The EDS results of relative elemental composition in each layer of the painting and the attribution from μ-RS analysis 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 layer sequence was numbered in the micrographs of cross sections in Fig. 3, 5 and 8 correspond to the same numbers used in EDS analysis. Organic layers that did not contain metal ions are: D1-2 L0; D1-9 L2, L5, L8, L10, L12, L14; and D1-16 L3. 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. 3 a and b, D1-1 has 14 layers from the statue surface to the unpainted clay ground, among which the pigment layers have 7 layers, and the layer 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 backscatter electron image (Fig.3 f). 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, but clearly visible under the coaxial light source and in the backscattered electron image (as shown in Fig. 3 c and d). 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 analysis 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. 5 a, 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 polychrome process. The reddish-brown pigment layer was covered with a dark translucent organic material (this translucent material is almost invisible in backscatter electron images).
The particle size distribution using the statistical analysis functions of the built-in software of the KEYENCE VHX-6000 ultradepthoffield 3D microscope were used to measure the thickness of the paint layers, the first pigment layers, the priming layers and the particles sizes of the pigment. The measurement results are shown in Table1.The thickness of the paint layers are different 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 the pigment layers and the priming layers are also different, with the thinnest only 20 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 μm. The pigment particles size of different pigment layers also varies greatly, for example, the average diameter of red pigment particles in D1-6 is approximately 5 μm, while the average diameter of green pigment particles in D1-15 is 40 μm. In Fig. 4 images of the 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.
Table 1 Summary table of the thickness of paint layers, the thickness of the first pigment layers and the particles size of pigment
Samples No
|
Thickness of paint layers /μm
|
Thickness of pigment layers and priming layers /μm
|
Particles size (minimum/maximum/average diameter /μm)
|
D1-1
|
580
|
L1 Reddish-brown pigment layer 55. L2 White priming layer 85
|
3/35/5
|
D1-2
|
205
|
L0 Organic layer 30. L1 Reddish-brown pigment layer 110. L2 Gray priming layer 60
|
3/35/10
|
D1-4
|
380
|
L1 Orange pigment layer 45. L2 White priming layer 190
|
3/40/20
|
D1-5
|
560
|
L1 Blue pigment layer 20. L2 Gray priming layer 95
|
2/15/10
|
D1-6
|
425
|
L1 Red pigment layer 30
|
2/30/5
|
D1-7
|
895
|
L1 Gold foil layer 2
|
-
|
D1-8
|
350
|
L1 Red pigment layer 40. L2 White priming layer 145
|
3/20/5
|
D1-9
|
370
|
L1 Gold foil layer 2. L2 Binding medium 35
|
-
|
D1-11
|
210
|
L1 Black pigment layer 80
|
-
|
D1-12
|
225
|
L1 Reddish-brown pigment layer 160
|
4/70/15
|
D1-13
|
250
|
L1 Red pigment layer 30. L2 Orange pigment layer 20
|
4/35/10
|
D1-14
|
345
|
L1 Yellow pigment layer 70
|
4/30/15
|
D1-15
|
745
|
L1 Green pigment layer 570
|
12/190/40
|
D1-16
|
225
|
L1 Red pigment layer 50. L2 White priming layer 40
|
3/65/10
|
D1-17
|
870
|
L1 White pigment layer 870
|
8/75/15
|
D1-18
|
290
|
L1Blue-green pigment layer 75
|
15/35/20
|
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 green pigment layer L1 of the D1-15 was applied directly over the red pigment layer L2 (Fig. 4 d).
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. 5 d, f, 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: Pink pigment layer→white priming layer→brown priming layer→white paper→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 application of polychromy.
Analysis of gilding layers
EDS analysis 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 are only 2μm 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. 3 d, f). EDS results showed 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. 6 a) showed characteristic bands at 147vs, 221m, 292m cm-1 similar to chlorargyrite (AgCl, Fig. 6b). Combined with the results of EDS, it can be inferred that the main phase of D1-7 L7 and L9 are chlorargyrite (AgCl). In terms of thickness and stratigraphic structure, D1-7 L7, L9 are only 1-2μm in thickness like other gold foil layers. Therefore, these two layers were silver foil that was pasted on the statue surface at two separate times. Corrosion later transformed the silver foils into chlorargyrite (AgCl).
