Conservation of Yuan Dynasty Caisson Paintings in the Puzhao Temple, Hancheng, Shaanxi Province, China

Abstract

Caisson paintings are an integral part of the unique interior decoration ceiling of traditional Chinese architecture. There are a large number of Yuan Dynasty caisson paintings in the Puzhao Temple, in Hancheng, Shaanxi Province, China. These caisson paintings have exquisite patterns and rich colors, which are rare artistic treasures of the Yuan Dynasty. In the history of nearly 700 years, due to various environmental and human factors, the caisson paintings have experienced various degradation; for example, the paper of the caisson paintings is acidified, the surface is polluted, the color is faded, mottled, and it is difficult to identify. Therefore, their protection is vital. In order to ensure the scientific and targeted development of the protection scheme, this study conducted a comprehensive and in-depth analysis of the paper fibers, pigments, adhesives, wood supports, and pollutants of the caisson paintings and carried out a series of protection experiments in the field and laboratory, providing a step-by-step review of the protection treatment application for the caisson paintings. Mechanical and wet cleaning were used to remove the pollutants. The caisson painting was deacidified with a barium hydroxide ethanol solution, and the paper and pigments of the caisson painting were strengthened with water-based fluorine. Several conservation problems, such as the removal of pollutants, the deacidification of acidified paper, and the reinforcement of flaking paper and pigments, were solved. Meanwhile, good conservation and restoration results for caisson paintings were obtained. This research method of combining theory and practice has greatly improved the scientificity and success rate of conservation work. These research results provide valuable experience and reference for other caisson paintings in similar environments.

1. Introduction

The caisson is a unique decorative element found in the interior ceilings of traditional Chinese architecture [1,2]. Its exquisite patterns and rich colors not only have a decorative effect but also contain deep cultural connotations. The origin of caisson paintings can be traced back to ancient times; it has been found in the earliest tomb in the Eastern Han Dynasty and Han Fu also described it. With the introduction of Buddhism and the opening of the Silk Road, caissette painting gradually integrated the elements of Central Plains culture and Western region culture, forming a unique artistic style.

On the basis of inheriting the style of the previous generation, the Yuan Dynasty caisson paintings showed many unique innovations, which were not only reflected at the technical level but also deeply into the artistic style and theme expression. In terms of technical innovation, the Yuan Dynasty caisson paintings often used the floating plastic gold plating technique in the well center. This technique made the caisson paintings more magnificent under the light by shaping three-dimensional patterns and pasted with gold foil, adding a rich visual level and three-dimensional sense. In terms of color application, the Yuan Dynasty caisson paintings were bold and rich in color application, with strong color contrast, mainly green, and a large number of bright colors such as gold and red, forming a strong decorative effect. The use of color not only reflects the aesthetic interest of the Yuan Dynasty but also enhances the artistic appeal of the caissons. In terms of artistic style, as a multi-ethnic dynasty, the Yuan Dynasty’s caisson paintings also incorporated multi-cultural elements. For example, hidden secret images, scrolls, and other foreign cultural elements have been widely used and expressed in caisson paintings, reflecting the openness and inclusiveness of the culture in the Yuan Dynasty. In terms of theme expression, although caisson paintings mainly served religious and court architecture, they also reflected the face of secular life in the Yuan Dynasty to a certain extent. For example, the dragon pattern, phoenix pattern, and other patterns in the caisson paintings not only have a decorative effect but also imply the majesty of imperial power and good wishes of good luck [3]. To sum up, the Yuan Dynasty caisson paintings showed many unique innovations in skills, artistic styles, and theme expression. These innovations not only reflect the exquisite skills and aesthetic taste of the craftsmen in the Yuan Dynasty but also provide important material materials for us to understand the social style and cultural characteristics of the time.

The caisson paintings are usually made as a well-shaped upward bulge with square, polygonal, or circular concavities and surrounded by various patterns, carvings, colored drawings, and paintings. Caisson paintings are based on wooden materials, and exquisite drawings are drawn on paper and mounted on wooden bases. There is a layer of large support paper between the substrate and the coating center, and the structure includes wood substrate, paper, pigment, and sizing material. Exquisite and distinctive caisson paintings can be seen in many existing ancient Chinese buildings, such as palaces, temples, and halls [4,5]. Puzhao Temple in Seoul, Shaanxi Province, was built in the third year of Yanyouyuan (AD 1316) and rebuilt in the Qing Dynasty [6,7]. In 1992, it was designated as a provincial key cultural relic protection unit and one of the five national key cultural relic protection units by the State Council (PZT located at Han City, Shaanxi Province, China, as shown in Figure 1).

