0026thinsp;\u0026plusmn;\u0026thinsp;2.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBack-gold\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e76.20\u0026thinsp;\u0026plusmn;\u0026thinsp;1.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e8.09\u0026thinsp;\u0026plusmn;\u0026thinsp;1.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e9.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e6.62\u0026thinsp;\u0026plusmn;\u0026thinsp;1.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e90.43\u0026thinsp;\u0026plusmn;\u0026thinsp;2.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBack-brown to black\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e69.44\u0026thinsp;\u0026plusmn;\u0026thinsp;5.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e13.87\u0026thinsp;\u0026plusmn;\u0026thinsp;3.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e8.84\u0026thinsp;\u0026plusmn;\u0026thinsp;1.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e7.85\u0026thinsp;\u0026plusmn;\u0026thinsp;3.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e83.31\u0026thinsp;\u0026plusmn;\u0026thinsp;4.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCross-section\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e71.94\u0026thinsp;\u0026plusmn;\u0026thinsp;0.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e1.45\u0026thinsp;\u0026plusmn;\u0026thinsp;0.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e21.37\u0026thinsp;\u0026plusmn;\u0026thinsp;1.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e5.23\u0026thinsp;\u0026plusmn;\u0026thinsp;1.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e98.03\u0026thinsp;\u0026plusmn;\u0026thinsp;0.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e* Quanta 200F for surface, TM3030 for cross-section.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAu, Ag, C, and O were detected on the gold foil (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). C and O are mainly due to adventitious carbon, organic contamination or embedding material (epoxy resin). SEM-EDS results show the average Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of front-gold (79.68\u0026thinsp;\u0026plusmn;\u0026thinsp;2.54) is lower than that of back-gold (90.43\u0026thinsp;\u0026plusmn;\u0026thinsp;2.22). The average Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of front-black (71.58\u0026thinsp;\u0026plusmn;\u0026thinsp;2.84) is lower than that of back-brown to black (83.31\u0026thinsp;\u0026plusmn;\u0026thinsp;4.02). Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of all the locations on the surface is lower than that of the cross-section (98.03\u0026thinsp;\u0026plusmn;\u0026thinsp;0.56)(Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). SEM-EDS results suggest tarnishing is related to low Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt%. On the same side, the lower the Au content, the darker the colour. However, Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% is not the determining factor of the colour, since Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of some back-brown to black sites is higher than that of front-gold sites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure 5 XPS high-resolution spectra for (a) Ag 3d, (b) Au 4f, (c) C 1s, (d) O 1s, (e) S 2p, (f) Cl 2p.\u003c/p\u003e \u003cp\u003eFigure 5c shows the XPS high-resolution spectrum for C 1s. C 1s peak for adventitious carbon (C-C/C-H) is centred at 284.80 eV, used to calibrate charge shift for all the XPS spectra[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. C 1s peaks for C-O-C are 285.84 eV (front-black), 286.23 eV (back-brown), and 286.34 eV (back-gold). C 1s peaks for O-C\u0026thinsp;=\u0026thinsp;O are 288.79 eV (front-black), 288.66 eV (back-brown), 288.71 eV (back-gold), which may also be assigned to metal carbonate (288\u0026ndash;290 eV, 288.93 eV for Ag\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e)[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the XPS survey spectra of the black site on the front, brown and gold sites on the back of the gold foil. O, C, Ag, S, Cl, Ca, Mg, Al, and Si were detected on the black and brown sites, while S and Cl were not detected on the gold site. The intensity of Au peaks is too low to annotate in the survey spectra, while it\u0026rsquo;s illustrated in the high-resolution spectrum. Ag is the major metal element on the surface, different from the bulk composition results of ESEM-EDS and XRD, and the intensity increases as the colour darkens (black\u0026thinsp;\u0026gt;\u0026thinsp;brown\u0026thinsp;\u0026gt;\u0026thinsp;gold), suggesting that tarnishing may be related to the existence of silver or silver compounds on the surface. High intensity of O and C mainly associates with surface contaminants. C, O, S, and Cl may be components of silver corrosion products, which needs further analysis through XPS high-resolution spectra. Ca, Mg, Al, and Si mainly associates with surface-adhered sand and soil.\u003c/p\u003e \u003cp\u003eFigure 5a shows the XPS high-resolution spectrum for Ag 3d. Ag 3d region of metal has well-separated spin-orbit components (Δ\u0026thinsp;=\u0026thinsp;6.0 eV), which are 368.2 eV for Ag 3d\u003csub\u003e5/2\u003c/sub\u003e and 374.2 eV for Ag 3d\u003csub\u003e3/2\u003c/sub\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. There exist small binding energy shifts for compounds, such as oxides or halides. Ag 3d peaks broaden relative to metal peaks. As Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents, Ag 3d\u003csub\u003e5/2\u003c/sub\u003e BEs of black, brown, and gold sites are 367.68 eV, 367.45 eV, 367.59 eV; Ag 3d\u003csub\u003e3/2\u003c/sub\u003e BEs of black, brown, and gold sites are 373.68 eV, 373.50 eV, 373.63 eV. The BEs of Ag (368.2 eV), Ag\u003csub\u003e2\u003c/sub\u003eS (368.1 eV), AgCl (368.1 eV) are similar and above 368.00 eV, a little higher than the BEs of the gold foil sample (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The BEs of AgO (367.6 eV), Ag\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e (367.7 eV), Ag\u003csub\u003e2\u003c/sub\u003eO (367.9 eV), Ag\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (367.9 eV) are below 368.00 eV (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), closer to the BEs of the gold foil sample. Specific components shall be determined by combining XPS high-resolution spectra for C 1s, O 1s, S 2p, Cl 2p.