A Methodology for Lacquer Gilding Restoration of Sandstone Sculptures: A Multidisciplinary Approach Combining Material Characterization and Environmental Adaptation

Abstract

The restoration of gold leaf on sandstone sculptures requires structural stability, aesthetic considerations, and compliance with the principles of cultural heritage preservation. A primary issue is achieving visual and material compatibility between newly restored and original areas. Based on the “Diagnosis–Analysis–Selection–Restoration” methodology, the research team developed a targeted restoration approach for gilded stone sculptures, using the Shakyamuni sculpture at Erfo Temple in Chongqing as a case study. Assessment of the current situation revealed that over 70% of the sculpture’s surface exhibited gold leaf delamination. The composition and structure of the gold-sizing lacquer, lacquer plaster filler, ground layers, and pigments were investigated using SEM-EDS, XRD, Raman spectroscopy, and THM-Py-GC/MS techniques. The results confirmed that the sculpture featured a typical multilayer gilding structure with clear evidence of historical restorations. Considering both material performance and interfacial compatibility, an NHL2/SiO₂/SF016 composite emulsion and traditional lacquer plaster were selected as the optimal materials for reattachment and infill, respectively. A scientific restoration protocol was developed, encompassing gentle cleaning, targeted reattachment and reinforcement, and region-specific repair methods. Principal Component Analysis (PCA) was used to evaluate the influence of temperature and humidity on the curing behavior of lacquer layers. Additionally, a non-invasive gold leaf color-matching technique was developed by controlling the surface roughness of the gold-sizing lacquer, effectively avoiding the damage caused by traditional color-matching methods.

1. Introduction

Gold has served as a significant decorative material since ancient times, with gold ornaments found in tombs dating back to the Shang dynasty in ancient China [1]. Due to its splendid color, sheen, and exceptional chemical stability, gold served not only as a symbol of currency and power in antiquity, but was also widely used in religious, architectural, artistic, and furniture contexts [2]. Gilding is a process that simulates the appearance of pure gold by applying adhesives to attach metal gold leaf or gold powder onto various surfaces, such as wood, metal, plaster, and glass, achieving localized or full-surface ornamental effects. This process has been widely practiced across different civilizations throughout history [3,4]. Gilding techniques can be categorized into four primary forms based on the type of adhesive: water gilding [5], glue gilding [6], oil gilding [7], and lacquer gilding [8]. Lacquer gilding uses a gold-sizing lacquer, formulated from raw lacquer and heated tung oil [9]. This process has been widely applied to achieve the surface decoration of wood artifacts [10], bronze artifacts [11], and stone Buddhist sculptures due to its strong adhesion and excellent durability.

Since the Han and Tang dynasties, lacquer gilding has served as a core technique for creating the “golden body” of Buddhist sculptures, especially common in the decoration of stone sculpture [12]. To ensure the stability and durability of the gilded layer, stone sculptures typically feature a multilayered lacquer gilding structure. First, a ground layer (commonly composed of gypsum and lime) is applied to the stone substrate to achieve surface smoothing and improve adherence. Subsequently, an organic binding material, such as raw lacquer, is applied to the gilded layer. Tung oil is sometimes used in the formulation of gold-sizing lacquer, which functions as the adhesive. This adhesive layer often incorporates red mineral pigments, such as cinnabar, to enhance the luster of the surface gold. The outermost layer consists of high-purity gold leaf, occasionally alloyed with small amounts of silver or copper to enhance durability [13,14]. For example, analytical studies of the Thousand-Armed Avalokiteshvara sculpture at Baoding Mountain in Dazu revealed the use of high-purity gold leaf, a lacquer–tung oil blend as the adhesive, and cinnabar within the gilding lacquer. Other constituents, including heat-thickened tung oil and benzoin resin, were also identified in the lacquer layer, suggesting that ancient artists utilized various additives to improve layer performance [15]. This process exhibits excellent durability across several periods and regions. Comparable multilayer gilding structures have been observed in stone sculptures excavated from Xi’an, Shaanxi Province [16], and the Longxing Temple site in Qingzhou, Shandong Province [17], indicating that lacquer gilding had developed into a mature and widely used technique in ancient China.

The lacquer-gilded layers of stone sculptures commonly display various degrees of deterioration due to long-term exposure to outdoor environments. The major forms of degradation include the following: (1) Stress fatigue within the gilding structure caused by environmental factors, such as fluctuations in temperature and humidity, ultraviolet radiation, and water-induced weathering, results in the delamination and detachment of the gold leaf. (2) Structural failure can be caused by the aging of constituent materials. The ground layer, typically composed of inorganic mineral materials, may undergo powdering and delamination over time, leading to weakened adhesion to both the lacquer layer and the stone substrate [18]. The lacquer layer is susceptible to oxidative embrittlement, resulting in a loss of elasticity and inducing cracking and detachment of the gold leaf. (3) Inappropriate interventions may have occurred during previous restoration efforts. For instance, repeated gilding can cause ageing and weakening of multilayered gold interfaces, leading to interface separation and aggravated structural failure. The Thousand-Hand Guanyin sculpture at Dazu has multiple overlapping gilding treatments, resulting in localized bulging and delamination, with certain sections including five or six layers of gold leaf [19]. These deterioration phenomena compromise the artistic integrity of the sculpture and diminish its symbolic religious value. Therefore, it is imperative to establish a scientific restoration approach that balances structural stability with aesthetic recovery for the conservation of lacquer-gilded stone sculptures.

Current restoration practices for gilded stone Buddhist sculptures face two primary challenges. First, material selection often emphasizes mechanical performance rather than compatibility. For example, high-strength adhesives, such as fish glue [20] or epoxy resin [21], are commonly used to join detached gold leaf, while moisture-sensitive materials like gypsum are employed to repair ground layers [22]. Although such materials may provide short-term structural stability, they often lack interfacial compatibility with the original substrate. Over time, this can result in secondary deterioration issues, such as discoloration, embrittlement, and detachment. Second, visual inconsistencies often arise due to differences in gloss and color between the newly applied and original gold leaf, as well as due to the different floors of gilding layers resulting from previous restorations. Although commonly used methods such as gold burnishing and potassium permanganate treatments can mitigate color differences, they pose potential risks of damaging the original gold leaf layer. Therefore, it is imperative to develop a material selection framework and restoration methodology guided by both material and chromatic compatibility, aiming to achieve a high degree of structural and visual coherence between new and original components. This process would contribute to the development of a scientific, stable, and sustainable methodology for restoring lacquer-gilded surfaces.

