Polychrome arhat figures dated from the Song Dynasty (960–1279 CE) at the Lingyan Temple, Changqing, Shandong, China

Scientific analysis revealed the materials and techniques used in the process of making polychrome sculptures providing a solid foundation for the protection and restoration of the painted statues. In addition, the analyses revealed changes in colour schemes applied to the sculptures can provide the basis for the virtual restoration of the painted statues. In order to carry out scientifically-informed protection and restoration of the Bodhidharma statue from the Lingyan Temple, Changqinq, Shandong, several analytical methods such as optical microscope (OM), Micro-Raman spectroscopy (μ-RS), scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM–EDS) and Fourier transform infrared spectroscopy (FTIR) were employed. Analyses clearly reveal the information including the stratigraphic structure and the composition of pigment. The use of silver foils and golden yellow pyrophyllite mineral to replace gold foils were found in the gilding paint layer in the later repainting after the Song Dynasty. This work reports the coexistence of emerald green (Cu(C2H3O2)2·3Cu(AsO2)2) and the degradation product lavendulan (NaCaCu5(AsO4)4Cl·5H2O) in large areas of the paint stratigraphy and on the surface confirming that the degradation of emerald green is related to the thickness of the paint layer; in thinner paint layers emerald green is transformed in lavendulan, while thicker layers of contain both lavendulan and emerald green, suggesting an environmental source of chlorides.


Introduction and the aims of research
The Lingyan Temple (灵岩寺) is located in the Lingyan mountain in Wande town (万德镇), Changqing district ( 长清区), Jinan, Shandong province, China, about 20 km from Mount Tai (Fig. 1). The temple is an important part of Mount Tai world natural and cultural heritage. As early as the Yuan-he period (806-820 CE) of the Tang Dynasty (618-907 CE), Li jifu (李吉甫), a knowledgeable prime minister, reports Lingyan Temple together with Qixia Temple (栖霞寺) in Jiangsu, Guoqing Temple (国清 寺) in Zhejiang and Yuquan Temple (玉泉寺) in Hubei as the "Four famous temples in Chinese domain". The extant stele inscriptions record that the Lingyan Temple ranked first among the four famous temples in the Song Dynasty. After the founding of the People's Republic of China, Lingyan Temple was classified in 1982 as a national key cultural relic. There are many precious historical relics in Lingyan Temple, such as Pizhi pagoda (辟支塔), the forest of stupas (墓塔林), stone inscriptions (there are more than 700 pieces) and many ancient buildings and ruins. Among the numerous precious historical relics, 40 painted arhat statues dating from the Song and Ming (1368-1644 CE) Dynasties are preserved in the Qianfo hall (千佛殿). These polychrome statues, especially the works of Song Dynasty, are skilfully sculpted with vivid and lifelike characters, and are recognized as rare masterpieces among ancient Chinese sculptures, and were once praised as the first famous statues in China by Mr. Liang Qichao (梁启超).
In May 1981, Jinan cultural relics management committee, Jinan museum and the Lingyan Temple cultural relics management institute of Changqing county jointly initiated the protection and repair of the arhat statues of Lingyan Temple [1]. In the process of maintenance, C-14 dating of the wooden structures within some of the arhat sculptures determined that of the 40 arhat statues, 27 were from the Song Dynasty and 13 date to the Ming Dynasty [1,2]. The objective of this paper is the Bodhidharma (菩提达摩) statue, which is a typical Song Dynasty work. This statue is colorful and rich, the expression of the characters is solemn and serene, tolerant and introspective. The statue is also well preserved and has extremely high artistic value. Bodhidharma was a native of India who came to China during the Northern and Southern Dynasties (420-589 CE) and founded Chinese Zen Buddhism (禅 宗). Because of the spread and popularity of Zen Buddhism in China, Bodhidharma was a household name in China. The statue of Bodhidharma was placed at the first place on the right hand side of the entrance to the Qianfo Hall, which meant the statue received more attention and favor from visitors than other statues.
For a long time, natural aging and many environmental factors (such as dust, smoke, roof leakage, the presence of insects, pollution.) lead to degradation of pigments, efflorescence and the flaking and peeling of paint. It is of great significance to analyse and study the paint layers of arhat statues for the subsequent conservation and virtual restoration. In 2018, Wang Chuanchang and others analysed samples from the sixth and seventeenth arhat statues in the east of the Qianfo Hall, and determined the mineral composition of various mineral pigments [3]. Besides that, there was little scientific analysis and research on the paint layers of arhat statues of Lingyan Temple. Complementary and multiple analytical techniques are widely used in the study of paint layers of painted cultural relics such as polychrome statues [4][5][6][7], mural paintings [8][9][10][11], painted pottery [12,13] and so on, which can successfully reveal the microscopic structure of paint layers, the composition of the pigment and the priming layers, the types of binding media found in paint layers, repair materials, and the paint stratigraphy. In this paper, multiple analytical techniques including ultra depth of field 3D microscopy (OM), Micro-Raman spectroscopy (μ-RS), scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR), etc. were applied to analyse the paint layers from the Bodhidharma statue. The purpose of this study is to reveal the materials and techniques used in the making the painted statues, and the changes to the composition of the paint layers as a consequence of aging, provide basic data for the protection and restoration of the painted statue and the virtual restoration of the colour texture.