The grand hall is five rooms wide. In the shrine of the Deep Six Oak Hall, there are many caissons above the alcove. There is a series of literary anecdotes, stories of historical figures, and stories of eight immortals crossing the sea. Among them, a series of interesting stories by writers are based on mountains, trees, bridges, and rivers. It depicts stories such as “violin” (Figure 2a), “playing chess” (Figure 2b), “appreciating flowers” (Figure 2c), and “searching for plum”, showing the rich and colorful lives of scholars at that time [8]. The top of the hall is a 13-by-8 horizontal wooden box, for a total of 104.

In front of the picture, each story is divided by a wooden square, and a large number of story plots are arranged in an orderly manner into the picture, and many characters are cleverly arranged in it so that the story is rich and coherent and visually gives a full and vivid feeling. They are rare artistic treasures of the Yuan Dynasty that have important historical and artistic value. However, due to the disrepair, the PZT caisson diagram showed different degrees of damage.

The caisson paintings of the Great Buddha Hall of the PZT are the most important part of the building. The site investigation showed that due to various factors, such as light, temperature, humidity, and air pollutants in the environment, the paper of the caisson painting was acidified, and the surface was polluted by dust, incense, and smoke, making it mottled and difficult to recognize. The paintings were seriously damaged (as shown in Figure 3). With the increasing awareness of cultural heritage protection, the protection of caisson paintings has attracted more and more attention from two aspects: preventive protection and remedial protection. The former aims to preserve paper relics in a controlled environment of temperature, humidity, light, and atmosphere, while the latter usually involves paper repair, washing, and deacidification of over-acidified paper. However, the protection of caisson paintings mostly depends on the experience of craftsmen and lacks a scientific basis. Therefore, it is urgent to study the essential chemical mechanism of the degradation of caisson painting paper so as to reasonably find effective protective materials and repair methods for caisson paintings.

In this paper, through the comprehensive use of a variety of modern analysis techniques, this study conducted a comprehensive and in-depth analysis of the caisson painting in Puzhao Temple, Hancheng, Shaanxi Province, China. SEM and XRD were used to detect and analyze the paper fiber, pigment composition, surface pollutants, and adhesives of the caisson painting, which laid a solid foundation for subsequent protective measures. At the same time, for the removal of pollutants, as well as the use of barium hydroxide ethanol solution deacidification, water-based fluorine reinforcement paper and pigments, and other specific measures, simulation tests were carried out to ensure its effectiveness and safety in the actual operation. The research results also provide valuable experience and reference for the cultural relics of caisson paintings in similar environments.

2. Materials Identification and Experimental Methods

2.1. Sample Information

In order to ensure the maximum appreciation value of the caisson painting, this paper selects the fallen fragments for the paper samples and the hidden locations that do not affect the overall picture for sampling.

2.2. Pigment

The caisson paintings of the PZT are rich in content, with patterns such as flowers, insects, fish, characters, and animals; the shapes are life-like, and the colors are also very rich, including green, red, black, etc. The SEM-EDS (SU3500, Hitachi High-Tech Co., Tokyo, Japan) was performed to detect the microscopic morphology, elemental composition, and distribution of the paper and pigments (Figure 4a). The samples were sputtered with gold, the magnification was 500 times, and the accelerating voltage was 15.0 kv.

2.3. pH Measurements

The pH of the paper was tested using a non-destructive acid meter (Sartorious PB-10, Sartorious Scientific Instruments Co., Ltd., Beijing, China); pH values were measured at three different points for each sample (Figure 4e–g). A drop of distilled water was placed on the surface of the paper, after which a flat-surfaced pH electrode was pressed against it.

