Analysis of Pigments and Damages for the 19th Century White-Robed Water-Moon Avalokitesvara Painting in Gongju Magoksa Temple, Korea

2.1. Object

This study was conducted prior to the conservation treatment of the White-Robed Water-Moon Avalokiteshvara, a Buddhist painting on the rear wall of main hall in Magoksa temple. It aimed to secure basic data can be used for conservation treatment through the precise analysis of coloring pigments by each color. Thus the surface of the painting as well as the integrity and damage status of the pigment layer were reflected in a drawing, which was intended to be preserved as a record of the repair and treatment areas of the colored layer.

This White-Robed Water-Moon Avalokiteshvara painting is a colossal mural measuring 515cm in length and 296cm in width, with Gwaneum Bosal (Avalokiteshvara) in a white robe sitting on a lotus pedestal above a rippling wave and a strange rocky stone, with her left leg hanging down. She is at the center of the painting surface and facing the front in a half-lotus position, her right leg placed on her left (Fig. 1a). Gwaneum Bosal wears a white robe from her head to below knees and red bottoms having a green bottom with red hem and tied with a pink belt inside the white robe (Fig. 1a).

In addition, Gwaneum Bosal wears a crown on her head with a standing Amitabha statue in the middle to indicate identity as Avalokiteshvara (Figs. 1b and 1c). Her right hand rests comfortably on right leg, and her right ankle is held with left hand. Her hands and feet are large and realistically depicted (Fig. 1d). Behind Gwaneum Bosal, four bamboo trees are rising from the rough rocks on the left side (Fig. 1e); Seonjae Dongja (priestling) stands thereunder, leaning toward Gwaneum Bosal (Fig. 1f); and a kundika, in which a willow branch is arranged, is placed on the opposite rocks (Fig. 1g).

2.2. Method

In the preservation status investigations, the damage types of the colored layer were subdivided to create a map, and empirical data were presented to increase the reliability. This study applied a method with recently proven reliability in the damage assessment in the field of cultural heritage conservation science, including stone artifacts [5–6].

Damage map creation and non-destructive investigation for the painting as a research object were conducted with special care to prevent any impact on the painting by using an installed scaffold. First, the preservation status was precisely recorded by checking the damage status at the site, and the analysis target was further selected after selecting the colors, such as blue, green, yellow, red, black, and white, by visual observation to confirm the color status.

As the analysis point for each color, a site was selected, whose measurement area of approximately 5mm in diameter was sufficient among the colors in which the difference in color was distinguished by visual observation, and where the pigment in the base layer with the target color was not disturbed (Fig. 2). Non-destructive chromaticity and composition measurements were performed for the selected analysis points, and a trace amount of pigment that had flaked off was collected by using a sample for precision analysis. The names of the analysis points are indicated in Table 1.

In the detail investigation, chromaticity measurement, observation through a portable stereoscopic microscope, and elemental analysis through a portable X-ray fluorescence analyzer were performed. All were conducted through non-destructive analysis, and the same area was analyzed for each measurement item. Furthermore, the field analysis was carefully conducted by minimizing the applied pressure by selecting only the part of the color pigment that was most closely adhered onto the base layer to avoid any loss from the split colored layer due to the contact-based analysis.

A total of 55 analysis points were used to identify the components of the color pigments in this painting for measurement under classification into blue, green, yellow, red, black, and white series. This study secured quantitative data based on the results of a color component analysis that had already been reported for this study, and the types of pigments used in this painting were identified in comparison to the data on previously studied traditional pigments [7–14].

A CM-2500d spectrophotometer (KONICA MINOLTA) was used to measure the chromaticity of the pigments, and three measurements were obtained at the same point with a measurement area of 3 mm through the light source D65. The chromaticity values of the analyzed pigments were expressed by the CIELab system defined by the International Commission on Illumination (CIE), and the color coordinates were represented by luminosity (L*), red to green value (a*), and blue to yellow value (b*). In addition, the color components can be more intuitively understood by indicating chromaticity (C*_ab) and hue (h*_ab) values on the polar coordinates. Chromaticity (Eq. 1) is expressed as a position between the center of the sphere (black) and the outer surface (white). Hue (Eq. 2) is expressed in angle, with red = 0°, yellow = 90°, green = 180°, and blue = 270° [15–16].