Materials
A total of 19 samples were collected from different colored paint on the Bodhidharma statue. The schematic diagram of sampling position was shown in Fig. 2. Except for D1-3, D1-10, D1-19, other samples were embedded in the epoxy resin and polished.

Optical microscopy (OM)
A KEYENCE VHX-6000 ultra depth of field 3D video microscope was used to observe and document the cross-sections, at magnifications from 20 to 2000× . The Leica DM2700 polarizing microscope was used for the polarizing microscopic analysis of individual pigments.

Scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDS)
A Tescan vega3 XMU scanning electron microscope equipped with a Bruker XFlash 610 M X-ray energy spectrometer was used to analyse the microstructural characteristics of paint layers and semi-quantitatively analyse the relative contents of major elements in pigment minerals in different layers. Analyses were carried out in a high vacuum environment, with a scanning voltage of 20 kv, 90 s of acquisition time at a 15 mm working distance.

Micro-Raman spectroscopy (μ-RS)
A HORIBA XploRAPLUS Raman spectrometer configured with an Olympus microscope and an integrated motorized stage were used to qualitatively analyse the mineral composition of different layers under 50× and 100× objective lenses. The blue and green pigments were analysed using a 532 nm laser, and the other pigment were analysed using the 785 nm laser. The laser output power ranged of 15-30 mW (532 nm) and 10-50 mW (785 nm), the spectral range of 50-2000 cm −1 , and the collection time was 15-25 s with 2 acculumations. The instrument was calibrated using the 520 cm −1 silicon Raman band.

Fourier transform infrared microscopy (FTIR)
A Thermo Nicolet iN10 MX Fourier transform infrared spectrometer was used to study the priming materials of paint layers based on the preparation of dispersions in KBr in the spectral range of 4000-400 cm −1 , at spectral resolution of 4 cm −1 .

Spectrophotometer
A X-Rite VS450 non-contact desktop spectrophotometer was used to measure the L*a*b* values of different colours at each sampling position on the statue surface. Measuring aperture 6 mm, measuring distance 1.5 inch.

Results and discussion
The EDS results of relative elemental composition in each layer of the painting and the attribution from μ-RS analyses are shown in Table 2. The elemental composition is given as the mass percentage after normalization treatment. In the processing of element relative content analysis, the free selection mode was used to select areas of interest within each layer. The layers sequence was numbered in the micrographs of cross-sections in Fig. 3, 4 and 8 correspond to the same numbers used in EDS analysis. The M lines were chosen to quantify the Pb. Organic layers that did not contain metal ions are: D1-2 L0; D1-9 L2, L5, L8, L10, L12, L14; and D1-16 L1, L3, etc. The elemental composition of these layers are not listed in Table 2.