The L3 and L6 of D1-7 and D1-9 paint layers appear golden yellow in cross-section with mineral granularity (Fig. 3 c, e). The four layers of golden yellow material were painted over gold foil layers. From the color 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. 3 e). The Raman spectrum of golden yellow particles in layer 3 (Fig. 6 c) showed characteristic bands at 216m, 260vs, 705m cm-1 similar to pyrophyllite (Al2Si4O10(OH)2, Fig. 6 d). 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 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 discovery of silver foil layers and golden yellow pyrophyllite mineral with gold 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 1/2-1/1.2) 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 mixtures showed no obvious Raman scattering except quartz particles. In order to determine the mineral phase of the priming layers mineral, three samples with simple layer structure, D1-2, D1-5, D1-6, were selected for FTIR analysis. Fig. 7 A, 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 [14]. Among them, the bands at 3696, 3651 cm-1 corresponds to the stretching vibration of Al-OH bond outside the kaolinite octahedron structure, the band at 3620 cm-1 corresponds to the stretching vibration of Al-OH bond within the kaolinite structure. The absorption band at 1630 cm-1 is the vibration band of attached water, 1113, 1033 and the band at 1007 cm-1 corresponds to the stretching vibration of Si-O bond, 937 cm-1 corresponds to the vibration of Si-O-Al bond, 912 cm-1 corresponds to the bending vibration of Al-O-H. The three bands of 796, 753 and 696 cm-1 are attributed to the stretching vibration absorption bands of Si-O, Si-Si and Si-Al generated by associated quartz and feldspar. The band near 538, 470, 432 cm-1 corresponds to the bending vibration of Si-O. In addition to the characteristic bands of kaolin, there are obvious infrared characteristic bands at 2924, 2851, 1710, 1406, 1385 and 1100 cm-1 in the infrared spectrum of D1-2 sample. These characteristic bands could be related to the organic layer of the sample L0.
The second type of priming layer mineral dominated by Al and Si elements(Al/Si ratio between 1/2-1/1.2) and containing Pb, including D1-4L2, D1-11L2, D1-13L4, L5 and L6, which contain 30%-45% Pb. Lead white (2PbCO3·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 proportion of lead white. The third type of priming layer mineral 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 (CaCO3) and lead white. D1-4 L4 is 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 D1-11 L4 and D1-15 L4, containing chiefly lead white with 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 1:3 respectively. 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 low aluminum and high calcium clay, while the brown priming layer is a low aluminum and high iron clay.