C^*_ab = (a^*2+b^*)^1/2 (1)

h_ab = arctan(b^*/a^*) (2)

The measurement range of L* is from 0 to 100, and 100 refers to nearly white, which is close to white light, and 0 refers to black. The measurement range of a* and b* for the colorimeter used in this study is -100 to + 100. The a* indicates a redder color in a positive direction and a greener color in a negative direction; b* indicates a yellower color in a positive direction and a bluer color in a negative direction.

In the component analysis for the color pigment at the site, an X-ray fluorescence spectrometer (P-XRF; Oxford Instruments, X-MET7500) was used for the part selected as the analysis location. The operating conditions of this spectrometer were an X-ray tube voltage of 15 to 40kV (40kV for heavy elements and 15kV for light elements) and a current of 10 to 50㎂. Measurement was performed in the ore mineral mode, the analysis area was approximately 5mm in diameter, and the measurement time was 20 seconds each in conducting the elemental analysis on an atomic number of 15 or higher.

The name of each pigment, even with the same components, may vary depending on the period, region, material, supply route, particle size, and areas of use. This study mainly recorded the names used based on previous studies regarding the color pigments of paintings. Furthermore, the estimation of black pigments and raw materials was performed through a comprehensive review of the results of scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) analysis and X-ray fluorescence analysis performed in the field investigation by collecting a trace amount of color pigment samples that had flaked off during the stripping process.

3.1. Preservation Status

The types of damage observed on the painting surface of White-Robed Water-Moon Avalokiteshvara are typically divided into microcracks (Fig. 3a), separation (Fig. 3b), fragmentation (Fig. 3c), contamination (Fig. 3d), and blistering and exfoliation of the pigment layer (Figs. 3e and 3f). Overall, although the contour lines and colored surfaces of this painting remain undamaged, damage appears on the painting surface along with blistering and exfoliation of some pigment layers, cracks and separation of the pigment layer, and cracks and separation of the wall (Fig. 3).

The blistering and exfoliation of the pigment layer are mainly observed around the face and on the red bottoms of the Gwaneum Bosal, which is believed to be accompanied by contraction, expansion, and cracking due to moisture. Moreover, after completely stripping the painting, the preservation condition of the wood materials on the rear wall were checked. As a result, it was found that the deterioration of the front lining paper and the wooden frame had severely progressed due to decomposition by wood-decay fungi and insect damage (Figs. 3g and 3h).

The structural condition of the wooden frame constituting the painting wall was found to be relatively stable, and while there were cracks and separation along the edge of the wall, the preservation condition of the mural painting as a whole was fair. In addition, because the blistering, exfoliation of the colored layer, contamination, and fragmentation were found to be scattered throughout the painting surface, it was recommended that appropriate measures for long-term preservation be taken.

3.2. Chromaticity Measurements

The pigments used to color this painting mainly include blue, green, yellow, red, black, and white, and violet and pink appear in smaller amounts (Fig. 4). This study measured the chromaticity of a total of 55 points for representative color pigments: six points for blue, eleven points for green, seven points for yellow, eight points for red, six points for black, eleven points for white, and ten points for other colors (Fig. 5 and Table 2).

The chromaticity values for all measurement points represent various luminosity values, which are distributed between red to yellow (h_ab: 0 to 90°) and yellow to green (h_ab: -90 to 0°) as a result of projection on the CIE Lab quadrant. Among the values, yellow and red pigments have relatively higher chromaticity and hue variability than blue and green pigments (Table 2, Fig. 5).

Regarding the chromaticity range of the blue-1 pigment, L*, a*, and b* range from 65.5 to 65.62 (mean 65.56), -1.77 to -0.8 (mean − 1.285), and 10.25 to 12.87 (mean 11.56), respectively. Regarding the chromaticity of the blue-2 pigment, L* on means is 70.71, a* is -5.07, and b* is 10.52, respectively. Blueness increases as b* increases in the negative direction, while the measured blue-2 pigments all have positive b* values, showing that the blueness of the blue-2 pigments is lower than that of the blue-1 pigments (Table 2 and Fig. 5).