Analysis of stratigraphy
The cross-sections observed under the optical and scanning electron microscope clearly show that the paint layers contains multiple layers of pigments and priming layers. D1-1, D1-7and D1-9 are samples with the most layers. As shown in Fig. 3a and b, D1-1 has 14 layers from the statue surface to the unpainted clay ground, among which are seven pigment layers, and the layers structure distribution are as follows: L1 reddish-brown pigment layer → L2 white priming layer → L3 red pigment layer → L4 white priming layer → L5 orange pigment layer → L6 white priming layer → L7 yellow pigment layer → L8 white priming layer (mixed with red pigment particles) → L9 orange pigment layer → L10 white priming layer → L11 red pigment layer → L12 white priming layer → L13 white pigment layer → L14 gray priming layer. The structural relationship between the layers of D1-9 gilding sample can be shown more clearly from the backscattered electron image (Fig. 3j). The sample from the statue surface to the unpainted clay idol totals 15 layers, including six gilding layers (L1, L4, L7, L9, L11, L13), six adhesive layers (L2, L5, L8, L10, L12, L14), two pigment layers (L3, L6), and one priming layer (L15). The layer structure of D1-7 gilding paint layer is not very clear under the optical microscope annular light source (dark field), but clearly visible under the coaxial light source (bright field) and in the backscattered electron image (as shown in Fig. 3e and f ). The layer structure is basically the same as the layer structure of D1-9, but there is an extra priming layer under the 12th layer of adhesive. According to our analyses, the D1-7 and D1-9 contain six gilding layers and two pigment layers demonstrating that the Bodhidharma statue has experienced at least 8 repainting treatments.
In other samples of paint there are only two layers. As seen from Fig. 3c and d, sample D1-2 located on the face of the statue has only one reddish-brown pigment layer and one priming layer, suggesting that the color of the face has always remained the same in the previous repainting process. The reddish-brown pigment layer was covered with a dark translucent organic material (this translucent material is almost invisible in backscattered electron images).
The statistical analysis function of the built-in software of KEYENCE VHX-6000 ultra depth of field 3D microscope was used to measure the thickness of paint layers, pigment layers, priming layers and particles sizes of the pigment. The measurement results are shown in Table 1. The thickness of paint layers are various in different positions of the sample locations. The D1-2 paint layer on the face is only 205 μm, while the paint layers at the gilding are relatively thick, such as the thickness of D1-7 is 895 μm. The thickness of pigment layers and priming layers are also different, with the thinnest only 10 μm and the thickest up to 570 μm. The thickness of gilding layers (gold foil) are only about 2 μm (as determined with SEM-EDX), while the thickness of adhesive layers are about 35-70 μm. The pigment particles size of different pigment layers also varies greatly, for example, the average diameter of black pigment particles in D1-11 L1 is less than 1 μm, while the average diameter of orange pigment particles in D1-14 L3 is 35 μm. Figure 4 are the micrographs of pigment particles observed by the microscope under different light sources and the distribution maps of pigment particles were fitted by the micrograph under coaxial light source (bright field).  The pigment layers and the priming layers usually appear alternatively, indicating that during repainting the statue was generally primed with a white layer that was applied over the last pigment layer. For the gilding layers very few priming layers are found. Instead new gold foil layers were applied directly over the last gold foil using an adhesive. In some places there is no priming layers between the old and new pigment layers. For example, the orange pigment layer L1 of the D1-12 was applied directly over the black pigment layer L2 (Fig. 4a).
Paint layers represented by D1-8 and D1-19 are clearly different from other paint layers in terms of layer structure. The whole paint layers were painted on a layer of white paper (Fig. 3t, x, paper fibers are attached to the back of the paint layers), and the white paper was pasted over a white ground layer. Taking D1-8 as an example, the layers from the surface to the unpainted clay is as follows: L1 pink pigment layer → L2 white priming layer → L3 brown priming layer → L4 white paper → L5 white ground layer → clay body. D1-8 and D1-19 come from the inside of the sleeves and the black cassock area and are associated with new painting technology in the latest polychromy. The application of this new painting technology was likely designed to realistically shape the texture of clothes worn by Bodhidharma statue.

