Introduction

The wall paintings in the Hall of Heavenly Kings at Guanyin Temple (37°50′N, 110°22′E) in Suide County, Shaanxi Province (Fig. 1a) represent precious Buddhist art, the cultural and military heritage complex of the Ming Dynasty (1368 A.D.–1644 A.D.) frontier regions. The frontier temple wall paintings of the Ming Dynasty encapsulate historical narratives of frontier governance, ethnic integration, and religious dissemination. Located in the ecological transition zone between the Loess Plateau and the Mao Wusu Desert, Suide County has historically been a vibrant junction between nomadic and agrarian civilisations. It also formed the core of Yansui Town, Suide County in northwestern China, and has long held critical strategic significance, particularly as a core component of the ‘Nine Military Border Garrisons’ system of the Ming Dynasty1, as a strategic buffer zone between the central government and the nomadic north of China’s history. It was strategically located in the middle section of the defence system of the Ming Great Wall of China2. The region’s semi-arid climate and unique loess geology fostered distinct construction techniques, evident in its vernacular architecture and wall painting heritage. The wall paintings in the Hall of Heavenly Kings at Majiawa Guanyin Temple serve as a material testament to this historical context and have important historical and artistic value.

Fig. 1: Photographs of the Guanyin Temple and the wall paintings.
figure 1

a Guanyin Temple in Suide County; b the wall paintings on the north wall in Heavenly Kings of the Guanyin Temple; c the wall paintings on the south wall in the Hall of Heavenly Kings of the Guanyin Temple.

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Spanning the north and south walls (Fig. 1b, c), the wall paintings are large and depict twenty-four Buddhist deities, whose significance is to pray for the prosperity of the country and the people, good weather, and the well-being of all living beings. The figures’ modelling, costume details, and scene designs reflect the local characteristics of religious art in the border areas of the Ming dynasty, and the social and cultural life of the people. However, centuries of exposure to geographical changes, historical upheavals, and natural disasters have placed these wall paintings in critical preservation jeopardy. Notably, the catastrophic floods of 25 July 2017, with precipitation up to 247.3 mm, exacerbated structural damage to the walls (maximum 10 cm cracks), accelerating their deterioration and underscoring the urgency for systematic research and conservation.

This study aims to characterise painting materials and explore degradation mechanisms, using multi-analytical techniques, including Raman spectroscopy, scanning electron microscopy (SEM), internal transcribed spacer (ITS) sequencing, etc., to investigate the composition of loess-based wall painting materials including pigments, adhesives, fibres, and whitewash layer, as well as painting degradation caused by soil grime, water-soluble salts and microbial colonisation. This multi-analytical approach3,4,5 addresses the limitations of single-method analyses in exploring material characteristics and degradation mechanisms of this loess matrix wall painting, thereby providing novel perspectives for the conservation of similar heritage objects. By integrating compositional analysis with currently available wall painting degradation pathways6,7, the research identifies the degradation mechanisms of these loess-based wall paintings in semi-arid environments, thus providing a scientific basis for targeted conservation interventions against salt mitigation and microbial inhibition. These results inform conservation strategies and provide an important case study for interpreting the Ming Dynasty’s frontier lifestyle.

Methods

Samples preparation

In this study, four fallen wall painting fragments were collected from wall paintings on the south and north walls of the Hall of Heavenly Kings, the pictures of these samples and their codes are shown in Fig. 2. The four samples have five types of colours, including green and black in SP-1, red in SP-2, reddish orange in SP-3, and white in SP-4.

Fig. 2
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Four fragments fallen from wall paintings and their codes.

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The cross-section sample preparation

To observe cross-sections of the samples, the samples were embedded in a resin mould (Transparent Cold Mount). After solidification at room temperature, the samples were extracted using a low-density cutter operating at 440 rpm (DTQ-5, Veiyee). Sandpaper (1000 mesh) was used to polish the surface to achieve a reflective and smooth surface. Then the structure of the sample cross-section was characterised by optical microscope.