2.4. Fiber Identification of Caisson Paintings

The Herzberg staining technique was used to extract, dye, and identify paper fibers to determine the fiber types. The preparation method for Herzberg staining was as follows: First, 25 mL of distilled water was added to 50 g of dry zinc chloride to make a saturated solution. Secondly, 0.25 g of iodine and 5.75 g of potassium iodide were dissolved in 12.5 mL of distilled water. The solutions obtained in the above two steps were mixed uniformly, and after standing for 12~24 h, the surface clear liquid was poured into a dark bottle, and a small amount of iodine (0.01 g) was added to obtain the Herzberg reagent [8,9,10]. This reagent was used for further detection and analysis. An XWY-VIII fiber analyzer equipped with different objectives was used to identify fiber morphologies.

2.5. Wooden Supports

Due to the preciousness of cultural relics, the on-site sampling was taken from a wood sample with a fracture length of 5 cm, a width of 1.2 cm, and a thickness of 1 cm (Figure 4b). With an optical microscope, we were then able to observe the structural characteristics of the wood; we then referred to the wood database Inside Wood, “Chinese Wood History”, and a wood atlas to identify the species of the wood samples.

2.6. Binders

The binders between the paper and the wooden supports of the caisson paintings were analyzed by XRD (Smart Lab 9, Rigaku Co., Tokyo, Japan). The test tube voltage was 40 kV, the tube current was 30 mA, and the scanning range of the diffraction angle was 5~90°, with a step angle of 0.02°.

2.7. Durability and Protection Performance

2.7.1. Simulated Samples of Acidified Paper

Six commercially available sheets of ramie paper (100 cm × 100 cm) were evenly coated with 3% alum water, dried naturally, and then pressed flat to obtain simulated samples of acidified paper. The pH of the paper was tested using a non-destructive acid meter (Sartorious PB-10, Sartorious Scientific Instruments Co., Ltd., Beijing, China).

2.7.2. Preparation of Barium Hydroxide

First, 7 g of barium hydroxide was weighed and dissolved in 100 mL of ethanol, and then it was refluxed for 2 h at 80 °C so that the barium hydroxide was completely dissolved. After cooling, ethanol was used to dilute it (dilution ratio: 1:2).

2.7.3. Simulated Sample of Acidified Paper Reinforcement by Inorganic Precipitation

A simulated sample of acidified paper was treated with a 3% barium hydroxide ethanol solution with a fine sprayer. After the solvent evaporates, excessive barium hydroxide and carbon dioxide in the air slowly form barium carbonate precipitation, so that the paper is kept at a pH of 7.2~7.5. Barium carbonate precipitation can protect the pigment and paper.

2.7.4. Simulated Sample Reinforcement by a Water-Based Fluorine Solution Solution

An appropriate amount of 1.5% water-based fluorine solution (Dalian Zhenbang Fluorine Paint Co., Ltd. (Dalian China)) was evenly applied to the surface of the inorganic precipitation-reinforced paper pattern. After treatment, the reinforced paper pattern was obtained after drying [11].

2.7.5. Color Difference Test and Sample Production Process

The surface of commercially available rice paper was evenly coated with 3% alum water and dried naturally. We weighed 4 g of red pigment, added 8 g and a 5% gelatin water solution, stirred evenly, brushed on one of the pieces of paper, dried, and cut into 2 pieces, named sample 1 and sample 2, respectively. We weighed 4 g of green pigment, added 8 g of 5% gelatin water solution, stirred evenly, brushed on another piece of paper, dried, and cut into 2 pieces, named sample 3 and sample 4, respectively. Sample 1 and sample 2 were uniformly coated with a 3% barium hydroxide ethanol solution for inorganic reinforcement. Sample No. 3 and sample No. 4 were uniformly coated with a 1.5% water-based fluorine hardening agent for reinforcement treatment.

2.7.6. Performance Testing

Tensile strength and folding resistance: The simulated samples of acidified paper pattern, inorganic precipitation reinforced paper pattern, and water-based fluorine reinforced paper pattern are cut into a sufficient number of strips with a width of 250 mm and 15 + 0.1 mm in vertical and horizontal directions to ensure that there are 10 valid data points in the longitudinal and horizontal directions. According to the standards ISO 1924-2:1994, the tensile strengths of the paintings were measured by using a universal testing machine (QT-1136, Dongguan Gaotai Testing Instrument Co., Ltd., Dongguan, China) at room temperature. The tensile speed was 5 mm/min, and 10 specimens were tested to obtain statistical data. According to the standards ISO 5626:1993, the folding resistance of the paintings was determined using a folding endurance tester (LB-MIT135, Shenzhen Lanbo Testing Instrument Co., Ltd., Shenzhen, China). The applied force was 4.9 N, and 10 specimens were examined [12,13,14,15,16,17].