Although the systematic restoration of lacquer-gilded stone sculptures remains in an exploratory stage, a relatively mature “Diagnosis–Analysis–Selection–Restoration” methodology has already been established in the conservation of immovable cultural heritage, such as murals, stone sculptures, and polychrome sculptures [23,24,25]. This restoration methodology emphasizes both structural stability and aesthetic integrity, offering valuable insights for the treatment of lacquer-gilded stone sculptures. The diagnosis phase relies on non-invasive techniques, such as high-resolution photography, 3D scanning, and hyperspectral imaging, to identify and record deterioration patterns [26]. In the analysis step, sampling-based laboratory techniques are employed to investigate the composition, manufacturing processes, and aging mechanisms of the original materials, clarifying their material properties and deterioration patterns. In material selection, the principles of compatibility and reversibility are prioritized, emphasizing the use of repair materials that closely match the physical and chemical properties of the original substrates. Restoration treatments follow the principles of “minimal intervention” and “preservation of original appearance,” and typically involve refined techniques such as micro-grouting, interlayer reinforcement, and localized inpainting. For example, in the conservation project of the frescoes in the underground chapel of the Church of Saint Paul in Matera, Italy, common issues included salt efflorescence, biological degradation, and pigment layer detachment. Analysis revealed that the plaster substrate consisted of a single layer of lime mortar, and the painting was executed using natural mineral pigments such as ochre. To address the limitations of traditional consolidants in high-humidity environments, the restoration team employed a water-based nanolime with strong penetrability and high material compatibility. Through a combination of layered consolidation and localized inpainting with natural mineral pigments, both structural reinforcement and aesthetic restoration of the mural were achieved [27]. In addition, exemplary restoration cases, such as the Marriage at Cana fresco in Ravenna [28], the mural paintings in Cave 85 of the Mogao Grottoes in Dunhuang [29], and the Four Fountains sculptures in Rome [30], further demonstrate the scientific validity and practical applicability of this methodology, offering valuable solutions for the conservation of lacquer-gilded Buddhist sculptures. Moreover, the reinforced strength of lacquer, bon glue, and poly(vinyl acetate) for the restoration of Chinese ancient lacquer sculptures was evaluated, and the lacquer achieved strong reinforcement with good permeation and film formation [31].

Inspired by previous research, this study proposes a systematic restoration strategy for lacquer-gilded stone sculptures and applies it to the conservation of the Shakyamuni cliffside sculpture at Erfo Temple in Chongqing. As a representative example of stone gilded Buddhist sculptures, the sculpture features a complex structural composition and exhibits severe gold leaf delamination and loss. Restoration must not only ensure the structural and material stability of the artifact, but also aim to recover its original appearance, achieving overall visual coherence and authentic expression of its cultural significance. A restoration methodology tailored to cliffside stone sculptures was developed based on the “Diagnosis–Analysis–Selection–Restoration” methodology [32]. This approach establishes six key objectives: (i) conduct on-site deterioration surveys to identify the primary forms of damage affecting lacquer-gilded layers; (ii) reconstruct original materials and manufacturing techniques using FTIR, THM-Py-GC/MS, Raman spectroscopy, and other analytical methods; (iii) select reattachment and ground repair materials based on interfacial compatibility as the core evaluation criterion; (iv) implement region-specific repair strategies according to different types and degrees of degradation, including tailored approaches for reattachment and ground consolidation; (v) optimize the lacquer-gilding technique, including temperature and humidity control and curing time adjustments, to ensure film stability and effective adhesion of the gold leaf; (vi) achieve visual integration between the original and restored gold leaf by controlling the surface roughness of the gold-sizing lacquer, thereby preventing secondary damage caused by traditional burnishing or chemical treatments. The results demonstrate the high applicability of the “Diagnosis–Analysis–Selection–Restoration” methodology in the conservation of lacquer-gilded stone artifacts.

2. Materials and Methods

2.1. Sandstone Sculpture

Most of the stone sculptures at Erfo Temple in Laitan were carved during the Southern Song Dynasty, over 850 years ago. The sculptures are carved from sandstone, as confirmed by X-ray diffraction (XRD) analysis (Figure S1). The site comprises 41 groups of sculptures with approximately 1700 individual figures (Figure 1a). The Laitan Erfo Temple cliff sculptures exemplify Chinese cliffside grotto art and possess significant historical, scientific, artistic, and social value. They offer an authentic record of the development and dissemination of Chan Buddhism, and through their unique ritual-oriented layout and exquisite carving techniques, exemplify the deep integration of Chinese Buddhist art and folk culture. Among these, the depiction of Shakyamuni sculpture is the central figure and holds significant historical and artistic value (Figure 1b).

According to preliminary investigations, the Shakyamuni sculpture displays extensive deterioration, including surface contamination, lifting of the gold leaf, large-scale gilding loss, pigment fading and delamination, weathering, and loss of the sandstone substrate. Statistical analysis of the deterioration areas shows that the majority of the gold leaf on the sculpture’s body has already detached. The remaining gilding is primarily concentrated on the face, chest, waist, right wrist, and legs (Figure 2a). Specifically, the facial area contains three layers of gold leaf, the chest and legs one layer each, the waist mostly one layer (with localized areas up to four layers), and the right wrist three layers. Surface contamination (Figure 2b), blistering (Figure 2c), and delamination. (Figure 2d) of the gold leaf are the most prominent types of deterioration, affecting approximately 70% of the total surface area. These issues have led to the gradual loss of historical information, have severely compromised the aesthetic value of the sculpture, and pose ongoing threats to its long-term preservation. Therefore, it is imperative to study the lacquer gilding techniques used in the sculpture and carry out restoration based on this understanding to faithfully recover its historical and artistic authenticity. Consequently, investigation of the lacquer gilding techniques used on the sculpture, along with scientifically guided restoration based on these findings, is essential to recovering and preserving its original historical and artistic significance.