Crystalline salt sample preparation

Four fragments fallen from the wall paintings were collected, and the wheat straw fibres were sifted. The soil in the mortar layer was mixed with ultrapure water in a ratio of 1:5, immersed in a beaker, and then treated with ultrasonic shaking for 30 min, and the soil conductivity was determined using a conductivity metre (ISO 11265: 1994). The soil solution was then filtered using a pump to obtain a salt solution, the water was evaporated to obtain crystalline salts, and their composition was determined using an energy X-ray fluorescence spectrometer and an X-ray diffractometer.

Analysis and identification of microorganisms

The sterile cotton swab was moistened with sterile physiological saline, and then microbiological samples were collected from the wall paintings and then placed on potato dextrose agar (PDA) medium, and incubated for 48 h at 28 °C (Aobox Biotechnology Co. Ltd.). After the microorganisms grew, single colonies were picked with an inoculating loop and placed in sterile PDA, then the above steps were repeated. When only one type of microorganism grew in the plate, i.e. the pure colonies were obtained. The morphology of the microorganisms was observed using scanning electron microscope, before which the mycelium and spores were adhered on the conductive adhesive and sprayed with gold.

Microbial identification based on ISO 18416: 2015 guidelines. Mould genomic DNA was extracted using a fungal DNA extraction kit (e.g., Qiagen DNeasy Plant Mini Kit) following mechanical disruption via liquid nitrogen grinding and spin-column purification. The internal transcribed spacer (ITS) region was amplified by PCR using ITS1/ITS4 primers in a 50 μL reaction with 2 × Taq Master Mix under thermal cycling conditions (95 °C/5 min → 35 cycles: 95 °C/30 s, 55 °C/30 s, 72 °C/1 min → 72 °C/10 min). Amplicons were validated via 1.5% agarose gel electrophoresis, and purified products with expected 500-700 bp bands were Sanger-sequenced. Trimmed sequences were analyzed by BLASTn against NCBI GenBank, with ≥ 97% identity confirming species identification. Phylogenetic analysis was performed using ClustalW and MEGA 11.

Spectrocolourimeter

To specify the areas to be analysed, the samples were first carefully observed by a spectrocolourimeter (DS-700D, CHNSpec) with a built-in high-definition camera. And the chromatic coordinates L, a, and b were determined using a portable spectrocolourimeter, running with the ColorQC 2 software. The illuminant is the D65 one, the observer geometry is set at 10°, and the test calibre is Φ3mm.

Optical microscope (OM)

The morphology characteristics of sample surfaces were observed by an optical microscope (BX53M, Olympus) equipped with 20 × and 50 × objective lenses. In addition, according to ISO 9184-3 1990, the morphology features of stained fibres from wall paintings with Herzberg stain were observed with 40 × objective lenses by OM. The sample cross-section was observed using OM with a 20 × objective lens.

Energy X-ray fluorescence spectrometer (EDX)

The elemental characterisation of samples was determined by a non-invasive EDX (EDX-7000, Shimadzu). It was equipped with an energy X-ray tube consisting of an Rh target and a silicon drift detector. The analysis was performed under atmospheric conditions, and each sample was characterised for approximately 5 min. The samples were placed on a Myra film for non-invasive testing with a test diameter of 1 mm.

Micro‑Raman spectrometer

Micro‑Raman spectroscope could be used to characterise a wide range of inorganic mineral pigments, due to the enhanced analytical properties of non-polar molecules8,9,10. The spectral properties of the pigment samples were non-invasively characterised using a laser micro-Raman spectroscopy (inVia Reflex, Renishaw), in the range of 100 cm−1 to 3500 cm−1 using a grating with 400 lines/mm and a spot size of 2 μm, the excitation wavelengths were set at 532 nm. The objective lens magnification was ×50 with an exposure time of 30 s. The accumulation period was 1 s using a laser power of 2 mW. All spectral processing was performed using the instrument Wire software v. 3.4.

X-ray diffractometer (XRD)

A high-resolution X-ray diffractometer (Smart Lab, Rigaku) was used to identify the composition of whitewash layer, crystalline salts and soil grime on the sample surface in wall paintings, utilizing Cu Kα radiation (λ = 1.5406 Å) over a 2θ range of 10°–90°, with an acceleration voltage of 45 kV, tube current of 200 mA, and a scanning speed of 5°/min.

Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectrometer

The wall painting fibre was identified by using an attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectrometer (PE-Frontier, Perkin Elmer) equipped with a diamond cell from 4000 to 400 cm−1 at a resolution of 4 cm−1 and a total of 32 scans.