Analysis of gilding layers
EDS analyses show that the gold layers of D1-7 and D1-9 contains only Au and Ag except some layers (the 11th layer of D1-7 contains 3.6% Cu). The gold content of most gold layers is more than 90%, which indicates that these gold layers are gold foils with high gold content. The thickness measured by the electron microscope is only 2 μm at the thickest point. The 7th and 9th layers of sample D1-7 can hardly be observed under the optical microscope, while they are clearly evidenced in the backscattered electron images (Fig. 3f ). EDS results show that the two layers mainly contain Cl and Ag, with a mass ratio of about 1/3.7 to 1/3.2. The Raman spectrum of white particles in layer 7 (Fig. 5a) shows characteristic bands at 147vs, 221 m, 292 m cm −1 similar to chlorargyrite (AgCl, Fig. 5b). Combined with the results of EDS, it can be inferred that the main phase of D1-7 L7 and L9 are chlorargyrite. In terms of thickness and stratigraphic structure, D1-7 L7, L9 are only 1-2 μm in thickness like other gold foil layers. Therefore, these two layers were silver foils that were pasted on the statue surface at two separate times. Corrosion later transformed the silver foils into chlorargyrite.
The L3 and L6 of D1-7 and D1-9 paint layers appear golden yellow in cross-section with mineral granularity (Fig. 3e, i). The four layers of golden yellow material were painted over gold foil layers. From the colour and its layer position, it can be confirmed that these layers were applied over gold foils. The results of EDS show that these four layers mainly contain Al, Si, K, Ca, Fe. In addition, D1-9 L3 contains 8% Au, which is due to the curious inclusion of gold particles in this layer (Fig. 3i). The Raman spectrum of golden yellow particles in layer 3 (Fig. 5c) shows characteristic bands at 216 m, 260vs, 705 m cm −1 similar to pyrophyllite (Al 2 Si 4 O 10 (OH) 2 , Fig. 5d). Combined with the results of EDS, it can be inferred that the main phase of the L3 and L6 of D1-7 and D1-9 are high iron pyrophyllite clay mineral.
In ancient China, especially since the Song Dynasty (960-1279 CE), the decorative technology of embossed painting and gilding (沥粉堆金) on Buddha statues was very popular. As the saying goes, "Clothes to people, Gold to Buddha". However, the use of silver foil and gold mixed with golden yellow mineral to replace gold foil used in the decorating Buddha statues is rarely mentioned in the literature. The decorative technology of paste silver foils were found in ancient murals [14,15] and architectural paintings [16] in small amounts. Pyrophyllite as a pigment was once found in bodhisattva statue of the Ming Dynasty (1368-1644 CE) grotto 51 in Maijishan (麦积 山) [17]. In addition, studies show that pyrophyllite yellow clay mineral is one of the iron-containing yellow pigments used in ancient China, in which the colour composition is iron yellow (ɑ -FeOOH) [18]. The discovery of silver foil layers and golden yellow pyrophyllite mineral with gold particles in D1-7 suggests that there were other alternatives to gold that were employed to imitate gilding.

Analysis of the priming layers
In terms of element distribution, the priming under the pigment layers can be roughly divided into five types. The first type is the white mineral dominated by Al, Si (Al/Si ratio between 1/2 and 1/1.2) and containing a small amount of K, Fe, Ca, Pb and other elements, such as D1-1 with priming layers, D1-2 L2, D1-5 L2 L4 L6, D1-6 L2 L4 L6 L8, D1-11 L5. These white minerals showed no obvious Raman scattering except quartz particles. In order to determine the mineral phase of the priming layers, three samples with simple layer structure, D1-2, D1-5, D1-6, were selected for FTIR analyses. Figure 6A, B and C are the infrared spectra of the above three samples respectively, and their main infrared characteristic absorption bands are basically one-to-one corresponding to the infrared characteristic absorption bands of kaolin [19]. The second type of priming layer mineral dominated by Al and Si (Al/Si ratio between 1/2 and 1/1.2) and containing Pb, including D1-4 L2, D1-11 L2, D1-13 L4, L5 and L6, which contain 30%-45% Pb. Lead white (2PbCO 3 ·Pb(OH) 2 ) particles were identified with Raman spectroscopy in layers D1-4 L2 and D1-13 L4. Therefore, it can be inferred that these leaded priming layers are mixtures of kaolin and different proportions of lead white. The third type of priming layer mineral is dominated by Ca and Pb. For example, the contents of Ca and Pb in D1-4 L4 are 57% and 38%, while the contents of Al and Si are only 2% and 3%. Micro-Raman spectroscopy revealed two white minerals, chalk (CaCO 3 ) and lead white in D1-4 L4. Combined with the results of EDS show that the priming layer composed of chalk and lead white, mixed with a small amount of kaolin. The fourth type of priming layer mineral dominated by Pb, such as D1 D1-4 L411 L4 and D1 D1-4 L415 L4, containing Pb content in the two layers as high as 85% and 60% respectively. μ-RS confirmed that the main mineral phase in these two layers is lead white, so this type of priming is lead white with a small amount of kaolin added. The fifth type of priming layer mineral dominated by Si, Al, Ca, and containing a certain amount Fe. For example, under the pink pigment layer of D1-8 sample, there are two priming layers of white and brown, and the Al/Si ratio of their chemical composition are about 1:3. Both of these two layers contain Fe (the white layer 13% and the brown layer 16%), which is different from other kaolin-based priming layers mineral in chemical composition. The priming of these two layers have no obvious Raman activity. According to the relative content of elements, it can be inferred that the white priming layer should be a clay with low aluminum and high calcium, while the brown priming layer is a clay with low aluminum and high iron.
In summary, the priming layers material used in each previous polychrome of the statue is mostly white kaolin or a mixture of other white minerals based on kaolin as the matrix. The use of a kind of priming material with low aluminum and high iron in D1-8 is obviously different from other paint layers.