According to the result of measuring the chromaticity for the green pigments, L*, a*, and b* range from 42.18 to 54.82 (mean 48.00), -15.7 to -3.87 (mean − 10.16), and − 3.75 to 6.57 (mean 2.71), respectively. As a* increases in the negative direction, greenness increases, and the a* values of all analysis points are negative. BG-7 shows the highest green level with a L* of 53.88, a* of -15.7, and b* of 11.75, and BG-10 shows the lowest green level with a L* of 44.14, a* of -3.87, and b* of 4.81 (Table 2 and Fig. 5).

Regarding the chromaticity of the yellow-1 pigments, L*, a*, and b* range from 54.53 to 60.56 (mean 57.41), 9.36 to 11.75 (mean 10.67), and 22.62 to 28.75 (mean 26.39), respectively. In the case of the yellow-2 pigments, L*, a*, and b* range from 56.64 to 61.02 (mean 57.41), 0.52 to 5.75 (mean 3.38), and 26.61 to 37.08 (mean 30.54), respectively. Yellowness increases as b* increases in the positive direction. Y-3 shows the highest yellowness, and OY-3 shows the lowest yellowness (Table 2 and Fig. 5).

The chromaticity of red pigments is widely distributed in the + a* and + b* coordinate planes. L*, a*, and b* range from 32.96 to 6.22 (mean 39.92), 12.71 to 30.39 (mean 20.67), and 14.45 to 19.68 (mean 17.28), respectively. Redness increases as a* increases in the positive direction. R-2 shows the highest redness with a L* of 38.38, a* of 30.39, and b* of 17.43, and R-8 shows the lowest redness with a L* of 46.22, a* of 12.71, and b* of 16.47 (Table 2 and Fig. 5).

The chromaticity range of black pigments is concentrated near the origin. L*, a*, and b* range from 25.42 to 58.65 (mean 34.51), 0.25 to 8.95 (mean 1.80), and 1.27 to 25.9 (mean 6.59), respectively. The analysis point closest to the origin is BL-1, where L* is 25.43, a* is 0.25, and b* is 1.27. The chromaticity range of white pigments is distributed vertically along the + b* axis. L*, a*, and b* range from 53.87 to 80.72 (mean 73.99), -2.55 to 2.89 (mean 1.02), and 8.36 to 21.78 (mean 16.09), respectively. The analysis point with the lightest color corresponds to W-8, and the analysis point with the darkest color corresponds to W-9 (Table 2 and Fig. 5).

Meanwhile, regarding the chromaticity range of violet pigments, L*, a*, and b* range from 51.42 to 53.28 (mean 52.35), -0.74 to -0.44 (mean − 0.57), and − 1.74 to 0.74 (mean − 1.24), respectively. Because a* and b* show negative values at both measurement points, the redness and yellowness are not high, which does not deviate significantly from the negative direction. Furthermore, regarding the chromaticity range of pink pigments, P-1 shows higher redness and yellowness than P-4 (Table 2 and Fig. 5).

3.3. Chemical Compositions by P-XRF

P-XRF was utilized to analyze the chemical composition of pigments in White-Robed Water-Moon Avalokiteshvara painting. The analysis locations were the same as the points for the chromaticity measurement and observation using the stereoscopic microscope. The analysis location for each colored pigment is shown in Fig. 4.

In analyzing a pigment layer through P-XRF, it is difficult to accurately measure only the pigment compositions because X-rays penetrate through the thin pigment layer and further analyze the underlying background. To overcome these shortcomings, studies have aimed to unravel only the components of the pigment from which the background component was removed. This study also detected the compositions of the pigment layer according to the method provided by Lee et al. (2012), Jeon et al. (2009), and Jeon and Lee (2011) [1–3].