Chromaticity measurement: The paper sample was cut into 3 cm by 3 cm square pieces and placed on a horizontal platform. The color changes of inorganic precipitates before and after deacidification treatment and water-based fluorine-reinforced treatment were recorded by VS-450 (X-Rite, CM, USA) from 400 to 700 nm, by the Commission Internationale de l’Eclairage (CIE) L*a*b* system according to ISO11 467:2000, and D65 standard light source was adopted [18,19].

3. Results and Discussion

3.1. pH Measurements

Acidification is the main factor in paper aging. The paper fibers are prone to acid degradation, resulting in the yellowing and low strength of the paper. In order to study the degree of acidification of the caisson painting, we tested the pH values of its different regions, and the results are shown in

Table 1. pH value of the paper samples.

Location	A	B	C	Average Value
No. 1	4.29	4.16	3.64	4.03
No. 2	3.10	3.24	4.04	3.46
No. 3	4.23	3.57	4.12	3.97

, with a pH range of 4.03 to 3.97. These results indicate that the caisson paper is seriously acidified, and it is urgent to protect the caisson painting.

3.2. Paper Fiber Characterization

We obtained pieces of paper fiber from the margin of the damaged, colored painting by using tweezers. Then, the fiber was boiled with ultra-pure water for 10 min. Three percent hydrogen peroxide was used for ultrasonic vibration in ultra-pure water at 60 °C for 3 h, so that the sample fiber was dispersed into a single fiber. Finally, the fibers were dyed with the Herzberg reagent. The morphological characteristics of the length, width, morphology, and color of fibers were investigated by the XWY-VIII fiber analyzer. As shown in Figure 5, the paper fiber has clear transverse nodes and longitudinal striations (Figure 5a). The lumen and membrane of cells in the fiber could be seen in the red label (Figure 5b). Furthermore, the paper fibers show twisted ribbons like cotton. The cross-markings were irregular, and dislocations could also be observed (Figure 5c) [16,20,21]. After dyeing, most of the fibers appeared red-purple in color. We finally conclude that the fiber of the caisson paintings of the PZT is ramie.

The statistical data on the length and width distribution of paper fibers were obtained using a fiber measuring instrument. As shown in Figure 6, it can be learned that paper fibers have a larger proportion of long fibers in the range of 0.4–2.2 mm (Figure 6a), and a larger proportion of wide fibers in the range of 12–22 μm (Figure 6b). This indicates that the paper fibers of caisson paintings of the PZT are long and wide, these characteristics are also consistent with the characteristics of ramie.

3.3. Pigments Analysis

The caisson paintings in the PZT are rich in content, with patterns of flowers, insects, fish, characters, and animals; the shapes are lifelike, and in reality, at the same time, the colors are also very rich, including green, red, black, and so on. In this research, we used SEM-DES to analyze some of the pigments in the PZT caisson paintings, as shown in Figure 7. Figure 7a shows that the red pigment contains not only significant amounts of Si, Al, and Ca but also Hg and S elements, inferring that the color phase of the red pigment is likely HgS, which was speculated to be vermilion. Figure 7c shows that the green pigment contains Cu and Cl elements, indicating that the color phase of the green pigment is likely CuCl₂·3Cu(OH), which could be atacamite. Figure 7e shows that the carbon content is relatively high, and we deduce that the color phase of the black pigment is carbon black.

3.4. Pollutant Analysis

We can see a large number of black pollutants on the surface of the caisson painting. It seriously affected the appearance of the caisson painting. The morphological analysis of pollutants by SEM imaging confirmed the presence of numerous morphological characteristics (Figure 8b). Flocculent particulates, fibrous particles, and clusters of irregular shapes, as shown in the images of contaminants, are highly aggregated together to form a lump. The EDS spectra clearly show that peaks for contaminants are a mixture of numerous elements, which is evident from Figure 8a. It can be observed that the contaminants on the surface of the caisson paintings in the PZT mainly contain Ca, P, Mg, K, S, N, Si, Al, and other elements. There is research reporting that the principal metallic components in wood ashes are calcium, potassium, and magnesium [11,22,23,24]. The most common acidiradicals are –CO₃, –PO₄, and –SO₃. Additionally, small amounts of aluminum, iron, and sulfates are almost invariably found in wood ashes. In addition, wood is mainly carbonated after burning, and we can also see from the energy spectrum that the carbon content is very high. This further shows that the pollutants on the caisson paintings are caused by burning incense. Additionally, burning incense releases many acid gases, such as SOx gas, NOx gas, and COx gas, which corrode the caisson painting paper.