Analysis of pigment layers
The results of μ-RS analyses of mineral pigments in each pigment layers are shown in Table 2. Raman analyses are based on published literature [20,21]. Raman spectra of some pigments are shown in Figs. 8 and 9. Table 3 shows the L*a*b* values of different colors at each sampling position on the statue surface measured by the X-Rite VS450 non-contact spectrophotometer.

Reddish-brown pigment layers
The reddish-brown pigment layers have three layers: D1-1 L1, D1-2 L1 and D1-12 L1. μ-RS analyses show that the D1-1 L1 and D1-2 L1 pigment layers are composed of hematite (Fe 2 O 3 ) and red lead (Pb 3 O 4 ). EDS analyses show that the Fe content in the two layers are significantly different (17%, 31%, respectively), indicating Fig. 6 a, b and c the back or exposed white priming layer of D1-2, D1-5, D1-6 before samples were embedded respectively; A, B and C correspond to the infrared spectra of the three samples respectively that the different colour requirements of the pigment layers at different locations (D1-2 L1 is darker in colour than D1-1 L1). The pedestal area represented by D1-12 L1 is reddish-brown (Fig. 2), while the pigment layer presents a bright orange-red on the cross-section (Fig. 4a).
Raman analysis shows that the orange-red mineral is red lead (Pb 3 O 4 ). The change from bright orange-red to reddish-brown on the exposed surface of the pigment layer should be related to the oxidation and deterioration of red lead pigments, and it is well known that red lead can be transformed into brown-black lead dioxide (PbO 2 ) in high humidity environment [22][23][24].

Red pigment layers
There are eight red pigment layers: D1-1 L3, L11, D1-5 L5, D1-6 L1, D1-13 L1, L3, D1-15 L3, L5. Raman analyses show that the mineral composition of the red pigment layers are mainly cinnabar (HgS) except for D1-5 L5. EDS analyses results also show that these pigment layers mainly contain Hg and S except for a small amount of Si, Al, Ca and other elements. D1-5 L5 is made of two minerals, cinnabar and red lead.

Orange pigment layers
There are seven orange pigment layers: D1-1 L5, L9, D1-4 L1, D1-11 L3, D1-13 L2, D1-14 L3 and D1-16 L4. μ-RS analyses show that the mineral phase of D1-16 L4 is hematite (Fe 2 O 3 ), and the rest of the pigment layers are all red lead (Pb 3 O 4 ). EDS analyses results show that the content of Pb in the pigment layers containing red lead are above 90%, which indicates that these pigment layers are basically composed of pure red lead.

Pink pigment layers
The pink pigment layers are D1-3 L1 and D1-8 L1. D1-3 L1 is composed of hematite (Fe 2 O 3 ) and anatase titanium dioxide (TiO 2 ), while D1-8 L1 is composed of hematite and chalk (CaCO 3 ) (the relative mass ratio of Ca(Mg) to Fe element is 2.4/1). The pink paint layer D1-3 L1 is directly painted on the unpainted clay idol (Fig. 3u). This is obviously different from the characteristics of paint layer painted over white paper in D1-8 L1, so it can be inferred that the pink area of hematite + anatase titanium dioxide in D1-3 L1 is a repair place after partial damage and shedding of the original hematite + chalk pink area.