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eXPS Ag 3d fitting parameters.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSite\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAg 3d\u003csub\u003e5/2\u003c/sub\u003e BE (eV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFWHM (eV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAg 3d\u003csub\u003e3/2\u003c/sub\u003e BE (eV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFWHM (eV)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBlack\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e367.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e373.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.93\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBrown\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e367.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e373.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGold\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e367.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e373.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.15\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAg 3d\u003csub\u003e5/2\u003c/sub\u003e chemical state BEs[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpecies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAg\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAg\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAgO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAg\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAg\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eAg\u003csub\u003e2\u003c/sub\u003eS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eAgCl\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBE (eV)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e368.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e367.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e367.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e367.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e367.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e368.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e368.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure 5b shows the XPS high-resolution spectrum for Au 4f. Au 4f region of metal has well-separated spin-orbit components, which are 83.96 eV for Au 4f\u003csub\u003e7/2\u003c/sub\u003e and 87.63 eV for Au 4f\u003csub\u003e5/2\u003c/sub\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Binding energy shifts may be observed with Au nanoparticles, leading to the BE for Au 4f\u003csub\u003e7/2\u003c/sub\u003e over 84 eV. Au 4f\u003csub\u003e7/2\u003c/sub\u003e BEs of black, brown, and gold sites are 83.53 eV, 83.27 eV, 83.55 eV; Au 4f\u003csub\u003e5/2\u003c/sub\u003e BEs of black, brown, gold sites are 87.26 eV, 87.44 eV, 87.58 eV, suggesting the chemical states of Au are metal.\u003c/p\u003e \u003cp\u003eFigure 5d shows the XPS high-resolution spectrum for O 1s. The O 1s BEs of many compounds and species fall within a narrow range, such as metal carbonates (531 eV), metal hydroxides (531.5 eV) and organic C-O (531.5\u0026ndash;532 eV)[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The O 1s BE of organic C\u0026thinsp;=\u0026thinsp;O is 533 eV. The O 1s BEs of Ag\u003csub\u003e2\u003c/sub\u003eO, AgO, Ag\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, Ag\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e are 531.27 eV[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], 528.63 eV[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], 531.39 eV[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], 531.41 eV[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Two components can be identified according to the fitting curves of the sample. The O 1s BEs of black, brown, and gold sites are 531.83 eV, 531.83 eV, 531.88 eV and 533.39 eV, 533.26 eV, 533.32 eV, which may probably be due to Ag\u003csub\u003e2\u003c/sub\u003eO, Ag\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, Ag\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e or organic contamination.\u003c/p\u003e \u003cp\u003eFigure 5e shows the XPS high-resolution spectrum for S 2p. S is not detected on the gold site. S 2p peak has closely spaced spin-orbit components (Δ\u0026thinsp;=\u0026thinsp;1.16 eV). The S 2p\u003csub\u003e3/2\u003c/sub\u003e BEs of Ag\u003csub\u003e2\u003c/sub\u003eS and Ag\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e are 160.7 eV and 168.2 eV[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. S 2p\u003csub\u003e3/2\u003c/sub\u003e BEs of black and brown sites are 160.77 eV and 160.55 eV; S 2p\u003csub\u003e1/2\u003c/sub\u003e BEs of black, and brown sites are 161.96 eV and 161.76 eV, which coincide with the BE of Ag\u003csub\u003e2\u003c/sub\u003eS. Ag\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e is ruled out. Thus, it\u0026rsquo;s confirmed that there exists Ag\u003csub\u003e2\u003c/sub\u003eS on the black and brown sites, but not on the gold site.\u003c/p\u003e \u003cp\u003eFigure 5f shows the XPS high-resolution spectrum for Cl 2p. Cl is not detected on the gold site. Cl 2p peak has spin-orbit components (Δ\u0026thinsp;=\u0026thinsp;1.6 eV). Each chemical state has two spin-orbit split peaks. Cl 2p\u003csub\u003e3/2\u003c/sub\u003e and 2p\u003csub\u003e1/2\u003c/sub\u003e BEs of AgCl are 197.7 eV and 199.3 eV[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Cl 2p\u003csub\u003e3/2\u003c/sub\u003e BEs of black and brown sites are 197.49 eV and 197.22 eV; Cl 2p\u003csub\u003e1/2\u003c/sub\u003e BEs of black, and brown sites are 199.15 eV and 199.15 eV, suggesting there exists AgCl on the black and brown sites, while not on the gold site.\u003c/p\u003e \u003cp\u003eCombining the XPS high-resolution spectra for Ag 3d, Au 4f, C 1s, O 1s, S 2p, Cl 2p, surface components of the back-gold site are mainly Ag\u003csub\u003e2\u003c/sub\u003eO or Ag\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e (interfered by organic contamination). Ag\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e easily blackens when exposed to light, so it\u0026rsquo;s less likely to be the main component on the surface of gold sites. Ag\u003csub\u003e2\u003c/sub\u003eS and AgCl also exist on the surface of front-black and back-brown sites, which may contribute to the tarnishing phenomenon of the gold foil.