2.2. Restoration Materials

Polyoxyethylene sorbitan monolaurate (Tween 20) and nano-silica (Nano-SiO₂, 20 nm) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). The acrylic emulsion SF016 was supplied by Deffner & Johann GmbH (Röthlein, Germany). Natural hydraulic lime (NHL2), meeting EN 459-1 CL 2.0 standards [33], was obtained from Hessler Kalkwerke GmbH (Nordrhein-Westfalen, Germany). Raw lacquer (Urushi) was provided by Futian Raw Lacquer Factory (Guangxi, China). Refined tung oil and tile ash, both traditional materials widely used in historical restoration, were purchased from Jinshan Jia Restoration Materials Studio (Shanghai, China). Silver vermilion pigment (cinnabar) was acquired from Yifeng Chemical Co., Ltd. (Jiangxi, China). Turpentine, used as a solvent and thinner, was purchased from Shanghai SIIC Marie Painting Materials Co., Ltd. (Shanghai, China). Gold leaf (23.75 karat, 99%) was purchased from Nanjing Jinxi Jinbo General Factory (Jiangsu, China). E-01 epoxy resin AB adhesive was provided by the Fujian Cultural Relics Protection Center (Fujian, China). Epoxy resin NYL-1 was supplied by Hxtal (Sparks, NV, USA). Acrylic resin Paraloid B72 was obtained from Dow Chemical Company (Midland, MI, USA). Jade Thick-1, a high-viscosity white polyvinyl acetate emulsion adhesive, was sourced from TALAS (New York, NY, USA).

2.3. Preparation of the NHL2/SiO₂/SF016 Composite Emulsion

Emulsion was prepared by gradually incorporating the dry components (NHL2 and nano SiO₂) into the SF016 emulsion under continuous stirring, followed by the addition of deionized water (

Table 1. Restoration materials.

Material Name	Molecular Formula	Function
Tween20 (Polyoxyethylene sorbitan monolaurate)	C58H114O28	Non-ionic surfactant for cleaning [34,35,36]
SF016	Acrylic copolymer emulsion	Adhesive for gold leaf reattachment [37,38]
NHL2	Hydraulic lime (Ca(OH)2, SiO2, Al2O3, etc.)	Inorganic filler and consolidant [39,40]
Nano-SiO2	SiO2	Nanofiller, strength modifier [41,42,43]
Raw lacquer	C15H24O2 (main component)	Traditional lacquer base [44,45]
Refined tung oil	C18H32O2	Binder in gold-sizing lacquer [46]
Cinnabar	HgS	Gold-enhancing ground [47]
Turpentine	C10H16 (main component)	Solvent and thinner for lacquer [48,49]

).

2.4. Preparation of Lacquer Materials

The primer lacquer was prepared by mixing turpentine and raw lacquer at a volume ratio of 2:8. During preparation, turpentine was poured into the lacquer with continuous stirring to ensure thorough blending. The mixture was then left to stand to eliminate air bubbles before subsequent use.

The lacquer plaster used for the ground layer was made by mixing raw lacquer, brick powder, and water at a mass ratio of 5:10:2. First, the brick powder was sieved and mixed with an appropriate amount of clean water to form a paste. Raw lacquer was then added and stirred evenly to obtain a smooth and delicate paste.

Raw lacquer was heated in an iron pot over a low flame with continuous stirring until all moisture was evaporated. The resulting heated lacquer was then blended with raw lacquer at a ratio of 3:7.

Gold-sizing lacquer was prepared by mixing raw lacquer, tung oil, and silver pearl powder at a ratio of 1:1:1. The mixture was thoroughly stirred and filtered at least twice through fine mesh cloth to remove impurities and particles, ensuring a fine lacquer texture and uniform color suitable for gilding.

2.5. Ground Layer Compatibility Test

Four types of simulated ground layer specimens were prepared: lacquer plaster ground layer, slaked lime ground layer, gypsum ground layer, and hydraulic lime ground layer. The general procedure included applying the ground layer, surface sanding, brushing with raw lacquer, sanding again, applying gold-sizing lacquer, and finally gilding with gold leaf. All specimens were prepared on site, naturally cured and maintained, then sampled for resin embedding and polishing to provide cross-sectional samples for subsequent microstructural analysis.

To further evaluate the compatibility between restoration materials and the original ground layer and sandstone substrate, the original ground layer was pre-consolidated using an NHL2/5% acrylic emulsion. Based on this treatment, hydraulic lime and lacquer plaster layers were applied separately to assess their interfacial bonding performance and structural compatibility with the existing materials.

2.6. 3D Digital Microscope

A Keyence VHX-6000 digital 3D video microscope (Keyence Corporation, Osaka, Japan) was used to observe the microstructures of the samples. Samples were embedded in a two-component epoxy resin mixed in proper proportions and cured at room temperature for 24 h, followed by grinding and polishing.

2.7. Scanning Electron Microscopy (SEM)

A TESCAN VEGA 3 XMU scanning electron microscope (TESCAN ORSAY HOLDING, Brno, Czech Republic) equipped with a Bruker Nano GmbH 610M energy-dispersive X-ray spectrometer(Bruker Nano GmbH, Berlin, Germany) was used to observe the cross-sections and structures of the ground layers and to perform elemental analysis. The parameters were set as follows: an accelerating voltage of 20 kV and a working distance of 15 mm. Samples were embedded in a two-component epoxy resin, cured at room temperature for 24 h, then ground and polished. Carbon coating was applied before analysis.

2.8. Pyrolysis–Gas Chromatography–Mass Spectrometry (Py-GC/MS)

To identify the organic components in the gold-sizing lacquer and lacquer plaster, Py-GC/MS analysis was conducted using a Frontier Lab PY-2020iD pyrolyzer (Frontier Laboratories Ltd., Fukushima, Japan) coupled with a Shimadzu GCMS-QP2010 PLUS system (Shimadzu Corporation, Kyoto, Japan). Approximately 50 mg of each sample was placed into a pyrolysis cup with 3 μL of 20% tetramethylammonium hydroxide (TMAH) solution, then pyrolyzed at 600 °C for 10 s. The resulting pyrolysates were injected into the gas chromatograph at an injector temperature of 250 °C. The gas chromatography conditions were as follows: an SLB-5MS column (30 m × 0.25 mm × 0.25 μm); an initial temperature of 40 °C, held for 5 min, then ramped at 6 °C/min to 292 °C, held for 3 min; carrier gas of helium; an inlet pressure of 15.4 kPa; a flow rate of 0.6 mL/min; and a split ratio of 1:100. The mass spectrometry conditions were as follows: an electron ionization voltage of 70 eV and a scan range of m/z 50–750. Product identification was performed using the NIST 05 and NIST 05s spectral libraries (National Institute of Standards and Technology, Gaithersburg, MD, USA).

2.9. X-Ray Diffraction (XRD)

X-ray diffraction analysis was conducted using a Rigaku RINT 2000 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) to determine the inorganic mineral composition of the lacquer plaster and ground layers. The testing conditions were as follows: a tube voltage of 40 kV, a tube current of 40 mA, a slit configuration of DS 1° and RS 0.15 mm, Cu Kα radiation, and a scanning range 2θ = 5°–60°.