3.5. Binders Analysis

In order to understand how the caisson painting and the board carrier are bonded together, we use XRD to analyze the white substances between the caisson painting and the board. The experimental results are shown in Figure 9. Through X-ray diffraction analysis, the white substances mainly contain kaolin, hydroxycalcite, anhydrite, quartz, illite, and manganese calcite, among which kaolinite has the highest content. This indicates that the white material is kaolin.

3.6. Identification of Wood

The description of the anatomical features and the identification of the specimens were performed according to the standards of the International Association of Wood Anatomists [25]. To identify wood species, their microntome sections (cross-section, longitudinal radial, and longitudinal tangential sections) were observed by means of a light microscope. After listing the observed features, the literature [26,27] was searched for probable matches to the unknown wood. It was identified that the base material of the caisson painting wooden board is Pinaceae pinus (Pinus spp.) hard pine wood [28,29]. The microstructural characteristics of the wood are shown in Figure 8. There is no tube hole; the growth ring is obvious, and the transition from the earlywood to the latewood changes slightly or rapidly. The latewood is dark brown, and the tracheids are arranged neatly in the radial direction. The earlywood is wide, and in the earlywood tracheids, the cavity is large and thin, and the cross-section is rectangular, square, and polygonal. The pores on the diameter wall are usually arranged in one column, sometimes two columns, and they are round or oval; the latewood belt is very narrow, only the width of the growth ring 1/8~1/6 (Figure 10a); the latewood tracheid has a small wall thickness, which is obviously denser than that of the earlywood; the cross section is polygonal and rectangular; and the diameter wall pit hole is 1 column. There are two types of wood rays: single columns and fusiform shape (Figure 10c). Single-column rays are mostly 5–15 cells or more; spindle-shaped rays have radial resin channels, and the upper and lower ends are gradually sharpened into single columns. Wood rays are composed of ray tracheids and ray parenchyma cells, the ray tracheids are located on the upper and lower edges of the ray, and the inner wall has a serrated thickening; the horizontal wall pits and nodular thickening of the end wall of the ray parenchymal cells are not obvious; the cross-field pit-pane type, 1–2 (mostly 1) pane, has the axial resin channel and radial resin channel (Figure 10b). According to the records of related trees, pinus pine is a hard pine tree species distributed in Shaanxi, and it is speculated that the wood-based materials for this batch of caisson wall paintings were taken on-site.

3.7. Water Resistance Property Text

The caisson paintings were wetted by spraying with distilled water (Figure 11a). Then we used a filter paper to rub the surface to check the adhesion pigments on the filter paper (Figure 11b). Figure 11c shows results from the water resistance property tests of the caisson painting. Any pigment on the filter paper was not observed, indicating the excellent water-resistance property of the pigments. This provides a theoretical basis for the choice of cleaning agent for caisson paintings.

4. Conservation Treatment

4.1. Laboratory and Spot Experiments

The caisson painting is poorly preserved and seriously polluted by acidification. Therefore, in order to fully preserve the caisson painting and show its artistic value, thorough restoration and protection are needed. At the same time, protection and restoration should be based on the field laboratory simulation sample treatment effect.

4.1.1. Cleaning and Removal Process of Surface Contaminants

Through detection and analysis, it was found that the pollutants on the surface of caisson paintings are mainly particulate matter from incense burning. Before cleaning, we tested the water resistance of caisson painting pigments and found that pigments in water do not fade, not diffusion, so we chose distilled water to clean it. As steps 2–7 are shown in Figure 12, the distilled water dissolved the colored groups existing in the pollutants on the surface of the paper. The cleaning effect was obvious when the color of the yellow-brown liquid became lighter. At the same time, the oil smoke on the surface of caisson painting is significantly reduced due to full contact with water, impregnation, softening, and adhesion strength, and then the bamboo skewer is used to slowly remove the fine effect to achieve the purpose of cleaning.