In summary, the priming layers material used in the 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 low aluminum and high iron priming in D1-8 is obviously different from other paint layers.
Table 2. Results of SEM-EDS and μ-RS analysis
Sample
No
|
Layers
No
|
Al
/Cr
|
Si
|
K
/Na
|
Ca
/Mg
|
Fe
/Ti
|
Pb
/Au
|
As
/Ag
|
S
/Cl
|
Hg
/Cu
|
μ-RS results
|
D1-1
|
L1 Reddish- brown
|
3.6
|
12.1
|
1.5
|
4.6
/0.9
|
17.5
|
59.8
|
-
|
-
|
-
|
Hematite, red lead
|
L2 Priming layer
|
23.6
|
37.3
|
3.7
|
8.7
|
4.4
|
20.0
|
2.3
|
-
|
-
|
-
|
L3 Red
|
0.7
|
0.9
|
-
|
-
|
-
|
-
|
-
|
14.9
|
83.5
|
Cinnabar
|
L4 Priming layer
|
18.8
|
35.5
|
3.9
|
17.6
|
7.0
|
17.2
|
-
|
-
|
-
|
-
|
L5 Orange
|
3.8
|
2.6
|
-
|
2.5
|
-
|
81.6
|
9.5
|
-
|
-
|
Red lead
|
L6 Priming layer
|
33.0
|
42.6
|
3.3
|
-
|
3.1
|
14.2
|
3.7
|
-
|
-
|
-
|
L7 Yellow
|
4.1
|
5.0
|
-
|
-
|
-
|
22.8
|
44.6
|
23.4
|
-
|
Orpiment
|
L8 Priming layer
|
29.8
|
35.2
|
-
|
6.8
|
3.8
|
16.1
|
8.3
|
-
|
-
|
-
|
L9 Orange
|
3.6
|
3.8
|
-
|
2.4
|
-
|
90.3
|
-
|
-
|
-
|
Red lead
|
L10 Priming layer
|
18.5
|
32.6
|
3.3
|
18.3
|
5.8
|
21.3
|
-
|
-
|
-
|
-
|
L11 Red
|
-
|
-
|
-
|
-
|
-
|
15.4
|
-
|
11.5
|
73.1
|
Cinnabar
|
L12 Priming layer
|
24.2
|
49.5
|
4.0
|
1.8
|
2.5
|
17.9
|
-
|
-
|
-
|
-
|
L13 White
|
-
|
3.7
|
-
|
12.9
/ 3.5
|
-
|
79.9
|
-
|
-
|
-
|
Lead white, chalk
|
D1-2
|
L1 Reddish- brown
|
-
|
8.4
|
-
|
1.6
|
26.4
|
63.6
|
-
|
-
|
-
|
Hematite, red lead
|
L2 Priming layer
|
39.1
|
49.5
|
1.8
|
2.9
|
6.7
|
-
|
-
|
-
|
-
|
-
|
D1-3
|
L1 Pink
|
25.3
|
45.1
|
4.3
|
8.1
/2.4
|
13.5
/1.3
|
-
|
-
|
-
|
-
|
Hematite, anatase titan white
|
D1-4
|
L1 Orange
|
2.9
|
3.2
|
1.0
|
1.2
|
-
|
91.7
|
-
|
-
|
-
|
Red lead
|
L2 Priming layer
|
22.9
|
25.2
|
2.2
|
2.8
|
-
|
46.9
|
-
|
-
|
-
|
Lead white,
|
L3 White
|
1.4
|
1.8
|
-
|
11.9
|
-
|
84.9
|
-
|
-
|
-
|
Lead white, chalk
|
L4 Priming layer
|
1.5
|
3.3
|
-
|
56.5
|
-
|
38.7
|
-
|
-
|
-
|
Chalk, lead white,
|
D1-5
|
L1 Blue
|
13.8
|
23.6
|
/1.5
|
43.8
|
4.0
|
10.5
|
|
|
/2.7
|
Azurite, chalk
|
L2 Priming layer
|
37.3
|
52.5
|
5.4
|
-
|
4.8
|
-
|
-
|
-
|
-
|
-
|
L3 White
|
0.6
|
0.7
|
-
|
-
|
-
|
98.7
|
-
|
-
|
-
|
Lead white
|
L4 Priming layer
|
27.1
|
53.3
|
7.4
|
3.0
|
2.7
|
6.5
|
-
|
-
|
-
|
-
|
L5 Red
|
6.5
|
6.8
|
3.5
|
9.8
/3.9
|
-
|
33.8
|
-
|