2.10. Confocal Raman Microspectroscopy (RAM)

Pigment components in the underlying layers were analyzed using a HORIBA XploRA confocal Raman microspectrometer (HORIBA Scientific, Kyoto, Japan), with a spectral detection range of 100–4000 cm⁻¹, a laser source of He-Cd (532 nm), and a spectral resolution ≤ 1.6 cm⁻¹.

2.11. Restoration of the Lacquered and Gilded Shakyamuni Sculpture

Based on the current preservation status of the Shakyamuni Buddha sculpture, an integrated restoration system was developed, incorporating surface cleaning, structural consolidation, reattachment of the delaminated gold leaf, and compensation for missing areas. Initially, a surface wet cleaning process was carried out using the Japanese tissue poultice method combined with a 5% Tween 20 solution. Multiple rounds of poulticing were performed until no visible pollutants remained on the absorbent layer. For areas with lifted gold leaf, a dropwise consolidation method using an NHL2/5% acrylic emulsion was applied. Once the consolidate fully penetrated and wetted the interface, surface residues were promptly removed. Local reactivation of the consolidated areas was conducted using warm air at a temperature below 50 °C, followed by reattachment of the original gold leaf using either a dropwise or thin-brush application of the same acrylic emulsion. To ensure stable adhesion, localized soft pad supports were applied. These supports were maintained for 24–48 h during the natural curing period. Upon removal, the adhesion quality was checked to confirm that the gold leaf was firmly bonded without signs of delamination, detachment, or blistering. For areas where the gold leaf was entirely lost, the restoration procedure began with a primer application to achieve penetrative consolidation of the base layer. Restoration materials prepared with raw lacquer as the main component were then applied to the missing areas. Once naturally dried to a semi-dry, non-tacky state, the surface was sanded. For areas where the gold leaf was entirely lost, the restoration procedure began with a primer application to achieve penetrative consolidation of the base layer. Restoration materials prepared with raw lacquer as the main component were then applied to the missing areas. Once naturally dried to a semi-dry, non-tacky state, the surface was sanded. Depending on specific conditions, either lacquer plaster or a composite hydraulic lime ground layer was applied evenly to the surface, compacted, and left to dry in the shade. After drying, it was sanded to achieve a flat surface. Raw lacquer was then brushed in a uniform direction; once dried, the area was polished again to ensure a clean, smooth finish. Finally, gold-sizing lacquer was applied evenly to the repaired areas. When the lacquer reached approximately 80% dryness, gold leaf was applied. During application, each adjacent leaf overlapped by approximately one-fourth to ensure continuity, completeness, and visual coherence of the gilded surface [50,51]. The overall restoration process is illustrated in Figure 3.

2.12. Methodology

A systematic restoration methodology was developed to address the gilding deterioration of the Shakyamuni sculpture at Erfo Temple in Laitan, Chongqing. This methodology consists of a sequence of procedures, each comprising several detailed tasks. The complete process is presented in Figure 4 and will be discussed in detail below.

3. Results

3.1. Reference Sample Characterization

The lacquer-gilded sample analyzed in this study was collected from the right ear of the Buddhist sculpture and features a three-layer gold leaf structure supported by an underlying ground layer. Figure 5 displays the microscopic image. From top to bottom, the sample comprises three alternating layers of gold-sizing lacquer and gold leaf, followed by an upper lacquer plaster layer, a primary lacquer layer, a pigmented underlayer, and a lime ground layer. Each layer is clearly distinguishable, forming a well-defined stratigraphic sequence. Layer thicknesses were measured using CAD software (Autodesk Inc., San Rafael, CA, USA). The gold leaf layers each had a thickness of approximately 1.7 μm, while the gold-sizing lacquer layers varied between 11.7 and 18.6 μm. The upper lacquer plaster layer was relatively thin at 8.1 μm, whereas the lower lacquer plaster layer attained a thickness of 83.2 μm. The primary lacquer layer was 6.7 μm thick, and the pigmented layer was approximately 9.8 μm thick. The alternating application of lacquer and gold leaf created an interfacial anchoring effect, which not only enhanced the adhesion of the gold leaf but also improved its surface gloss through the optical refraction properties of the lacquer layers. The lacquer plaster layer exhibited a typical granular–binder microstructure, providing surface smoothing and buffering functions to relieve interfacial stress. The ground layer at the bottom offered structural support for the entire multilayer system. Overall, this composite structure combines both decorative and functional attributes. It improves the aesthetic quality of the sculpture while significantly improving the mechanical stability and environmental adaptability of the lacquer-gilded layer, thereby mitigating damage caused by fluctuations in ambient temperature and humidity.

The sample was analyzed using SEM-EDS to determine the elemental composition of each structural layer. The SEM image is shown in Figure 6, and the corresponding EDS results are listed in

Table 2. Composition of NHL2/SiO2/SF016.

Name	Formulation (% by Mass)				Effect
	SF016	NHL2	Nano SiO2	Deionized Water
5%NHL2/SiO2/SF016	5	1	0.3	93.7	Preliminary consolidation of ground layer
15%NHL2/SiO2/SF016	15	45	15	15	Reattachment of original gold leaf
20%NHL2/SiO2/SF016	20	60	20	0	Preparation of hydraulic lime ground layer

. The SEM-EDX spectrum of the upper gold leaf is shown in Figure S2. The gold contents at positions J-1, J-2, and J-3 were 58.4%, 75.8%, and 74.2%, respectively. The surface gold leaf (J-1) exhibited the lowest gold content, likely due to long-term environmental exposure, leading to surface contamination and dust deposition. In contrast, the middle and lower layers showed higher gold content, indicating that the inner gold leaves were well preserved, with no significant elemental migration. Beneath the gold leaf, gold-sizing lacquer (Y-1) revealed high levels of mercury (62.8%) and sulfur (12.4%), with an atomic ratio close to 1:1, suggesting the presence of cinnabar pigment in the lacquer.

Py-GC/MS was employed to further identify the organic components present in the gold-sizing lacquer. The total ion chromatogram (TIC) of the three gold-sizing lacquer layers is shown in Figure 7, and the corresponding pyrolysis products are listed in

Table 3. Energy spectrum elemental analysis results of gold leaf sample.