4.1.2. Deacidifying and Reinforcing Caisson Paintings

Acidification is one of the most important reasons for aging papers, so deacidification is the indispensable method to restore ancient papers, which involves the neutralization of the acidic materials presented on the surface and interior of the paper, as well as the sedimentation of alkaline substances on the surface of the paper to prevent or delay further acidification with natural aging [30]. Generally, the deacidification reagents mostly applied were magnesium salt, calcium salt, and barium salt, such as magnesium hydroxide (Mg(OH)₂), and calcium hydroxide (Ca(OH)₂). These metal hydroxide particles can cause a neutralization reaction with the proton of the acid species. Meanwhile, the superfluous metal hydroxide particles in neutralization further react with CO₂ from the atmosphere, converting it into carbonate as an alkaline reserve, which prevents or delays further acidification in natural aging [31,32]. Therefore, we used an ethanol solution of barium hydroxide to deacidify the caisson painting and a water-based fluorine-reinforced material to strengthen it. To test its effects in the lab. As can be seen from

Table 2. Physical properties of paper-simulated samples before and after treatment.

Physical Properties Samples	Tensile Strength (N)		Folding Strength (Double Fold) Experimental Conditions: 4.9 N		pH
	Lengthways	Crosswise	Lengthways	Crosswise
Rice paper	18.67	13.92	28.6	15.4	3.95
After inorganic precipitation reinforced	18.71	15.63	33.4	21.8	8.03
After water-based fluorine reinforced	23.84	15.68	61.3	24.3	7.56

and

Table 3. Color difference changes before and after inorganic precipitation and water-based fluoride reinforcement treatment.

Color Sample	Red				Green
	L	a	b	ΔE	L	a	b	ΔE
No. 1 (after treatment)	47.5	42.34	22.71		65.54	−23.92	11.00
No. 1 (after treatment)	46.47	41.6	21.49	1.55	65.86	−23.57	10.65	1.14
No. 2 (blank)	48.26	42.85	22.36		66.42	−23.73	10.79
No. 2 (after treatment)	47.05	42.05	21.4	1.64	63.08	−24.84	10.16	1.49
No. 3 (before)	43.56	45.67	25.83		68.07	−30.89	10.65
No. 4 (after treatment)	43.47	45.74	28.13	0.66	67.94	−31.05	11.17	0.33
No. 4 (before)	43.65	46.21	28.07		68.39	−30.31	10.11
No. 4 (after treatment)	43.77	45.9	29.31	0.45	67.95	−31.21	11.34	0.83

, the simulated paper sample produced in the experiment is acid paper, which meets the requirements of the experiment. The experimental results show that the pH of the simulated sample of the acidified paper has been raised from 3.95 to 8.1, which achieves the purpose of deacidification. This method achieves the same effect as the method of calcium hydroxide and magnesium hydroxide for the paper cultural relics deacidification [33,34]. At the same time, it can be seen from Table 2 that, after the simulated samples were treated with barium hydroxide ethanol solution, the tensile strength and folding strength were significantly improved, indicating that it is feasible to use barium hydroxide ethanol for the deacidification and reinforcement of caisson paintings. We think that Ba(OH)₂ has a carbonization reaction in the paper fiber, as shown in Formula (1). The barium carbonate is filled in the pores of the paper fiber, which not only plays the role of deacidification but also plays the role of reinforcement.

$${\mathrm{Ba}\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{BaCO}}_3+{\mathrm{H}}_2\mathrm{O}$$

(1)

At the same time, the tensile strength of the paper was also significantly improved after being strengthened with a 1.5% water-based fluorine reinforcing agent, which indicates that it is feasible to strengthen the caisson painting by using a water-based fluorine reinforcing agent. The ΔE of the paintings was tested by VS-450 (X-Rite, USA). The calculation formula for ΔE is given by

$$\Delta E*=\sqrt{\Delta L^2+\Delta {\mathrm{a}}^2+\Delta {\mathrm{b}}^2}$$

(2)

In Equation (1), ΔE* is the total color change, ΔL, Δa, and Δb are the changes in the color lightness (L*), position between red and green (a*), and position between yellow and blue (b*), respectively. The smaller the value of ΔE, the less color difference there is between treated and untreated pigment samples [35]. As can be seen from Table 3, after the inorganic precipitation of barium hydroxide ethanol solution, and the protection of 1.5% water-based fluorine material, the color of the blank simulated sample has no obvious change, ΔE < 2 [19,36,37,38,39,40], which is in line with the basic principle of preserving the original appearance of cultural relics to meet the requirements of cultural relic protection.