\u003c/p\u003e \u003cp\u003eComparing the results of SEM-EDS, XRD, and XPS, it is worth pointing out the differences in depth and lateral resolution of the 3 methods (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The depth resolution of EDS is about 0.3\u0026thinsp;~\u0026thinsp;5 \u0026micro;m, which means the chemical composition obtained by EDS is information of bulk. The lateral resolution of EDS is about 10 nm, far smaller than the other two, which means EDS is the most precise to obtain regional heterogeneous information. The lateral resolution of XPS is about 10 nm, so that XPS is used to detect surface information. However, the lateral resolution of XPS is about 15 \u0026micro;m, making it hard to analyse the chemical composition of a single crystal interested. The lateral resolution of XRD is 0.5 mm, and the depth resolution of XRD is from ~\u0026thinsp;2 nm to ~\u0026thinsp;30 \u0026micro;m, depending on material properties and X-ray incidence angle, which means XRD also provides information of bulk.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDepth and lateral resolution of EDS, XRD and XPS.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTechnique\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSignal Detected\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDepth Resolution\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLateral Resolution\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eEDS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCharacteristic X-rays\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.3\u0026thinsp;~\u0026thinsp;5 \u0026micro;m[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e~\u0026thinsp;10 nm[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eXRD\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDiffracted X-rays\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e~\u0026thinsp;2 nm to ~\u0026thinsp;30 \u0026micro;m[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.5 mm[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eXPS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePhotoelectrons from near-surface atoms\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10 nm[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15 \u0026micro;m[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eBased on the comparison of depth and lateral resolution of SEM-EDS, XRD, and XPS, differences in results can be explained reasonably. Since XRD detects the information of the bulk, the peaks of black, brown, and grey sites are the same. Only the intensity of the grey site is lower than the others, which may be due to the amorphous pattern of the grey build-ups (discussed later). SEM-EDS results reveal a relevance between Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% and the tarnishing degree, which is the lower Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt%, the more severe the tarnishing degree generally. Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of the cross-section is much higher than that of the surface, suggesting a possibility of selective corrosion of the Au-Ag alloy, which is further supported by the high XPS signal intensity of silver compounds rather than Au on the very surface (\u0026lt;\u0026thinsp;10 nm).\u003c/p\u003e \u003cp\u003eCombining the results of SEM-EDS, XRD, and XPS, the gold foil is an Au-Ag alloy, with a cross-section Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of 98%. Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% on the front is generally lower than that on the back. On the same side, Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% decreases as the colour darkens. The dominant element on the very surface (\u0026lt;\u0026thinsp;10 nm) is Ag, mainly existing as Ag\u003csub\u003e2\u003c/sub\u003eO, while Au is nearly undetectable. Ag\u003csub\u003e2\u003c/sub\u003eS and AgCl also exist on the surface of front-black and back-brown sites, which may contribute to the tarnishing phenomenon of the gold foil. However, there still remains a problem unresolved: why Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of back-brown is higher than that of front-gold, while displaying a darker colour? According to band theory [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], the colour of bulk metal is determined by the width of the energy band, which means the colour is closely related to the chemical composition of the metal. The paradox here indicates that chemical composition merely is insufficient to explain the problem. So we turned our attention to structural colours, looked into surface nanostructures, and tried to figure out the optical mechanism of gold tarnishing.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Optical mechanism of gold tarnishing\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eESEM-EDS was conducted to observe the surface nanostructures of the gold foil. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows an ESEM-EDS map scan of the front-gold site. Nanoparticles of about 50 nm are visible on the surface. In the map scan area, Au (74.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 wt%) almost disperses homogeneously, except for a particle marked in the red circle, where Ag (16.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 wt%) concentrates, suggesting the enrichment of silver compounds.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eESEM-EDS (Quattro) results of the gold foil.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSite/ Element\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eFront-gold wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBack-gold wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBack-brown* wt%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003eFront-black wt% (at%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSpectrum 41\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSpectrum 38\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eSpectrum 27\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAu\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e74.