Serial Number	Elements/wt.%
	C	O	Al	S	Ca	Au	Hg
J-1	31.4	3.2	0.6			58.4
J-2	18.8	4.6			0.8	75.8
J-3	21.7	3.3			0.7	74.2
Y-1	21.9	3.0		12.4			62.8

. The analytical results indicate that the pyrolysis compositions of the three lacquer layers are mostly consistent. Prominent pyrolysis markers identified include 1,2-dimethoxy-3-pentadecylbenzene, 3,4-dimethylphenol, and 2-methylphenol, alongside with a range of alkanes and alkenes, notably pentadecane and 1-tetradecene, which were the most abundant, consistent with the conventional pyrolytic profile of traditional Chinese lacquer [52]. Figure 7b–d display the relative fatty acid content. The gold-sizing lacquer exhibited a significant concentration of glycerol, monocarboxylic acids (C5–C9, C14–C18, C20, C22), and dicarboxylic fatty acids (C4–C12), which aligns with the characteristics of drying oils. The molar ratio of palmitic acid to stearic acid (P/S) varied from 1.03 and 1.16. The detection of phenolic derivatives indicates the incorporation of processed tung oil in the lacquer formulation [53]. The sample is a typical composite system of urushi lacquer and refined tung oil, added with cinnabar pigment (HgS) to improve the visual richness of the gold leaf. This technique is consistent with the “Zhu Tian Jin” method described in the Xiushi Lu (Treatise on Lacquering) [54]. The lacquer gilding system exhibits excellent adhesion and flexibility, significantly improving the stability and durability of the gilded layers, and reflects the adaptability and continuity of traditional Chinese lacquer gilding techniques.

The lacquer plaster layer is typically composed of a mixture of organic adhesives and inorganic fillers. XRD analysis was conducted on the sample to determine the composition of the inorganic phase, with the findings presented in Figure 8. The diffraction pattern indicates that the components of the sample are calcite, quartz, gypsum, illite, and anorthite. Silicate minerals are comparatively plentiful, displaying the usual properties of clay minerals. According to records in Xiushi Lu (Treatise on Lacquering), traditional lacquer ash was typically produced from inorganic materials of a single origin, such as animal bone plaster, porcelain shards, or brick plaster. Brick plaster is produced by high-temperature firing of clay and has a mineral composition similar to that of natural clay minerals [55]. The XRD data suggest that the inorganic filler in this sample is likely brick ash rich in calcite. The calcite may have originated from lime and formed through long-term natural carbonation processes [56]. As the principal skeletal components of the inorganic phase, calcite and quartz enhance the structural strength of the lacquer ash layer and provide stable support for the overlying lacquer gilding system.

Py-GC/MS analysis was conducted on the lacquer plaster specimen The TIC of the sample is depicted in Figure 9a, and the associated pyrolysis products are listed in

Table 4. The analysis results of THM-PY-GC/MS.

Serial No.	Retention Time	Component	Serial No.	Retention Time	Component
1	2.18	Methyl pentanoate	10	6.73	1-Tetradecene
2	2.77	Methyl 5-octenoate	11	7.22	Dimethyl azelaate
3	3.01	Methyl hexanoate	12	7.68	Pentadecane
4	3.57	Decane	13	8.07	Dimethyl azelate
5	3.96	2-Methylphenol	14	8.28	Dimethyl pelargonate
6	4.19	3,4-Dimethylphenol	15	9.86	Dimethyl decanoate
7	4.45	Methyl octanoate	16	11.08	Methyl hexadecanoate
8	5.25	Methyl nonanoate	17	12.45	Methyl stearate
9	6.27	Dimethyl sebacate	18	15.23	1,2-Dimethoxy-3-pentadecylbenzene

. Distinct pyrolysis markers of Chinese lacquer, including 3-heptadecylcatechol, 3-methylcatechol, 1-tetradecene (C14), and pentadecane (C15), were identified in the sample, confirming consistency with the pyrolytic behavior of Chinese lacquer [50]. Additionally, the presence of trimethyl phosphate, glycine, amino acids, and protein-derived markers indicates the addition of egg white as a binder in the lacquer plaster. Studies have shown that egg white can improve interfacial adhesion through the disulfide (-S-S-) crosslinking network formed by ovomucoid proteins, a technique historically employed in traditional lacquer practices [57]. Figure 9b illustrates the distribution of fatty acid pyrolysis products. Palmitic acid (C16) and high levels of stearic acid (C18, C18:1) were detected, while no dicarboxylic acids, such as azelaic acid were observed, suggesting that drying oils were not used in the lacquer plaster formulation. This composite formulation of lacquer and egg white not only ensures strong adhesion, but also effectively avoids the embrittlement risks associated with oxidative degradation of oil-based materials.

A pigment layer was detected between the ground layer and the lacquer base layer. Micro-Raman spectroscopy was employed to identify its composition. As shown in Figure 10, the Raman spectrum of the pigment layer reveals characteristic Fe–O stretching vibration peaks at 225 cm⁻¹, 290 cm⁻¹, 410 cm⁻¹, and 612 cm⁻¹ [58]. These results indicate that the main component of the pigment layer is natural hematite, suggesting that the pigment is iron red. Based on its stratigraphic position, the pigment layer is presumed to be a remnant of an earlier polychrome decoration applied prior to the lacquer gilding process. This suggests that it holds significant value for reconstructing historical artistic practices and documenting technical evolution in religious sculpture production.

The ground layer serves as the interfacial layer between the lacquer layers and the stone substrate in the gilding process, crucially providing mechanical support and facilitating materials transfer. The composition of the ground layer was analyzed by XRD, and the results are shown in Figure 11. The diffraction pattern indicates that the mineral components of the layer are calcite, gypsum, and quartz. Among them, calcite exhibits the strongest diffraction peak at 29.4°, indicating that the presence of the calcite may be due to the carbonation of slaked lime or added as a micro-filler in the ground layer [59]. The SiO₂ diffraction peaks observed in the spectrum may originate from natural impurities in the lime raw materials or result from the incorporation of composite materials such as brick ash and sandy soil. In addition, several diffraction peaks associated with gypsum were identified. Based on the common materials used in repairs and supporting field investigation data, it is inferred that these gypsum components were introduced during later restoration processes. Gypsum, as a typical hygroscopic material, is prone to hygroscopic–dehydration reactions in high-humidity environments (e.g., Chongqing), leading to volumetric expansion and contraction. This may result in powdering, delamination, and other forms of structural deterioration in the ground layer. Therefore, while the lime-ground layer can offer mechanical strength, improved adhesion, and humidity regulation, the presence of gypsum may pose a potential threat to the long-term conservation of gilded sculptures.