Blue pigment layers
The blue pigment layers are D1-5 L1 and D1-10 L1. Raman analyses shows that the blue mineral in D1-5 L1 is azurite (Cu 3 (CO 3 ) 2 (OH) 2 ), and there are white chalk (CaCO 3 ) particles mixed in L1. D1-10 L1 mineral Raman characteristic bands at 258w, 548vs, 1091 m cm −1 (Fig. 8j) are completely consistent with the characteristic bands of ultramarine. Raman spectrum and chemical composition cannot distinguish natural and synthetic ultramarine pigment, but polarizing microscope analysis is an effective and simple method. Natural ultramarine pigment is made of lapis lazuli mineral grinding, because lapis lazuli mineral is often accompanied by diopside, calcite, pyrite, so there are more impurities in the pigment; particles shape are irregular, angular, with particles diameter in the 10-20 μm interval. But artificial ultramarine pigment is pure in texture, and the particles shape are regular, uniform and fine (average diameter is 5 μm) [25]. D1-10 L1 blue pigment is bright and colourful under plane polarized light (Fig. 7a), with pure texture, smooth particle edge, particles diameter are 3-6 μm, completely extinction under orthogonal polarized light (Fig. 7b), which correspond to the characteristics of artificial ultramarine pigment (Na 6-10 Al 6 Si 6 O 24 S 2-4 ).

Yellow pigment layers
There are six yellow pigment layers: D1-1 L7, D1-14 L1, D1-7 L3, L6, D1-9 L3, L6. As discussed in the "Analysis of gilding layers" section, the golden minerals in D1-7 and D1-9 L3, L6 were high iron pyrophyllite clay minerals that were used to replace gold foil. D1-1 L7 mainly contains As, S, Pb, and Raman analysis shows that the main mineral phase is orpiment (As 2 S 3 ). In cross-sections of D1-14 L1 (Fig. 9d), it can be clearly seen that yellow mineral particles and white mineral particles are mixed together. According to Raman analyses, yellow pigment particles are chrome yellow (PbCrO 4 , Fig. 9e), and white pigment particles are chalk (CaCO 3 ). Chrome yellow is a synthetic pigment that appeared in the early nineteenth century [26].
Lavendulan (NaCaCu 5 (AsO 4 ) 4 Cl·5H 2 O) is a rare supergene arsenate mineral in oxidation zones of Cu-and As-bearing ore bodies [29]. It was first described by Breithaupt in 1837 and named as a mineral containing As, Co, Ni; Vogl describes it as blue coating from Jáchymov; Foshag published a detailed description of lavendulan and gave its refractive index [30]. Guillemin first used X-ray diffraction analysis to study lavendulan and proposed its original rhombic crystal structure [29]. The mineral, which has a vivid blue-green color and a small spherical or wafer-like microstructure, is often found in arid climates, and in caves derived from copper sulfides in cave walls. Also, it has been reported as an alteration products in ancient slag heaps. Further, it is certain that some of these minerals are of archaeological significance and it is apparent that they were used for cosmetics in ancient Egypt [31].
Lavendulan has also been found sporadically in Chinese polychromy such as the painted clay statue in Yungang Grottoes (云冈石窟), the coloured drawing on timber structure in Kumbum (塔尔寺) [32], the wall painting in Anyue Grottoes (安岳石窟) [33], the painted statues on Baoding Mountain in Dazu (大足宝顶山) [34][35][36], and the paintings of the Maidservant in the Saint Mother's Hall of Jinci Temple (晋祠) [36]. The distribution of lavendulan is extremely rare in nature. Due to its physical microscopic structure and the fact that the chemical composition is similar to the widely-used emerald green pigment from the 1830s to the early 20th century. Chinese scholars believe that the lavendulan found in polychromy is most likely an alteration product from Fig. 8 a, c, e, g cross-sections of D1-4, D1-5, D1-6, D1-13, respectively, and b, d, f, h are the corresponding backscattered images, i Raman spectrum of orpiment, hematite, red lead and cinnabar, j Raman spectrum of chalk, lead white, ultramarine and azurite emerald green pigment, and was not intentionally used as the blue-green pigment [32][33][34][35][36].
In literature [36], the mechanism of corrosion conversion of emerald green pigment into lavendulan has been studied in depth. It is suggested that emerald green (Cu(C 2 H 3 O 2 ) 2 ·3Cu(AsO 2 ) 2 ) is first dissociated into copper ions (Cu 2+ ) and arsenite ions ((AsO 2 ) − ) in a slightly acidic and humidity environment. The (AsO 2 ) − is oxidized to arsenate ion ((AsO 4 ) 3− ), and finally Cu 2+ , (AsO 4 ) 3− reacts with soluble K + , Ca 2+ , Cl − from the environment to produce lavendulan. There are no residual emerald green pigment particles were found in the blue-green pigment layer D1-18 L1. However, the simultaneous presence of layers of green and blue-green pigments can be seen in other blue-green pigment areas (Fig. 9a), which shows the signs of green emerald green transformation into blue-green lavendulan. Therefore, it can be inferred that the blue-green pigment layer D1-18 L1 (the part of plant and flower pattern above the black pigment layer of the cassock of the Bodhidharma statue, as shown in Fig. 2) may have transformed from emerald green. The thickness of the pigment layer in D1-18 L1 is 75 μm, and its mineral phase was almost completely transformed into blue-green lavendulan, while the emerald green pigment layer D1-15 L1, with a thickness of 570 µm has no sign of alteration. This observation suggests that the transformation of emerald green pigment depends on the thickness of the pigment layer, with thinner pigment layers exhibiting lavendulan. This could be because the thinner the pigment layer, the easier it is to contact with the moisture and acid substances in the air, and also easier it is for the soluble salt ions to migrate in the pigment