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e77.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e73.69\u0026thinsp;\u0026plusmn;\u0026thinsp;0.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e69.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.66 (21.92)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e30.88\u0026thinsp;\u0026plusmn;\u0026thinsp;0.62\u003c/p\u003e \u003cp\u003e(7.55)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e23.81\u0026thinsp;\u0026plusmn;\u0026thinsp;0.55\u003c/p\u003e \u003cp\u003e(6.26)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e16.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e11.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e9.82\u0026thinsp;\u0026plusmn;\u0026thinsp;0.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e16.77\u0026thinsp;\u0026plusmn;\u0026thinsp;0.46 (9.72)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e44.74\u0026thinsp;\u0026plusmn;\u0026thinsp;0.62\u003c/p\u003e \u003cp\u003e(19.97)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e53.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.65\u003c/p\u003e \u003cp\u003e(25.71)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e7.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e8.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e9.24\u0026thinsp;\u0026plusmn;\u0026thinsp;0.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e10.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.56 (53.05)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e10.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.62\u003c/p\u003e \u003cp\u003e(43.75)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.56\u003c/p\u003e \u003cp\u003e(26.56)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e2.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e7.26\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e3.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.36 (15.31)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e5.64\u0026thinsp;\u0026plusmn;\u0026thinsp;0.45\u003c/p\u003e \u003cp\u003e(16.98)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e9.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.52\u003c/p\u003e \u003cp\u003e(30.74)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e7.82\u0026thinsp;\u0026plusmn;\u0026thinsp;0.24\u003c/p\u003e \u003cp\u003e(11.75)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e5.46\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e \u003cp\u003e(7.98)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003cp\u003e(1.67)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.59\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e \u003cp\u003e(1.08)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e* Data collected by Quanta 200F.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows ESEM-EDS of front-black sites. The size of nanoparticles is about 140 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). There are two kinds of larger particles (grains and polygons) above nanoparticles. The diameter of grains is about 1200 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). Ag (19.97 at%) to S (11.75 at%) is approximately 2:1 (Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), suggesting the chemical composition of grains is Ag\u003csub\u003e2\u003c/sub\u003eS, which coincides with the result of XPS (Fig.\u0026nbsp;5). The diameter of grains inside polygons is about 274 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eg). Ag (25.71 at%) and Cl (7.98 at%) suggest the existence of AgCl (Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), which coincides with the result of XPS (Fig.\u0026nbsp;5). The diameters of nanoparticles of Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed \u0026amp; g are about 130 nm, similar to those of Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea. Since only Au, Ag, C, and O are detected on the sites of nanoparticles, nanoparticles are probably Ag\u003csub\u003e2\u003c/sub\u003eO combined with XPS results (Fig.\u0026nbsp;5).\u003c/p\u003e \u003cp\u003eIn summary, there exist Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles (~\u0026thinsp;140 nm) on the surface of front-black sites. Above nanoparticles, Ag\u003csub\u003e2\u003c/sub\u003eS (~\u0026thinsp;1200 nm) grains and AgCl (~\u0026thinsp;274 nm) grains inside polygons are scattered on the surface layer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows ESEM comparison and frequency counts of nanoparticle sizes of front-gold, back-gold, front-black, and back-brown sites. Average nanoparticle sizes of front-gold, back-gold, front-black, and back-brown sites are 36.44\u0026thinsp;\u0026plusmn;\u0026thinsp;6.22 nm, 44.46\u0026thinsp;\u0026plusmn;\u0026thinsp;9.47 nm, 123.10\u0026thinsp;\u0026plusmn;\u0026thinsp;32.63 nm, 82.02\u0026thinsp;\u0026plusmn;\u0026thinsp;11.97 nm, suggesting a strict correlation between colour depth and nanoparticle sizes, which is the larger nanoparticle sizes, the deeper colours.\u003c/p\u003e \u003cp\u003eThe phenomenon can be explained by localized surface plasmon resonance (LSPR) of metal nanoparticles. Localized surface plasmon resonance is the collective electron oscillation phenomenon between metal nanoparticles at the time when the frequency of the incident light is equal to the vibration frequency of the conduction electrons[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The frequency of LSPR is dependent on the shape, size, spacing, composition of the nanometric features and the dielectric constants of the surrounding medium[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. LSPR of nanoparticles causes strong absorption at specific wavelengths, which can be detected by UV-vis spectrum. The wavelength of the maximum absorbance (\u0026#120582;\u003csub\u003emax\u003c/sub\u003e) of the surface plasmon (SP) band increases with the increase of nanoparticle size, which coincides with the simulation of Mie theory[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The red shift is also accompanied by a small broadening of the SP band in the long wavelength