Two types of blue pigments were used for the basic coloring of this painting. According to the results of chromaticity measurement and observation using the stereoscopic microscope, an analysis was conducted by classifying the results into Blue-1 and Blue-2. If grayish blue ((Co,Ni)As₃) is used as a blue pigment, cobalt and nickel compositions are detected. However, these were not detected in this painting. On average, 9,037 ppm (6,363 to 11,710 ppm) and 33,763 ppm (12,844 to 59,991 ppm) of Cu were detected in Blue-1 and Blue-2 analysis points, respectively.

Thus, the results suggest that the color depth was raised by mixing a white pigment with azurite (Cu₃(CO₃)₂(OH)₂), which is one of the traditional blue pigments. The detection of the high concentration of Ca, Pb, and S compositions at all analysis points further suggests that the color shade could have been adjusted by oyster shell white (CaCO₃) or white lead (2PbCO₃·Pb(OH)₂) (Table 3 and Fig. 6).

According to the results of the P-XRF analysis regarding the green pigment layer measured at a total of 11 points, the major compositions detected were elements such as S, Ca, Ti, Fe, Cu, As, and Pb. However, the average content of Cu and As showed a significant difference:149,915 ppm (13,321 to 311,848 ppm) and 110,568 ppm (13,716 to 296,732 ppm). Typically, when Cu and As are simultaneously detected in the composition of a green pigment, it is considered emerald green (Cu(C₂H₃O₂)₂·3Cu(AsO₂)₂). However, in the relative content ratios of emerald green, the content of As is much higher than that of Cu. Although a high As content was detected in the green pigment used in this painting, it was difficult to determine it as emerald green because the ratio of As was mostly lower than that of Cu (Table 3 and Fig. 6).

The difference in the Cu and As content at each analysis point may have been affected by the thickness of the overlapped color. Furthermore, the pigments used for the face of Buddha and Keyura accessories remained relatively undamaged, whereas those used on the feet and the lower parts fell off, resulting in a relatively small amount. Thus, these results suggest that clinoatacamite (Cu₂(OH)₃Cl) made up of copper-based raw materials was mainly used for the green pigment layer, and it was likely painted over by modern pigments given the result that the Ca and Cl content is relatively high at some points (Table 3 and Fig. 6).

The points classified as yellow were analyzed by dividing them into Yellow-1 and Yellow-2 according to the results of chromaticity measurement and observation using the stereoscopic microscope. Among the main constituent elements detected, Al and Si appear to have been simultaneously detected as a dead color paint component using a white pigment, such as ūrnā (Table 3). The content of Pb was the highest at all points with an average of 288,094 ppm (246,801 to 323,689 ppm) and 252,850 ppm (187,988 to 289,484 ppm), in Yellow-1 and Yellow-2, respectively, followed by S with an average of 90,707 ppm (59,470 to 132,885 ppm) and 140,291 ppm (133,392 to 151,7544) (Table 3 and Fig. 6). Thus, the color yellow can be assumed to lower the chromaticity by mixing white pigments with massicot.

However, the detection of As (6,014ppm) only in Y-1 suggests that orpiment (As₂S₃) and massicot (PbO) were mixed with a white pigment for coloring. Regarding the white pigment used in the color combination, the Ti content was relatively high at 72,827 ppm (58,638 to 81,718 ppm) and 72,400 ppm (51,038 to 83,346 ppm) in Yellow-1 and Yellow-2, respectively, which can be assumed to be titanium dioxide (TiO₂). In addition, compared to other color pigments, Ba is highly likely to be a modern pigment due to the detection of high levels of Ba with an average of 55,919 ppm (46,032 to 74,7499 ppm) and 67,742 ppm (56,966 to 82,836 ppm) in Yellow-1 and Yellow-2, respectively (Table 3 and Fig. 6).

According to the results of the component analysis on red pigments, the main composition were Pb, S, and Hg, and Si and Al were detected at all measured points. It can be assumed that the colored white clay under the red layer was detected together with the pigments. According to the results of the P-XRF analysis regarding a total of eight points, the patterns of constituent elements were classified into two, depending on the presence and content of Fe, Pb, and Hg.