Modern rose red paint layer
The rose red paint layer is only distributed in the bottom step area of the statue pedestal represented by D1-16. From the preserved photos [37], this area was originally reddish brown before restoration and protection in 1981, and the paint layer was largely peeled off (see Fig. 10a). According to the interview with the staff of Lingyan Temple, the rose red area was painted in the restoration and protection of the Arhat statues in 1981; in addition, a layer of varnish was also painted in the bottom step area of pedestal in order to isolate moisture [1].
EDS analysis shows that the mass percentage of C and O elements in D1-16 L1 is more than 75% consistent with an organic pigment. The characteristic bands in the Raman spectrum (Fig. 11b) Fig. 11c). The infrared spectrum of the D1-16 is shown in Fig. 11d, in which the absorption bands around 3538, 3405 cm −1 and 1685, 1621 cm −1 correspond to the stretching and bending vibration of water; the absorption bands around 1141, 1115 cm −1 correspond to the symmetric and asymmetric stretching vibration of (SO 4 ) 2− , and the bands around 671, 602 cm −1 correspond to the bending vibration of (SO 4 ) 2− . The characteristic bands correspond to the infrared characteristic absorption bands of gypsum (CaSO 4 ·2H 2 O) [39]. D1-16 L3 appears Fig. 10 Comparison between the current state of the Bodhidharma Statue and the state before the conservation and restoration in 1981. a The state before 1981, and the picture is from literature [37], b the current state brown-black, and the content of C and O elements in this layer is more than 85%, which should be some kind of organic matter. The infrared spectrum of D1-16, in addition to the characteristic absorption bands of gypsum, exhibits bands in the vicinity of 2926, 2855, 1581, 1385 (Symmetrically deformed vibration of dimethyl in bisphenol A), 1456, 1305 (stretching vibration of fatty aromatic ether C-O), 913 (stretch vibration of epoxy ring C-O) cm −1 , etc. These bands basically match with the infrared characteristic bands of an epoxy resin [40], so it can be inferred that L3 was coated with epoxy resin on the bottom step of pedestal for moisture insulation in the previous restoration. Raman analysis shows that the orange-red mineral pigment in L4 is ochre (Fe 2 O 3 ).
The determination of the minerals composition of the outermost pigment layers may also provide a basis for the estimation of the repainting time of the latest paint of the Bodhidharma statue. According to the results of Raman analyses, the yellow pigment layer D1-14 L1 contains chrome yellow and chalk, and the green pigment L2 is emerald green. The L1 yellow pigment layer is the floral motif painted over the green pigment layer. It can be seen from the cross-sections that there is no priming layer between the yellow pigment layer and the green pigment layer, indicating that the two layers of pigment were painted on the surface of the statue at the same time. Chrome yellow appeared in the early 19th century [26], emerald green (chemical name "copper acetoarsenit"), was first synthesized in Germany in 1814, mainly used in the 1830s and early twentieth century [41]. The use of chrome yellow and emerald green in the outermost pigment layer confirms that the statue was repainted later than 1814. Chinese scholars' textual research on the existing inscriptions and other materials in Lingyan Temple have unanimously concluded that the last polychrome of the arhat statues of Lingyan Temple was in the thirteenth year of the reign of Tongzhi (1874CE) [2,42] in the Qing Dynasty(1639-1912 CE). Chrome yellow and emerald green were most likely painted on the surface of the statue in the 1874 redecoration. The use of anatase titanium dioxide pigment in D1-3 pink pigment layer and D1-16 L2 indicated that the statue had also been treated after 1874, because the commercial titanium dioxide pigment appeared in 1916 [43]. There is no mention of the polychrome repainting of the arhat statues in Lingyan Temple after 1874 in various Chinese documents. From the photographs taken before the restoration of the Bodhidharma statue in 1981 (Fig. 10a), it can be seen that the state of the pink area from D1-3 (the inside of the hat) before 1981 is similar to its current condition. Based on the use time of anatase titanium dioxide, it can be inferred that the pink area containing anatase titanium dioxide should has been painted between 1920s to the 1980s. In addition, the Bodhidharma statue was painted with a rose red organic pigment on the previously reddish-brown step area. In summary, the most recent large-scale repainting of the statue took place in 1874, and there have been at least two partial repainting treatments since then.