region[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. For Au nanoparticle sizes larger than 25 nm, the red shift of \u0026#120582;\u003csub\u003emax\u003c/sub\u003e is about 0.7 nm per 1 nm increase in particle size[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. When the size of Ag is above 53 nm, a new peak starts to develop at shorter wavelengths, which is characteristic of the quadrupole component of the plasmon resonance[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. LSPR also exists in Ag-Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles. As the oxidation of Ag, the size of nanoparticles and the dielectric of the surrounding medium increase, leading to the red shift of \u0026#120582;\u003csub\u003emax\u003c/sub\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Based on the linear fitting of \u0026#120582;\u003csub\u003emax\u003c/sub\u003e-size (\u003cem\u003ed\u003c/em\u003e) data of Ag nanoparticles in water[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] (\u0026#120582;\u003csub\u003emax\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.9591\u003cem\u003ed\u003c/em\u003e\u0026thinsp;+\u0026thinsp;384.94, \u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.9951, 415 nm\u0026lt; \u0026#120582;\u003csub\u003emax\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;510 nm), calculated \u0026#120582;\u003csub\u003emax\u003c/sub\u003e of front-gold (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;36.44 nm), back-gold (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;44.46 nm), back-brown (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;82.02 nm), front-black (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;123.10 nm) are 419.89 nm (yellow), 427.58 nm (yellow), 463.61 nm (orange), 503.01 nm (brown), coincided with measured \u0026#120582;\u003csub\u003emax\u003c/sub\u003e of 430 nm for Ag\u003csub\u003e2\u003c/sub\u003eO (\u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;46.6 nm)[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt\u0026rsquo;s a pity that we were not able to obtain the UV-vis spectrum of the gold foil sample before returning the artefact. Hence, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, we used Mieplot software[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] to calculate the extinction efficiency (Q\u003csub\u003eext\u003c/sub\u003e = C\u003csub\u003eext\u003c/sub\u003e / (π\u003cem\u003er\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e), C\u003csub\u003eext\u003c/sub\u003e: extinction cross-section), scattering efficiency (Q\u003csub\u003esca\u003c/sub\u003e = C\u003csub\u003esca\u003c/sub\u003e / (π\u003cem\u003er\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e), C\u003csub\u003esca\u003c/sub\u003e: scattering cross-section), absorption efficiency (Q\u003csub\u003eabs\u003c/sub\u003e = C\u003csub\u003eabs\u003c/sub\u003e / (π\u003cem\u003er\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e), C\u003csub\u003eabs\u003c/sub\u003e: absorption cross-section) of Ag\u003csub\u003e2\u003c/sub\u003eO spheres[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] with the same mean size and size deviation as measured in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e in the air and water. As Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticle size increases, Q\u003csub\u003esca\u003c/sub\u003e decreases, Q\u003csub\u003eabs\u003c/sub\u003e increases except for the black site, Q\u003csub\u003eext\u003c/sub\u003e = Q\u003csub\u003esca\u003c/sub\u003e + Q\u003csub\u003eabs\u003c/sub\u003e increases, the wavelength of the maximum extinction (\u0026#120582;\u003csub\u003emax\u003c/sub\u003e) increases, and the range of extinction broadens. \u0026#120582;\u003csub\u003emax\u003c/sub\u003e of Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles in water is about 100 nm larger than that in air. CIE \u003cem\u003ex\u003c/em\u003e - \u003cem\u003ey\u003c/em\u003e chromaticity diagrams of black, brown, back-gold, and front-gold sites were converted from Q\u003csub\u003eext\u003c/sub\u003e-wavelength spectra. The colours in CIE \u003cem\u003ex\u003c/em\u003e - \u003cem\u003ey\u003c/em\u003e chromaticity diagrams of black, brown, back-gold, and front-gold sites are orange, yellowish, whitish, whitish in air and red-purple, orange, yellowish, yellowish in water. Simulated colours in the air are kind of different from real colours, which may be due to Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles being surrounded half by air and half by metal, not completely by air. However, the trend towards darker colours with the increase of nanoparticle size coincides with the tarnishing degrees observed.\u003c/p\u003e \u003cp\u003eIn summary, optical mechanism of gold tarnishing is localized surface plasmon resonance (LSPR) of Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles on the surface. With the increase of nanoparticle size, the wavelength of the maximum extinction (\u0026#120582;\u003csub\u003emax\u003c/sub\u003e) of the surface plasmon band increases, leading to the change of hues; the range of absorbance broadens, leading to the decrease of saturation, i.e. tarnishing. Results of other similar tarnishing phenomena and corrosion simulation samples will be discussed in more detail in our coming work.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Grey build-ups on the back\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eESEM-EDS (Quanta 200) of grey build-ups (at%).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElement/Site\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e28 Grey\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e32 Brown\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e44 Grey polyhedra\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e47 Grey layers\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAu\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e29.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e33.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e78.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e54.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e26.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e45.90\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e31.