Points R-1 to R-4 show a high Pb content with an average of 366,476 ppm (360,982 to 373,432 ppm). Points R-5 to R-8 show a relatively high content of Fe, with an average of 94,592 ppm (63,070 to 144,556 ppm). Thus, the red colors appear to be a mixture of minium (Pb₃O₄) or minium and hematite (Fe₂O₃), and it can be assumed that cinnabar (HgS) was used in smaller amounts together with these (Table 3 and Fig. 6).

According to the analysis results regarding the black layer, elements such as P, Fe, K, S, Ca, Ti, Cl, and Cu were detected. Most of these elements are not components of black pigments, which suggests the effect of dead color paints. However, no element indicating black color was detected (Table 3 and Fig. 6). The traditional black pigment is an Chinese ink, which is a carbon compound. Because Chinese ink is composed of light elements, it is difficult to detect with P-XRF. Thus, because only the same components as the white pigment used in the base layer were mainly detected at the measurement point of the black pigment, Chinese ink was likely used for black color development, requiring additional review.

In this black layer, the detection of slightly high levels of Pb and Ca, with an average of 59,164 ppm (0 to 331,475 ppm) and 85,953 ppm (from 16,859 to 194,497 ppm) through P-XRF, suggests that white lead and oyster shell white were mixed to lower the concentration of black pigment. Among the analysis points, the highest detection of Pb at 331,475 ppm in the right pupil of the Gwaneum Bosal statue suggests that the pigment was reapplied several times (Table 3 and Fig. 6).

The white color is mainly used on the Gwaneum Bosal. statue and cloth, in which elements such as Si, K, and Al were detected through the P-XRF analysis. Among these elements, the average contents of Si, K, and Al were 151,220 ppm (58,740 to 244,566 ppm), 11,979 ppm (5,722 to 24,628 ppm), and 46,856 ppm (7,846 to 89,153 ppm), respectively, and these high levels suggest the high likelihood of the use of kaolin (Al₂Si₂O₅(OH)₄). The detection of Ca and S with an average of 225,922 ppm (95,963 to 341,179 ppm) and 52,471 ppm (36,456 to 98,436 ppm) suggests that gypsum was likely mixed with feldspathic kaolin (Table 3 and Fig. 6).

The color violet was mainly used for coloring the daeui part of Gwaneum Bosal. According to the result of the P-XRF analysis, Cu and Fe were detected with an average of 27,799 ppm (22,263 to 33,334 ppm) and 26,454 ppm (23,255 to 29,653 ppm), respectively, as the main compositions. Because the composition is similar to that of the blue pigment, and high levels of Fe and Pb are detected, the mixture of azurite, hematite, and minium can be assumed (Table 3 and Fig. 6).

In addition, Ca, S, and Mg were detected here at 34,920 ppm (32,165 to 37,674 ppm), 72,511 ppm (66,265 to 78,756 ppm), and 72,223 ppm (71,575 to 74,871 ppm), respectively. This result suggests the addition of gypsum or talc (Mg₃Si₄O₁₀(OH)₂), which is a white pigment, or the detection of constituent minerals in the base layer to lower the chromaticity (Table 3 and Fig. 6).

The color pink is painted on the chest top part of Gwaneum Bosal as well as the ears of Buddha. According to the results of the P-XRF analysis, Si, Mg, Ca, and S were detected with an average of 177,186 ppm (43,800 to 230,1666), 128,059 ppm (104,302 to 158,080 ppm), 109,155 ppm (34,669 to 304,902 ppm), and 44,710 ppm (23,002 to 71,002 ppm), respectively. This result becomes the basis for assuming the use of kaolin or talc as a white pigment in combination with pink. Moreover, the detection of high levels of Pb and Fe with an average of 99,309 ppm (2,228 to 277,324 ppm) and 11,374 ppm (7,0378 to 16,438 ppm) suggests that the color depth was raised by mixing kaolin or talc with minium and hematite in terms of components (Table 3 and Fig. 6).