4.1.3. Restoration of the Caisson Paintings

After field investigation, testing, and analysis, we have established protective measures for caisson painting on the basis of small-scale experiments in the laboratory and PZT caisson painting: removal of the dirt layer, deacidification, and reinforcement.

(1) As step 1 in Figure 12 shows, the nondestructive testing of the acidity of caisson painting paper.

(2) As steps 1–7 in Figure 12 show for cleaning the surface contaminants: First, immerse three layers of chromatography paper in distilled water, then remove and squeeze out excess water to make it into a semi-wet state and lay it flat on the contaminated screen; secondly, lay a layer of auxiliary paper on it (the front side is smooth, the back side is rough) face down and align with pens; then, spread plastic wrap on it to reduce water evaporation and ensure that the water is fully in contact with the contaminants on the sample. By fully contacting the wet chromatographic paper with the sample, the contaminants on the sample can be adsorbed, and the effect of cleaning the sample can be achieved. The yellow-brown liquid is adsorbed onto the surface of the chromatography paper because the distilled water dissolves the colored groups that are present on the surface of the paper. When the color of the yellow-brown liquid becomes lighter, the cleaning work is basically over. The chromatographic paper can be changed repeatedly according to the extent of the stain. For the dirt that has not been absorbed, due to full contact with water, impregnation, softening, and adhesion strength are significantly reduced, and then the bamboo skewer is used to slowly remove it until it returns to its original appearance.

(3) As step 8 in Figure 8 shows for deacidification: The absorbent cotton is used to brush barium hydroxide methanol-ethanol solution on the caisson painting paper to neutralize the acid in the paper. At the same time, the excessive barium hydroxide and carbon dioxide in the air slowly form barium carbonate precipitation, which forms a strong buffer layer and keeps the paper at a pH of 7.2~7.5. The barium carbonate precipitation can protect the pigment and paper.

(4) As step 9 in Figure 12 shows for reinforcement: Use the absorbent cotton ball dipped in the curing agent to smear the painting pigment on the caisson painting. When the reinforcing agent solution is fully diffused into the pigment and paper (i.e., after 10 min of curing), press on the surface to make it adhere firmly. The cleaning and restoration effects of surface contaminants on ancient paintings of the PZT are shown in Figure 13.

5. Conclusions

In this paper, the ancient caisson paintings of the Puzhao Temple in Hancheng, Shaanxi, China, were studied. Multiple analytical methods were used to analyze the paper fibers, pigments, binders, wooden supports, and pollutants of the caisson paintings. The results show that the paper fiber of caisson paintings was made from ramie fiber. The red, green, and black pigments on the caisson paintings were presumed to be vermilion, atacamite, and carbon black. The base material of the caisson painting wooden board is Pinaceae pinus (Pinus spp.) hard pine wood. Surface contaminants are mainly caused by the burning incense. These research results provide accurate data support for subsequent protection processing. This comprehensive and in-depth analysis ensures the scientificity and pertinence of the protection plan. In the process of developing the protection treatment scheme, various technical means are verified by simulation experiments to ensure their effectiveness and security in practical operation.

Mechanical cleaning and wet cleaning were used to remove contaminants on the surface of caisson paintings. The caisson paintings were deacidified with a barium hydroxide ethanol solution, and the paper and pigment of caisson paintings were strengthened with water-based fluorine. The caisson paintings have been well protected and restored. This research method of combining theory and practice has greatly improved the scientificity and success rate of conservation work. This study not only successfully protects the Yuan dynasty caisson paintings in Puzhao Temple but also provides valuable experience and references for other caisson paintings in similar environments. Especially in the comprehensive protection of complex cultural relics, the methodology and technical means of this study have a wide range of applicability.