9.1
|
26.7
|
Cinnabar, red lead
|
L6 Priming layer
|
31.1
|
51.6
|
8.6
|
4.5
|
4.2
|
-
|
-
|
-
|
-
|
|
L7 Green
|
4.1
|
22.4
|
-
|
29.1
/12.1
|
-
|
-
|
-
|
-
|
/32.4
|
|
L8 Priming layer
|
18.5
|
23.6
|
2.8
|
52.0
|
3.1
|
-
|
-
|
-
|
|
|
D1-6
|
L1 Red
|
-
|
-
|
-
|
1.7
|
-
|
-
|
-
|
21.3
|
77.0
|
Cinnabar
|
|
L2 Priming layer
|
30.7
|
50.3
|
8.4
|
10.6
|
-
|
-
|
-
|
-
|
-
|
|
|
L3 White
|
-
|
4.6
|
-
|
29.2
/13.6
|
-
|
52.6
|
-
|
-
|
-
|
Chalk, magnesite,
lead white
|
|
L4 Priming layer
|
38.0
|
51.9
|
6.6
|
3.6
|
-
|
-
|
-
|
-
|
-
|
|
|
L5 White
|
0.8
|
-
|
-
|
1.3
|
-
|
97.9
|
-
|
-
|
-
|
Lead white
|
|
L6 Priming layer
|
27.7
|
59.2
|
8.3
|
1.8
|
3.0
|
-
|
-
|
-
|
-
|
|
|
L7 White
|
1.4
|
0.9
|
-
|
1.0
|
-
|
96.7
|
-
|
-
|
-
|
Lead white
|
|
L8 Priming layer
|
37.9
|
50.5
|
5.9
|
-
|
5.7
|
-
|
-
|
-
|
-
|
|
D1-7
|
L1 Gold
|
-
|
-
|
-
|
-
|
-
|
/89.1
|
/10.9
|
|
-
|
|
L3 Golden yellow
|
23.6
|
43.1
|
5.7
|
4.0
|
23.6
|
-
|
-
|
-
|
-
|
Pyrophyllite
|
L4 Gold
|
-
|
-
|
-
|
-
|
-
|
/93.1
|
/6.9
|
|
-
|
|
L6 Golden yellow
|
27.1
|
37.1
|
5.5
|
3.2
|
25.3
|
|
|
|
/1.8
|
|
L7 Silver
|
-
|
-
|
-
|
-
|
-
|
-
|
/78.6
|
/21.4
|
|
|
L9 Silver
|
-
|
-
|
-
|
-
|
-
|
-
|
/73.6
|
/23.0
|
/3.4
|
|
L11 Gold
|
-
|
-
|
-
|
-
|
-
|
/89.1
|
/7.3
|
|
/3.6
|
|
D1-8
|
L1 Pink
|
8.5
|
24.0
|
-
|
37.1
/8.4
|
19.0
|
-
|
-
|
3.0
|
-
|
Hematite, chalk
|
L2 Priming layer
|
12.8
|
42.0
|
5.5
|
26.7
|
13.0
|
-
|
-
|
-
|
-
|
|
L3 Priming layer
|
16.5
|
49.1
|
7.9
|
10.3
|
16.2
|
-
|
-
|
-
|
-
|
|
D1-9
|
L1 Gold
|
-
|
-
|
-
|
-
|
-
|
/97.4
|
/2.6
|
|
-
|
|
L3 Golden yellow
|
18.9
|
45.6
|
4.2
|
3.6
|
19.5
|
/8.2
|
-
|
|
-
|
|
L4 Gold
|
-
|
-
|
-
|
-
|
-
|
/96.7
|
/3.3
|
|
-
|
|
L6 Golden yellow
|
28.8
|
38.2
|
5.8
|
5.2
|
22.0
|
-
|
-
|
-
|
-
|
|
L7 Gold
|
-
|
-
|
-
|
-
|
-
|
/96.6
|
5.4
|
|
-
|
|
L13 Gold
|
-
|
-
|
-
|
-
|
-
|
/90.2
|
9.8
|
|
-
|
|
D1-10
|
L1 Blue
|
19.9
|
34.1
|
2.8/19.4
|
1.1
|
-
|
-
|
-
|
21.1
/1.6
|
-
|
Ultramarine
|
D1-11
|
L1 Black
|
|
|
|
|
|
|
|
|
|
Lamp black
|
L2 Priming layer
|
27.7
|
32.8
|
-
|
9.2
|
/0.5
|
29.8
|
-
|
-
|
-
|
|
L3 Orange
|
1.9
|
1.5
|
-
|
1.4
|
-
|
88.4
|
-
|
/6.8
|
-
|
Red lead
|
L4 Priming layer
|
3.3
|
3.6
|
-
|
-
|
-
|
83.4
|
-
|
/9.7
|
-
|
Lead white
|
L5 Priming layer
|
28.1
|
48.3
|
3.9
|
-
|
/0.7
|
19.1
|
-
|
-
|
-
|
|
D1-12
|
L1 Reddish- brown
|
0.5
|
-
|
-
|
0.4
|
-
|
95.4
|
-
|
/3.7
|
-
|
Red lead
|
L2 Black
|
|
|
|
|
|
|
|
|
|
Lamp black
|
L3 Priming layer
|
10.9
|
23.0
|
-
|
16.2
/8.5
|
-
|
41.4
|
-
|
-
|
-
|
|
D1-13
|
L1 Red
|
1.8
|
2.9
|
-
|
9.6
|
-
|
-
|
-
|
19.8
|
65.9
|
Cinnabar
|
L2 Orange
|
0.7