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e47.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e20.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e16.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThere are 15 irregular holes in the gold foil sample, mainly distributing in the lower body part of the gold foil tiger, with the largest lengths ranging from 41 \u0026micro;m to 732 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Near the holes on the back, there exist grey build-ups, which also exist on brown tarnishing areas. According to ESEM photos (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb-f), grey build-ups consist of multiple amorphous layers, in which embedded some polyhedra. Clear gaps exist between polyhedra and layers. EDS results show grey polyhedra are Ag, and grey layers are AgCl (Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The surface under grey build-ups is brown, suggesting the formation of grey build-ups is related to corrosion. Ag polyhedra are well encapsulated in the layers, suggesting Ag polyhedra are converted from AgCl layers, which may probably be due to the photodecomposition of AgCl[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Corrosion mechanism of the gold foil\u003c/h2\u003e \u003cp\u003eAccording to analyses of chemical composition and ESEM-EDS observation of the gold foil, it has already been confirmed that tarnishing is due to the corrosion of the less noble metal Ag in the Au-Ag alloy. The darkening degree is associated with the size of the corrosion product Ag\u003csub\u003e2\u003c/sub\u003eO. Optical mechanism of tarnishing is localized surface plasmon resonance (LSPR) of Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles on the surface. The corrosion mechanism of tarnishing is probably surface segregation, or rather selective corrosion of Ag[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In binary alloys, one element tends to move to the surface driven by thermodynamics, leading to the lowest surface free energy. The Enthalpy (\u003cem\u003eH\u003c/em\u003e) and Gibbs Free Energy (\u003cem\u003eG\u003c/em\u003e) of Au and Ag are 0, so surface segregation of Au-Ag alloy will not happen in the vacuum. However, O\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eS, carboxylic sulphide (COS), chloride etc. in the air promote the formation of silver compounds (\u003cem\u003eE\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0, \u003cem\u003eG\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0), such as Ag\u003csub\u003e2\u003c/sub\u003eO, Ag\u003csub\u003e2\u003c/sub\u003eS, AgCl, facilitating silver separation to the surface (Table\u0026nbsp;\u003cspan refid=\"Tab7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of surface ranges from 66.72\u0026ndash;93.59%, all much lower than that of cross-section (98.03%). On the very surface (\u0026lt;\u0026thinsp;10 nm), Au is even nearly undetectable (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e). As Ag moves to the surface, Au is enriched internally.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab7\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 7\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEnthalpy and Gibbs free energy values for Au, Ag and silver compounds[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpecies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAu\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAg\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAgCl\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAg\u003csub\u003e2\u003c/sub\u003eS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAg\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEnthalpy (\u003cem\u003eH\u003c/em\u003e, kJ/mol)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u0026minus;127.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026minus;29.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u0026minus;31.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGibbs Free Energy (\u003cem\u003eG\u003c/em\u003e, kJ/mol)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u0026minus;109.80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026minus;39.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u0026minus;11.21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eNevertheless, since \u003cem\u003eH\u003c/em\u003e of AgCl, Ag\u003csub\u003e2\u003c/sub\u003eS and \u003cem\u003eG\u003c/em\u003e of AgCl are lower than Ag\u003csub\u003e2\u003c/sub\u003eO, why the component of nanoparticles on the surface is Ag\u003csub\u003e2\u003c/sub\u003eO not the more stable silver compounds? According to the analysis of the former work[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], the gold foil was attached to the surface (probably the lateral side) of the lacquerware, which was usually placed in the atmosphere, not in direct contact with the human body as much as gold jewellery. O\u003csub\u003e2\u003c/sub\u003e concentration is much higher than chloride or sulphide in the atmosphere environment, resulting in a full exposure and a higher reaction rate. O\u003csub\u003e2\u003c/sub\u003e seizes the initiative to occupy reaction sites, forming a uniform layer of Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles, above which Ag\u003csub\u003e2\u003c/sub\u003eS and AgCl form afterwards as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed\u0026amp;g.\u003c/p\u003e \u003cp\u003eIt\u0026rsquo;s worth noticing that this gold foil is the only one piece with the severe tarnishing phenomenon, grey build-ups on the back and irregular holes of all the seven pieces. Since the burial environment is similar, the difference is more likely due to experience before burial. A possible speculation is the gold foil may undergo flaming by an oil lamp or other flaming devices (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e). Gold foil applied on the bamboo slice was close to the flaming side for 20 s. After being flamed, the surface turned from gold to reddish, similar to the brown site of the archaeological gold foil sample. Heating accelerates the oxidation of Ag[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e] and leads to the deformation of the substrate wood, resulting in the separation of the gold foil edges and the lacquerware. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the gold foil edges are brown on the back, suggesting a fuller exposure to O\u003csub\u003e2\u003c/sub\u003e and more severe corrosion than the centre area.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePitting corrosion is also only observed on this gold foil, which is probably related to the edge-off phenomenon due to the flaming. Crevice exists between the gold foil and the lacquerware surface. During burial, rainwater infiltration and groundwater circulation provide a liquid microenvironment. Due to the restricted mass diffusion, dissolved O\u003csub\u003e2\u003c/sub\u003e and pH decrease, while Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e accumulates inside the crevice[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], offering an electrolyte environment. Enrichment of Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e also facilitates Ag dissolution by reducing reduction potential (\u003cem\u003eE\u003c/em\u003e\u003csup\u003e0\u003c/sup\u003e): Ag\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; Ag (\u003cem\u003eE\u003c/em\u003e\u003csup\u003e0\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.799 eV), AgCl\u0026thinsp;+\u0026thinsp;e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; Ag\u0026thinsp;+\u0026thinsp;Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u003cem\u003eE\u003c/em\u003e\u003csup\u003e0\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.222 eV)[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The gold foil surface inside the crevice has a low O\u003csub\u003e2\u003c/sub\u003e concentration, becoming the anode, while the surface outside the crevice is well-oxygenated, becoming the cathode (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea). Gold foil with Ag\u003csub\u003e2\u003c/sub\u003eO passive film outside crevice acts as an inert electrode, where the cathodic reaction occurs: O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;4e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; 4OH\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u003cem\u003eE\u003c/em\u003e\u003csup\u003e0\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.401 eV) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. AgCl forms and builds up inside the crevice. Pit grows under AgCl build-ups. The AgCl layer passivates the sidewall of the pit, while Ag continues to dissolve at the tip of the pit, eventually resulting in the perforation (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003eb). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, some sites of AgCl build-ups are in the stage of Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea or b, with AgCl build-ups while not perforated. Holes with AgCl build-ups on the back are in the next stage of Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003eb, where pitting corrosion has been accomplished. Gold foil surface besides AgCl build-ups is brown for full exposure to O\u003csub\u003e2\u003c/sub\u003e. Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticle size of the back-brown site is not as large as that on the front (exposed to O\u003csub\u003e2\u003c/sub\u003e while being heated), but larger than that closely attached to the lacquerware (back-gold site).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eCombining the chemical composition results of SEM-EDS, XRD, and XPS, the gold foil is an Au-Ag alloy, with a cross-section Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% of 98.03%. Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% on the front (66.72%-82.94%) is generally lower than that on the back (75.92%-93.56%). On the same side, Au/(Au\u0026thinsp;+\u0026thinsp;Ag) wt% decreases as the colour darkens. The dominant element on the very surface (\u0026lt;\u0026thinsp;10 nm) is Ag, mainly existing as Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles, while Au is nearly undetectable. Selective corrosion results in Ag moving to the surface, while Au is enriched internally. Ag\u003csub\u003e2\u003c/sub\u003eS grains (~\u0026thinsp;1200 nm) and AgCl grains (~\u0026thinsp;300 nm) inside polygons (~\u0026thinsp;1500 nm) also exist on the surface of front-black and back-brown sites, above the Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticle layer.\u003c/p\u003e \u003cp\u003eAverage nanoparticle sizes of front-gold, back-gold, front-black, and back-brown sites are 36.44\u0026thinsp;\u0026plusmn;\u0026thinsp;6.22 nm, 44.46\u0026thinsp;\u0026plusmn;\u0026thinsp;9.47 nm, 123.10\u0026thinsp;\u0026plusmn;\u0026thinsp;32.63 nm, 82.02\u0026thinsp;\u0026plusmn;\u0026thinsp;11.97 nm, suggesting a strict correlation between colour depth and nanoparticle sizes, which is the larger nanoparticle sizes, the deeper colours. Optical mechanism of gold tarnishing is localized surface plasmon resonance (LSPR) of Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles on the surface. With the increase of nanoparticle size, the wavelength of the maximum extinction (\u0026#120582;\u003csub\u003emax\u003c/sub\u003e) of the surface plasmon band increases, leading to the change of hues; the range of absorbance broadens, leading to the decrease of saturation, i.e. tarnishing.\u003c/p\u003e \u003cp\u003eTarnishing and pitting corrosion only happened in this gold foil, which is probably due to the experience of being flamed in the atmosphere. Heating accelerates the oxidation of Ag[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e] and leads to the deformation of the substrate wood, resulting in the separation of the gold foil edges and the lacquerware. The edge-off phenomenon leads to crevice corrosion during burial, which develops into pitting corrosion and eventually results in the perforation of the gold foil. AgCl build-ups near holes are electrochemical corrosion products. Ag polyhedra are well encapsulated in the AgCl layers, which may probably be due to the photodecomposition of AgCl.\u003c/p\u003e \u003cp\u003eA comprehensive analysis of the extraordinary corrosion phenomena of the gold foil puts forward the understanding of optical and chemical mechanisms of gold corrosion and offers guidance for gold foil conservation.