Analytical results indicate that the facial area of the main Shakyamuni sculpture at Erfo Temple in Laitan was originally decorated with polychrome painting, utilizing mineral pigments directly onto a lime-ground layer. In subsequent restoration phases, a lacquer gilding method was applied over the original pigment layer, involving the sequential application of a lacquer layer, a lacquer plaster layer, and three sets of alternating lacquer and gold leaf layers. The lacquer plaster layer has a granular, gel-binding microstructure and consists of brick plaster filler blended with raw lacquer and egg white, showcasing excellent mechanical strength and adhesion. The gilded structure is well preserved, with three distinct combinations of gold leaf and lacquer layers clearly identifiable, each corresponding to a separate phase of relacquering and restoration. The three lacquer-gilded layers possess the same composition, comprising raw lacquer, cooked tung oil, and cinnabar. This is consistent with the “vermilion under gold” (zhu tian jin) method described in the “Treatise on Lacquer Decoration”. Field investigation confirmed that the gilding structure of other body parts of the sculpture is generally consistent with that of the face. Four gilding layers were identified in the waist area, each utilizing similar binding materials. This finding aligns with historical records describing four phases of regilding, further verifying the consistency and continuity of restoration materials and techniques. This study not only reveals the main technical characteristics of the lacquered and gilded craftsmanship of the sculpture, but also provides a scientific basis for material selection and structural strategies in future conservation and restoration efforts.

3.2. Study of Restoration Techniques

3.2.1. Surface Cleaning

The surface of the stone-carved sculpture was found to be heavily contaminated, covered with a grayish-black deposit layer. The primary components of the pollutants were identified as silicon dioxide and sodium aluminum silicate, which may have originated from mineral dust generated during the grinding of the substrate layer in prior treatments. The XRD pattern of the pollutants is shown in Figure S2. In addition, traces of carbon black were detected, presumably due to prolonged exposure to incense smoke within the temple. This inorganic–organic composite contamination not only obscures the luster of the gold leaf but may also compromise surface stability. Therefore, the cleaning of gold leaf surfaces constitutes a critical initial step in the restoration of gilded artifacts. Traditional manual cleaning methods often involve mechanical techniques such as brushing or wiping with degreased cotton. However, in areas where the gold leaf has already delamination or detached, these methods can pose a risk of secondary damage. To evaluate a gentler yet effective cleaning approach, this study conducted a comparative test under laboratory conditions between mechanical cleaning and poultice-based cleaning using Tween 20. The detailed procedure is described in Section 2.10. Figure 12 presents microscopic comparisons of the cleaning effects: Figure 12a shows the untreated surface, where substantial particle accumulation is observed. Figure 12b shows the mechanically cleaned area, with some particle removal but reduced gold luster and slight edge damage. Figure 12c shows the area cleaned with Tween 20, where the gold surface appears clean and intact, with a significantly restored metallic sheen and no visible scratches or damage. Tween 20 is a widely used non-ionic surfactant. The amphiphilic molecular structure forms micelles in aqueous solutions, encapsulating contaminants and reducing interfacial tension, thereby facilitating the gentle wetting, dispersion, and removal of surface pollutants [60]. This method offers controlled removal and safe cleaning, making it particularly suitable for treating highly fragile and vulnerable areas of gilded artifacts.

3.2.2. Screening of Reattachment and Consolidation Materials

Severe delamination of gold leaf was observed on the surface of the sculpture, with the impacted area accounting for approximately 70% of the overall decorative surface. Due to the extreme thinness and fragility of gold leaf, successful reattachment relies heavily on the formation of a stable interfacial bond between the adhesive and the substrate. The reattachment process is complex. To address this issue, it is essential to identify consolidates with good adhesion and compatibility with the underlying layers. A series of commonly used adhesives were tested in a small trial area on site, including Gold-sizing lacquer, raw lacquer (Futian Raw Lacquer Factory, Guangxi, China), NYL-1 epoxy resin (Hxtal Sparks, NV, USA), Paraloid B72 (Dow Chemical Company Midland, MI, USA), acrylic emulsion SF016 (Deffner & Johann GmbH, Röthlein, Germany), and polyvinyl acetate emulsion (TALAS, New York, NY, USA) [61].

Table 5. The analysis results of PY-GC/MS.

Serial No.	Retention Time	Component	Serial No.	Retention Time	Component
1	2.32	Glycine	11	7.67	Pentadecane
2	2.73	Alanine	12	8.31	Octadecadienoic acid
3	3.17	Trimethyl phosphate	13	9.19	1-Heptadecene
4	4.45	Methyl octanoate	14	11.04	Methyl palmitate
5	4.68	Gelatin protein marker	15	11.25	n-Hexadecanoic acid
6	5.02	Pentylbenzene	16	12.28	Methyl stearate
7	5.25	Methyl methoxypropionate	17	12.53	Oleic Acid
8	5.38	Isoleucine amine	18	12.67	Stearic acid
9	5.58	3-methylcatechol	19	13.26	Oleic Acid
10	6.73	1-Tetradecene	20	16.69	3-Heptadecylcatechol

indicates that the gold-sizing lacquer and lacquer exhibited inadequate adhesive properties, failing to stabilize the gold leaf effectively. The epoxy resin exhibited moderate bonding strength; nevertheless, its brittleness and limited reversibility posed practical challenges. Paraloid B72 showed negligible adhesion enhancement, with minimal re-adhesion noted. Conversely, the SF016 acrylic emulsion achieved exceptional consolidation performance. However, its elevated fluidity led to undesirable dripping when applied to vertical surfaces. A filler system was introduced into the emulsion formulation to enhance its operational stability.

Studies have shown that hydraulic lime combined with nano-SiO₂ as inorganic fillers can enhance the adhesive strength and penetration of colloids while regulating fluidity, exhibiting excellent compatibility and mechanical performance in the consolidation of mural grounds and stone cultural heritage. Following the formulation described in Section 2.2, a 15% NHL2/SF016 composite emulsion was prepared and applied for the reattachment of the gold leaf (

Table 6. Adhesion performance of different gold leaf reattachment materials.

Material	Concentration (%)
	5	20	40	50	60	70	80	Original Solution
gold-sizing lacquer	-							×
lacquer	-							√
epoxy resin	-							√√
Paraloid B72	-		×	×	×	-		-
SF016	×	√	√	√	√	√√	√√	√√
polyvinyl acetate emulsion	-							×

Evaluation criteria (gold leaf retention): √√ indicates strong adhesion, √ indicates weak adhesion, × indicates no adhesion, and - indicates not tested.

). Figure 13a shows the lifting state of the gold leaf of the sculpture. Figure 13b presents a cross-sectional microscopic image prior to consolidation, where noticeable lifting of the gold leaf and interfacial separation are observed. Figure 13c shows the cross-section after reattachment using the NHL2/SF016 composite emulsion, in which the gold leaf is tightly bonded to the lacquer layer, demonstrating strong interfacial adhesion and a stable reattachment outcome.