|
0.8
|
-
|
-
|
-
|
98.5
|
-
|
|
|
Red lead
|
L3 Red
|
2.9
|
4.0
|
-
|
3.6
|
-
|
-
|
-
|
8.2
|
81.3
|
Cinnabar
|
L4 White
|
25.6
|
33.7
|
3.2
|
3.0
|
-
|
34.5
|
-
|
-
|
-
|
Lead white
|
L5 Priming layer
|
18.1
|
35.4
|
5.0
|
/2.1
|
5.8
|
33.6
|
-
|
-
|
-
|
|
L6 Priming layer
|
18.9
|
28.0
|
3.0
|
8.9
|
-
|
39.2
|
-
|
-
|
-
|
|
D1-14
|
L1 Yellow
|
/0.5
|
1.4
|
-
|
30.3
|
-
|
60.4
|
7.4
|
-
|
-
|
Chrome yellow, chalk
|
L2 Green
|
1.6
|
5.3
|
-
|
3.9
|
-
|
48.3
|
26.0
|
/3.0
|
/11.9
|
Emerald green
|
L3 Orange
|
0.7
|
0.9
|
-
|
-
|
-
|
98.4
|
-
|
-
|
-
|
Red lead
|
L4 Green
|
2.2
|
2.3
|
|
2.6
|
|
74.8
|
9.9
|
/2.06
|
/6.1
|
Emerald green
|
D1-15
|
L1 Green
|
-
|
6.1
|
-
|
4.2
|
-
|
-
|
58.9
|
-
|
/30.8
|
Emerald green
|
L2 Red
|
-
|
5.0
|
-
|
4.2
|
-
|
-
|
4.2
|
36.2
|
50.4
|
Cinnabar
|
L3 Priming layer
|
10.7
|
21.2
|
-
|
-
|
-
|
59.7
|
8.4
|
-
|
-
|
|
L4 Red
|
1.0
|
1.4
|
-
|
3.1
|
-
|
48.9
|
7.1
|
12.7
|
25.8
|
Cinnabar
|
D1-16
|
L1 Rose
|
|
|
|
|
|
|
|
|
|
|
L2 White
|
2.3
|
4.7
|
-
|
20.6
|
/54.6
|
-
|
-
|
17.8
|
-
|
Anatase titan white
|
L4 Orange
|
6.1
|
12.6
|
-
|
11.9
/6.9
|
59.7
|
-
|
-
|
2.8
|
-
|
Hematite
|
D1-17
|
L1 White
|
8.0
|
16.4
|
1.9
|
70.8
/2.9
|
-
|
-
|
-
|
-
|
-
|
Chalk
|
D1-18
|
L1 Blue-green
|
|
|
/3.6
|
4.8
|
|
|
53.3
|
/0.4
|
/37.9
|
Lavendulan
|
|
L2 Black
|
|
|
|
|
|
|
|
|
|
Lamp black
|
D1-19
|
L1 Black
|
|
|
|
|
|
|
|
|
|
Lamp black
|
Analysis of pigment layers
The results of μ-RS analysis of mineral pigments in each pigment layers were shown in Table 2. Raman analysis were based on published literature [15-16]. Raman spectra of some pigments are shown in Fig. 9 and 10.
Reddish-brown pigment layers
The reddish-brown pigment layers have three layers: D1-1 L1, D1-2 L1 and D1-12 L1. μ-RS analysis show that the D1-1 L1 and D1-2 L1 pigment layers are composed of hematite (Fe2O3) and red lead (Pb3O4). EDS analysis show that the Fe content in the two layers are significantly different (17%, 31%, respectively), indicates that the mineral ratio of pigments was different due to the different color requirements of the pigment layers at different locations (D1-2 L1 is darker in color than D1-1 L1). The pedestal area represented by D1-12 L1 is reddish-brown (Fig. 1), while the pigment layer presents a bright orange-red on the cross section (Fig. 4 a). Raman analysis shows that the orange-red mineral is red lead (Pb3O4). The change from bright orange-red color to reddish-brown color 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 alter to brown-black lead dioxide (PbO2) in high humidity environment[17-19].