Conclusions
Based on the scientific analyses of the painted layers of Lingyan Temple's Bodhidharma statue, there are some insights.
The statue has been painted at least eight times and there have been at least two partial repainting treatments in the 20th C. Before each new polychrome, white wash was applied over the last pigment layer. In the most recent polychrome, a new technique of pasting white paper on the clay figure surface and then painting the priming layer and the pigment layer was used in some areas.
The gold layer in the gilding decoration area mostly uses gold foil with a high content of gold, and its thickness is only 1-2 μm. In different periods of history, silver foil and gold particles mixed with golden yellow mineral pigments were used to replace the original gold foil layers in the gilding area. The priming layers material used in the each previous polychrome of the statue was mostly white kaolin or a mixture of other white minerals based on kaolin as the matrix. Sometimes lead white, chalk or a minerals mixture of the two white pigments were used as the priming layers material. In addition, the clay with a low aluminum and high iron content has been used as priming layers material in the most recent repainting.
The pigment layers of the statue were mostly composed of a single inorganic mineral, and the individual pigment layers were made of 2-3 kinds of minerals to achieve different colours. The red series mineral pigments used in the statue are cinnabar (HgS), hematite (Fe 2 O 3 ) and red lead (Pb 3 O 4 ); Yellow pigments include orpiment (As 2 S 3 ) and chrome yellow (PbCrO 4 ); White pigments include lead white (2PbCO 3 ·Pb(OH) 2 ), chalk (CaCO 3 ) and magnesite (MgCO 3 ); Blue pigments include azurite (Cu 3 (CO 3 ) 2 (OH) 2 ) and artificial ultramarine (Na 6-10 Al 6 Si 6 O 24 S 2-4 ); Green pigments include 19th C. emerald green (Cu (C 2 H 3 O 2 ) 2 ·3Cu (AsO 2 ) 2 ) and coppercontaining pigments in a glassy state; Black pigments were only lamp black (C). In addition, organic pigments and anatase titanium dioxide (TiO 2 ) have been used in partial repainting more recently. The blue-green lavendulan mineral on the surface of the statue is not intended to be used as a blue-green pigment, but rather is a degradation product of emerald green pigment. The transformation of emerald green pigment seems related to the thickness of the pigment layer. The thinner the pigment layer, the easier it is to transform.
Based on the scientific analyses, a new understanding of the structure, material, technology and other aspects of the painted layers of Bodhidharma statue were formed.
There are still open questions related to the identity of the green copper-containing pigments in glassy state. What kind of technology and materials were used in ancient China to make this kind of pigment? In addition, the determination of organic materials such as the oily substance covering the face and neck of Bodhidharma statue, the adhesive materials used to apply the gold foil in the gilding paint layers, and the pigment binder are also crucial for our original intention. Further research is needed to identify binders, and also identify the source of the chloride ions which have led to the degradation of silver and arsenic based pigments.
Abbreviations OM: Optical microscopy; SEM-EDS: Scanning electron microscopy coupled with energy dispersive X-ray analysis; FTIR: Fourier transform infrared spectroscopy.