Furthermore, prolonged natural weathering has led to surface powdering of the lime plaster layer, resulting in structural degradation and reduced cohesion, thereby compromising the stability of the gilded layers. A 15% NHL2/SiO₂/SF016 composite emulsion was applied to improve structural density and mechanical properties by consolidate the powdery areas through capillary penetration [62]. Scanning electron microscopy (SEM) images of the plaster layer before and after consolidation are presented in Figure 14. The untreated area (Figure 14a) exhibits prominent pores and loosely packed particles, whereas the consolidated area (Figure 14b) shows significantly reduced porosity and a densified microstructure. The acrylic-based polymer in the composite emulsion can effectively infiltrate the pores of the ground layer, forming adhesive bonds that enhance interparticle cohesion. Meanwhile, nano-SiO₂ and hydraulic lime act as inorganic fillers that penetrate and fill internal voids within the lime-based ground layer, while the nano-silica component additionally promotes the formation of calcium silicate hydrate (C–S–H) [63]. These materials not only improve the mechanical strength and durability of the emulsion but also exhibit excellent material compatibility due to their compositional similarity to the original lime-based ground. The NHL2/SiO₂/SF016 composite emulsion fulfills dual functions of both gold leaf reattachment and ground layer consolidation. Aligned with the principle of minimal intervention in cultural heritage conservation, this formulation allows for flexible adjustment between adhesion and penetration through compositional tuning. This homogeneous repair strategy, leveraging material compatibility, provides a reliable technical solution for the large-scale restoration of gilded artifacts.

3.2.3. Selection of Ground Layer Materials

The selection of ground layer materials focuses on interfacial compatibility as the core evaluation criterion to ensure optimal adaptation between repair and the original materials. In this study, four types of materials were selected for testing: lacquer–plaster, lime, gypsum, and a 20% NHL2/SiO₂/SF016 composite emulsion [64,65]. Their compatibility with the gold-sizing lacquer was evaluated. Four simulated ground layers were prepared and subjected to lacquer gilding applications. The interfacial bonding conditions of cross-sectional samples were systematically analyzed using 3D digital microscopy and SEM, with the results illustrated in Figure 15. The results demonstrate that the lacquer plaster ground exhibits a dense structure with a seamless interfacial transition to the gilded lacquer layer, forming localized mechanical interlocking structures that enhance physical anchoring efficacy. Both materials utilize raw lacquer as their matrix, ensuring high chemical compatibility. In contrast, the gypsum ground displays uneven interfaces and localized cracks when combined with organic gold lacquer. Due to its inorganic crystalline nature, gypsum lacks chemical compatibility with the organic binder, relying solely on physical adhesion, which results in inferior bonding performance. The composite hydraulic lime ground achieves full contact with the gold lacquer surface without significant defects, developing microscopic interlocking structures that contribute to robust interfacial bonding. Adhesion is largely governed by polar interactions between the organic moieties of the acrylic emulsion and the gold lacquer, further supported by the skeletal role of nano-SiO₂ [66,67]. Although the lime ground maintains a relatively intact interface with the gold lacquer, it exhibits porosity, loose regions, and partial delamination. Dominated by calcium carbonate with coarse particles and a loosely packed structure, the lime ground lacks chemical affinity with the gold lacquer and fails to establish effective penetration or anchoring at the interface, leading to weak interfacial cohesion. In summary, based on interfacial integrity and material compatibility, both the lacquer plaster ground and modified hydraulic lime ground demonstrate superior interfacial bonding performance in gilded structures, making them more suitable as restoration materials for ground layers.

The restoration of new ground layers is often accomplished on the original substrates. Therefore, repair materials must exhibit compatibility with both the lacquer-gilded layers and the original substrates. Based on the previous experiments results, lacquer plaster and composite hydraulic lime grounds were selected as candidate repair materials, and their interfacial bonding characteristics with the original ground layers were evaluated. Three-dimensional digital microscopy revealed the interfacial bonding properties (Figure 16), demonstrating that both materials formed tight interfaces with the original ground. This behavior arises from the inherent compatibility between lacquer plaster and gypsum-based grounds, as well as the synergistic interaction of organic–inorganic components within the hydraulic lime system. These findings confirm the adaptability and application potential of both materials in ground layer restoration. During application, grinding is required to ensure surface flatness. However, the lacquer plaster layer, which exhibits a black appearance, generates dark dust during polishing. This dust adheres stubbornly to residual gold leaf surfaces, posing challenges for removal and risking secondary contamination. Consequently, lacquer plaster is primarily employed in areas with extensive gold leaf loss. In contrast, the hydraulic lime ground produces minimal dust during grinding, which is easily removable, making it suitable for regions with better-preserved gold leaf, such as the face and chest of the sculpture. Therefore, during the restoration process, lacquer–ash or hydraulic lime ground should be selected appropriately based on the condition of the surviving gilding, enabling targeted and region-specific ground layer repair.

3.3. On-Site Restoration

The Shakyamuni sculpture primarily suffers from surface contamination, delamination of gold leaf, and localized loss of gilding. To address these issues, a conservation protocol was implemented, comprising three sequential phases: (1) removal of surface contaminants, (2) re-adhesion of detached gold leaf, (3) infilling of degraded ground layers followed by relacquering and regilding. The restoration workflow is illustrated in Figure 17. Figure 17a illustrates the cleaning process applied to the contaminated surface areas of the sculpture. A gentle cleaning method combining Tween 20 with Japanese tissue poulticing was employed to remove surface pollutants. This approach effectively minimizes the risk of damaging the remaining gold leaf. Figure 17b depicts the ground layer restoration phase. For areas of gold leaf loss on the face, a pre-consolidation treatment was first conducted using a 5% NHL2/SF016 composite emulsion. A layer of base lacquer was then applied as an interfacial treatment, followed by the application of a 20% NHL2/SF016 composite emulsion to complete the ground layer. Figure 17c illustrates the lacquer application process on the newly restored ground layer, where raw lacquer and gold-sizing lacquer were applied to ensure that the thickness and structure of the repaired area are consistent with the original lacquer gilding layer. Figure 17d presents the final gilding step, in which gold leaf was reapplied to the restored regions. The result is a visually integrated surface consistent with the appearance of the original gilded areas. During the restoration process, the distinction between original and newly applied gold leaf was primarily based on visual examination. Differences in color tone, edge fracturing, and surface texture were used as criteria to identify original gilding from restored areas.