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAvailability of data and materials\u003c/h2\u003e\n\u003cp\u003eThe data presented in this study are available in the article.\u003c/p\u003e\n\u003ch2\u003eCompeting interests\u003c/h2\u003e\n\u003cp\u003eNo competing interests.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThis research received no external funding.\u003c/p\u003e\n\u003ch2\u003eAuthor\u0026rsquo;s contributions\u003c/h2\u003e\n\u003cp\u003eZisang Gong: Writing \u0026ndash; original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.\u003c/p\u003e\n\u003cp\u003eSiyuan Sun: Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003ePei Hu: Methodology, Conceptualization, Project administration, Supervision, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003eGang Hu: Methodology, Supervision, Conceptualization, Project administration,\u0026nbsp;Writing \u0026ndash; review \u0026amp; editing,\u0026nbsp;Funding acquisition.\u003c/p\u003e\n\u003cp\u003eJingrong Pei: Resources.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThanks to Li Chen in School of Physics, Peking University, for the support of ESEM-EDS. Thanks to Jianfeng Cui in School of Archaeology and Museology, Peking University, for the support of SEM-EDS. Thanks to Minqiao Ren in SINOPEC Beijing Research Institute of Chemical Industry, for the support of XRD. Thanks to Jinglin Xie in Analytical Instrumentation Center, Peking University, for the support of XPS. Thanks to Taiyuan Institute of Cultural Relics and Archaeology for providing the samples in this work. Thanks to Xin Wang in School of Archaeology and Museology, Peking University, for the preparation of lacquer. Thanks to Xiaolin Yang in Nation Museum of China, Jian Kang in Feifanshangshi Studio, Xinrui Zhang in College of Chemistry and Molecular Engineering, Shengyu Liu, Kaiwen Zhao, Minghao Jia, and Bo Tang in School of Archaeology and Museology of Peking University, for the discussion of corrosion phenomena.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBastidas DM, Cano E, Gonz\u0026aacute;lez AG, Fajardo S, Lleras-P\u0026eacute;rez R, Campo-Montero E, et al. An XPS study of tarnishing of a gold mask from a pre-Columbian culture. Corros Sci. 2008;50:1785\u0026ndash;8.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGard F, Daizo M, Santos D, Halac E, Freire E, Reinoso M et al. Application of surface science techniques to study a gilded Egyptian funerary mask: A multi-analytical approach. 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Metals for Biomedical Devices [Internet]. Woodhead Publishing; 2019 [cited 2024 Aug 28]. pp. 131\u0026ndash;52. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.sciencedirect.com/science/article/pii/B9780081026663000043\u003c/span\u003e\u003cspan address=\"https://www.sciencedirect.com/science/article/pii/B9780081026663000043\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-heritage-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"hsci","sideBox":"Learn more about [Heritage Science](http://heritagesciencejournal.springeropen.com)","snPcode":"40494","submissionUrl":"https://submission.nature.com/new-submission/40494/3","title":"npj Heritage Science","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Gold foil, tarnishing, pitting corrosion, ESEM-EDS, XPS, XRD, SPR, nanoparticle","lastPublishedDoi":"10.21203/rs.3.rs-5316041/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5316041/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo figure out optical and corrosion mechanism of tarnishing and pitting corrosion of the archaeological gold foil, ESEM-EDS, XRD, XPS were used to obtain chemical composition of different depths and observe surface topography. The results show Ag is enriched on the surface, mainly as Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles, and Au is enriched internally due to the selective corrosion of Ag. Optical mechanism of gold tarnishing is surface plasmon resonance (SPR) of Ag\u003csub\u003e2\u003c/sub\u003eO nanoparticles. The colour darkens as nanoparticle size increases. Flaming in the atmosphere results in accelerated oxidation of Ag and edge-off, leading to electrochemical crevice, pitting corrosion and perforation during burial.\u003c/p\u003e","manuscriptTitle":"Tarnishing and Pitting Corrosion Mechanism Revealed by Nanoparticles on the Gold Foil of Western Han Dynasty in Taiyuan, Shanxi, China through a Multi-analytical Approach","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-08 04:41:08","doi":"10.21203/rs.3.rs-5316041/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-11-20T22:10:31+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-11-08T12:05:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"336058633229030984382483768147500058633","date":"2024-11-08T08:30:42+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-11-07T21:56:32+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-11-05T13:34:21+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-11-05T13:32:13+00:00","index":"","fulltext":""},{"type":"submitted","content":"Heritage Science","date":"2024-10-23T06:01:10+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-heritage-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"hsci","sideBox":"Learn more about [Heritage Science](http://heritagesciencejournal.springeropen.com)","snPcode":"40494","submissionUrl":"https://submission.nature.com/new-submission/40494/3","title":"npj Heritage Science","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"08be6b07-7285-4efd-8035-febd92cd0cea","owner":[],"postedDate":"November 8th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-03-31T16:01:45+00:00","versionOfRecord":{"articleIdentity":"rs-5316041","link":"https://doi.org/10.1038/s40494-025-01616-y","journal":{"identity":"npj-heritage-science","isVorOnly":false,"title":"npj Heritage Science"},"publishedOn":"2025-03-27 15:57:03","publishedOnDateReadable":"March 27th, 2025"},"versionCreatedAt":"2024-11-08 04:41:08","video":"","vorDoi":"10.1038/s40494-025-01616-y","vorDoiUrl":"https://doi.org/10.1038/s40494-025-01616-y","workflowStages":[]},"version":"v1","identity":"rs-5316041","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5316041","identity":"rs-5316041","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}