Red pigment layers
There are 8 layers of red pigment layers: D1-1 L3, L11, D1-5 L5, D1-6 L1, D1-13 L1, L3, D1-15 L3, L5. Raman analysis show that the mineral composition of red pigment layers are all single mineral of cinnabar (HgS) except for D1-5 L5. EDS analysis results also showed that these pigment layers mainly contain Hg and S except for a small amount of Si, Al, Ca and other elements. D1-5 L5 was made of two minerals, cinnabar and red lead.
Orange pigment layers
There are 7 layers of 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 analysis show that the mineral phase of D1-16 L4 is hematite (Fe2O3), and the rest of the pigment layers are all red lead (Pb3O4). EDS analysis 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 was composed of hematite (Fe2O3) and anatase titan white (TiO2), while D1-8 L1 was composed of hematite and chalk (CaCO3) (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 (See additional Fig. 1). 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 titan white in D1-3 L1 is the repair area 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 analysis shows that the blue mineral in D1-5 L1 is azurite (Cu3(CO3)2(OH)2), and there are white chalk (CaCO3) particles mixed in L1. D1-10 L1 blue pigment’s Raman characteristic bands at 258w, 548vs, 1091m cm-1 (Fig. 9 j) are completely consistent with the bands of ultramarine. Raman spectroscopy and chemical composition can not 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; particle shape is irregular, angular, particle diameter in 10~20 μm. But artificial ultramarine pigment is pure in texture, and the particle shape is regular, uniform and fine (average diameter is 5 μm) [20]. D1-10 L1 blue pigment is bright and colorful under plane polarized light (Fig. 8 a), with pure texture, smooth particle edge, particle diameter of 3-6 μm, completely extinction under orthogonal polarized light (Fig. 8 b), which corresponds to the characteristics of artificial ultramarine pigment (Na6-10Al6Si6O24 S2-4).
Yellow pigment layers
There are 6 layers of 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 gold in D1-7 and D1-9 L3, L6 is a clay with a high iron content that was used to replace gold foil. D1-1 L7 mainly contains As, S, Pb, and Raman analysis shows that the main mineral phase is orpiment (As2S3). In cross sections of D1-14 L1(Fig. 10 d), it can be clearly seen that yellow mineral particles and white mineral particles are mixed together. According to Raman analysis, yellow pigment particles are lead chrome yellow (PbCrO4, Fig. 10 e), and white pigment particles are chalk (CaCO3).
White pigment layers
There are 7 layers of white pigment layers: D1-1 L13, D1-4 L3, D1-5 L3, D1-6 L3, L5, L7, D1-17 L1. The Ca element in D1-17 L1 is as high as 70.8%, and Raman analysis shows that the main mineral is chalk (CaCO3). D1-6 L3 mainly contains Pb, Ca and Mg. Raman analysis shows that the main mineral phase in layers are lead white (2PbCO3·Pb(OH)2) , chalk, and magnesite (MgCO3). Both D1-1 L13 and D1-4 L3 mainly contain Pb, Ca, and the relative contents of the two elements are about 80% and 12%. Combined with Raman analysis, these two layers are lead white and chalk. The relative content of Pb in the other white pigment layers (D1-5 L3, D1-6 L5, L7) are as high as 96%, and Raman analysis shows that the main mineral phase is lead white.
Green pigment layers
There are 5 layers of green pigment layers: D1-5 L7, D1-14 L2, D1-14 L4, D1-15 L1, D1-18 L1. D1-5 L7 mainly contains Cu, Ca, Mg and Si, among which Cu content is the highest (32%). The green pigment layer showed no obvious Raman activity. Because the pigment layer appears glassy under microscope (Fig. 9 c), it is likely that the layer is a copper-containing pigment in a glassy state. D1-14 L2, L4 and D1-15 L1 mainly contain Cu and As, with characteristic Raman bands at 122w, 154vs, 175vs, 217vs, 242vs, 294m, 325m, 371m, 429m, 492m, 539m, 637vs, 685w, 760w, 835w, 951m cm-1 (Fig. 10 f) corresponding to bands emerald green pigment (Cu(C2H3O2)2·3Cu(AsO2)2). D1-18 L1 minerals are blue-green in color and contain As, Cu, Ca, Na and Cl. The Raman spectrum(Fig. 10 h) shows bands at 178w, 544m and 856vs cm-1,and is identified as Lavendulan (RRUFF ID: R141212).