3.3.1. Optimization of the Lacquer Gilding Technique

Traditional lacquer gilding methods predominantly depend on the artisan’s individual experience. Fluctuations in temperature and humidity can significantly affect the curing performance of raw lacquer, thereby influencing both the construction schedule and final surface quality. To enable more scientific control over the lacquer gilding process, temperature and humidity parameters during each application stage, along with the curing time of each lacquer layer, were recorded. PCA was then employed to explore the correlation between environmental variables and the curing behavior of different layers. As shown in the PCA results (Figure 18), the first two principal components, PC1 and PC2, together explain 90.6% of the total variance, indicating that the model effectively captures the differences in curing behavior across the various layers. Temperature (PC1, 67.4% contribution) was identified as the primary factor governing curing behavior, exhibiting a significant negative correlation with curing time. This suggests that lower temperatures necessitate longer curing periods to ensure adequate cross-linking of the lacquer. Humidity (PC2, 23.2% contribution) served as a secondary factor, exerting varied effects on the curing efficiency of different layers. The differential responses of lacquer layers to temperature and humidity are mainly attributed to their compositional differences and curing mechanisms. The lacquer–ash and gold-sizing lacquer contain gelatinous substances or inorganic fillers, making them more sensitive to humidity fluctuations. The primer layer, composed of raw lacquer and diluents with a higher proportion of low-molecular-weight compounds, is more responsive to temperature variations. In contrast, the main lacquer layer, composed primarily of high-purity raw lacquer, cures through oxidative polymerization and possesses a more stable structure, rendering it relatively less susceptible to environmental changes. Therefore, by appropriately adjusting temperature and humidity during application, the curing time and quality of each layer can be optimized. Furthermore, this relationship can be employed to adjust the surface roughness of the gold-sizing lacquer, hence improving the optical blending effect between gold leaves.

3.3.2. Color Matching

The glossiness of gold leaf is influenced by the surface roughness of the gold-sizing lacquer. Smooth, well-cured lacquer films yield high-gloss gold surfaces, while uneven or under-cured layers produce matte finishes. Based on this principle, temperature and humidity conditions were carefully controlled during the restoration process to regulate lacquer curing behavior. In combination with targeted polishing of the ground layer, this enabled precise adjustment of the final gold leaf appearance. Figure 19a,c show the sculpture before and after restoration. The gilded surface appears continuous and complete, with facial expressions and drapery details effectively restored. Figure 19b,d present the CIE chromaticity diagrams of selected surface regions before and after treatment.

Table 7. Lab values before and after restoration of Buddha sculpture.

Position	Before Repair			After Repair			ΔE
	L	a	b	L	a	b
1	2.98	0.48	−2.98	3.07	2.11	−8.86	6.1
2	3.43	1.36	−5.22	3.16	2.12	−8.51	3.39
3	3.34	1.34	−8.20	3.07	2.11	−7.73	0.94
4	2.98	0.48	−6.56	3.16	2.12	−10.48	4.25
5	3.25	1.33	−11.32	3.16	1.72	−9.96	1.42
6	3.43	1.76	−10.68	3.16	1.72	−9.79	0.93
7	3.43	2.97	−7.67	3.16	2.12	−7.19	1.01
8	2.98	1.28	−6.00	3.16	2.12	−10.13	4.22
9	2.98	0.88	−2.41	3.25	2.94	−6.66	4.73
10	3.61	5.42	−2.07	3.16	4.95	−3.99	2.03
11	2.98	0.48	−6.00	3.25	2.14	−9.27	3.68
12	3.34	2.96	−4.24	3.25	3.35	−5.91	1.72
13	3.16	3.33	−3.61	3.34	2.55	−6.32	2.83
14	2.89	0.87	−7.47	3.25	2.54	−7.23	1.73
15	3.34	4.57	−5.18	3.25	2.94	−4.40	1.81
16	3.25	2.94	−5.15	3.25	2.54	−8.91	3.78
17	3.43	2.97	−5.78	3.25	2.54	−6.66	1.00

shows the chromaticity changes before and after restoration. The relatively small shifts in ΔE indicate a high degree of consistency in hue between the original and restored areas. Compared with traditional approaches such as burnishing the gold leaf or chemical oxidation using potassium permanganate solution, this method achieves color matching between the newly applied and original gold through controlled adjustment of lacquer curing and ground smoothness. It avoids any damage to the original gold surface, offering a safer and more controllable process. This technique reflects the principle of minimal intervention while effectively restoring the visual integrity of the original gilded surface.

4. Conclusions

The lacquered and gilded interface of stone sculptures is structurally complex and susceptible to environmental factors, such as fluctuations in temperature and humidity, as well as weathering and erosion. These circumstances frequently result in deterioration phenomena including delamination, detachment, and surface contamination, posing significant challenges to the long-term preservation and visual integrity of such cultural heritage. To achieve safe, scientific, and effective restoration of these artifacts, this study introduces a comprehensive restoration methodology based on the “Diagnosis–Analysis–Selection–Restoration” framework, applying it to the conservation of the lacquer-gilded layer of the Shakyamuni cliffside sculpture at Erfo Temple in Chongqing, China.

(1)

On-site investigations and deterioration mapping revealed that the sculpture suffers primarily from surface contamination, gold leaf delamination, and gold leaf loss, with over 70% of the surface area affected.

(2)

A combination of analytical techniques including SEM-EDS, XRD, Raman spectroscopy, and Py-GC/MS was employed to systematically examine the composition and microstructural characteristics of key materials such as gold-sizing lacquer, lacquer–ash, ground layers, and pigments. The gilding structure was confirmed to exhibit a characteristic multilayer composition consistent with historical records in Xiushi Lu (Treatise on Lacquering) and to have undergone three to four phases of repair.

(3)

For material selection, various concentrations of NHL2/SiO₂/SF016 composite emulsions were evaluated for both ground consolidation and gold leaf reattachment. The incorporation of NHL2 and nano-SiO₂ effectively mitigated flowability issues and showed favorable performance. Taking interfacial compatibility as the core selection criterion, a 20% NHL2/SiO₂/SF016 composite emulsion and traditional lacquer–ash were identified as the most suitable ground layer repair materials, and their compatibility with original components was thoroughly assessed.

(4)

During the restoration phase, a targeted treatment protocol was established based on identified deterioration types, including gentle cleaning, reattachment and consolidation of the gold leaf, and region-specific ground layer reconstruction. PCA was used to analyze the relationship between lacquer composition and curing behavior, supporting a strategy of modulating lacquer film quality and ground smoothness to visually harmonize the newly applied and original gold leaf.

This approach avoids mechanical abrasion or chemical alteration of the gilded surface and aligns with the conservation principles of “minimal intervention” and “authentic appearance restoration”.