Lavendulan(NaCaCu5(AsO4)4Cl·5H2O) is a rare supergene arsenate mineral in oxidation zones of Cu- and As-bearing ore bodies [21]. It was first described by Breithaupt in 1837 and named as a mineral containing As, Co, Ni; Vogl (1853) describes it as blue coating from Jáchymov; Foshag (1924a) published a detailed description of lavendulan and gave its refractive index [22]. Guillemin (1956a) first used X-ray diffraction analysis to study lavendulan and proposed its original rhombic crystal structure. 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 [23].
Lavendulan has also been found sporadically in Chinese polychromy such as the painted clay statue in Yungang Grottoes (云冈石窟), the colored drawing on timber structure in Kumbum (塔尔寺) [24], the wall painting in Anyue Grottoes (安岳石窟) [25], the painted statues on Baoding Mountain in Dazu (大足宝顶山) [26-28], and the paintings of the Maidservant in the Saint Mother’s Hall of Jinci Temple (晋祠) [28]. 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 emerald green pigment, and was not intentionally used as the blue-green pigment [24-28].
In literature [28], the mechanism of corrosion conversion of emerald green pigment into lavendulan has been studied in depth. It is suggested that emerald green (Cu(C2H3O2)2·3Cu(AsO2)2) was firstly dissociated copper ions (Cu2+) and arsenite ions ((AsO2)-) in a slightly acidic and humidity environment. The (AsO2)-was oxidized to arsenate ion ((AsO4)3-), and finally Cu2+, (AsO4)3- reacts with soluble K+, Ca2+, 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. 10 a), 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 80 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 the easier it is for the soluble salt ions to migrate in the pigment layer, accelerating the transformation of emerald green to lavendulan.
Black pigment layers
There are 4 layers of black pigment layers: D1-11 L1, D1-12 L2, D1-18 L2 and D1-19 L1. The Raman spectra of the black pigment shows two broad bands near 1325 cm-1 and 1580 cm-1(Fig. 10 i), which is consistent with the standard Raman spectra of lamp black (C). The black pigment appears gelatinous under the microscope, and has no granular sensation (Fig. 4 a). Therefore, it can be inferred that the black pigment is the Chinese ink, which the most commonly used black pigment in ancient China.
Rose red painted layer
The rose red painted layers are only distributed in the bottom step area of the statue pedestal in sample D1-16. From the preserved photos [29], this area was originally reddish brown before restoration and protection in 1981, and the painted layer was largely peeled off (see Fig. 11 a). According to the interview with the staff of Lingyan Temple, the rose red area was painted in the restoration and protection of the Arhat statue 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. 12 b) near 618, 732, 1000, 1233, 1283, 1361, 1493, 1547, 1595 cm-1 match the Rhodamine B (C28H31ClN2O3) [30]. The white priming layer of D1-16 L2 mainly contains Ti, Ca and S. Raman analysis confirms that the layer contains anatase titan white (TiO2, Fig. 12 c). The infrared spectrum of the D1-16 was shown in Fig. 12 d, in which the absorption bands around 3538, 3405 cm-1 and 1685, 1621 cm-1correspond 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 (SO4)2-, and the bands around 671, 602 cm-1 correspond to the bending vibration of (SO4)2-. The characteristic bands correspond to the infrared characteristic absorption bands of gypsum (CaSO4·2H2O) [31]. D1-16 L3 appears 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 [32], 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 (Fe2O3).
The determination of the mineral composition of the outermost pigment layers may also provide a basis for the estimation of the time of the application of the latest paint of the Bodhidharma statue. According to the results of Raman analysis, the yellow pigment layer L1 of D1-14 contains lead 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 [33, 34], emerald green (chemical name "copper acetoarsenit"), was first synthesized in Germany in 1814, mainly used in the 1830s and early 20th century [35]. The use of lead 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 has 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, 36] in the Qing Dynasty(1639-1912 CE). Lead chrome yellow and emerald green were most likely painted on the surface of the statue in the 1874 redecoration. The use of anatase titan white 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 [37]. 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. 11 a), 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 titan white, it can be inferred that the pink area containing anatase titan white 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.