Scanning electron microscope (SEM)

The micromorphology of fibres and microorganisms was characterised using SEM (SU3500, Hitachi). The tests were carried out in a vacuum environment with an operating voltage of 10 kV. The magnification range was adjusted between 600-2000. The sample surfaces were sprayed with gold for 120 s.

Pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS)

A small content of pigments were analysed by pyrolysis-gas chromatography-mass spectrometry (EGA/PY-3030D, Frontier Labs) equipped with a GC-MS instrument (QP2010Ultra, Shimadzu) and the column SLB-5MS (5% diphenyl/95% dimethyl siloxane) was 30 m in length with an internal diameter of 0.25 mm and a film thickness of 0.25 μm (Supelco). For the analysis, 0.5 mg of sample was ground into powder and placed into a thermal cracking sample cup. 2 μL TMAH solution (four methyl ammonium hydroxide, 25%) was added to the cup and left for 1 h to make them fully dissolve. The parameters for analysis were set as follows: the thermal cracking temperature, the cracker interface temperature, and the inlet temperatures were equivalent to 600 °C 300 °C, and 250 °C, respectively. The initial column temperature was set to 40 °C for 20 min, then up to 280 °C at a rate of 10 °C/min. High-purity helium was the carrier gas for GC-MS with an inlet pressure of 15.5 kPa and a split ratio of 1:100. The electronic pressure control system was operated in the constant flow mode. The mass spectrometer used electron ionization at an energy of 70 eV. Subsequently, the compounds were characterised with NIST14 libraries and other mass spectrometry databases.

Conductivity metre

The soil conductivity was tested by a conductivity metre (DDSJ-308A, LeiCi). The DJS-1D conductivity electrode head and the T-818-B-6 temperature electrode head were placed in an aqueous soil solution and tested at room temperature, with an electrode constant of 0.01 cm−1.

Results

Wall painting structure and material composition

Field investigations and sample analysis results indicated that the ancient wall paintings consist of five distinct layers (Fig. 3a, b). The five layers from the bottom to top are support body, constructed of rammed earth or adobe providing the structural foundation, coarse layer, composed of clay mixed with crude fibre to achieve a flat substrate while enhancing mechanical adhesion to the support body4, fine layer, comprised of refined clay reinforced with fine fibres to improve tensile strength and interlayer cohesion4, whitewash layer applied to create a bright, uniform surface for pigment application. The top layer is the painted layer, which is the decorative layer containing mineral pigments bound with binders.

Fig. 3: Structure of the wall painting.
figure 3

a Schematic image of wall painting structure; b cross-sectional structure of the samples.

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This construction sequence started from the load-bearing support body and proceeds sequentially, with layer-by-layer refinement, reflecting the traditional Dunhuang wall painting techniques11, to ensure long-term structural stability and visual integrity, due to the Loess Plateau’s friable geology. It is also similar to the production of the Shanxi Xianqing Temple ancient wall paintings in the Ming Dynasty4.

Microscopic examination of the SP-3 sample’s fine layer (Fig. 4a) revealed fibrous structures embedded within the fine matrix (Fig. 4b). SEM images indicated cylindrical fibre morphology lacking spiral or lamellar features (Fig. 4c1–c2), consistent with hemp fibres (Cannabis sativa) as previously reported4. FTIR spectroscopy confirmed the cellulosic nature12 of these fibres (Fig. 4d). Key spectral features, including O-H out-of-plane bending at 664 cm−1, C-H asymmetric bending vibrations at 1429 and 1314 cm−1, C-O bond stretching at 1023 cm−1, water absorption at 1630 cm−1 13,14, O-H stretching at 3332 cm−1 and C-H stretching at 2901 cm−1 15. The lack of a characteristic 1243 cm−1 peak, which typically corresponds to C-O stretching or NH₂ deformation in other fibres, further confirmed the identification of hemp4. Herzberg staining analysis (Fig. 4e) also supported this finding, showing wine-red stained fibres with long, unbranched structures and parenchyma cells consistent with the database16. In summary, SEM, FTIR, and fibre analysis evidence confirm the presence of hemp fibres in the wall painting’s fine layer. Meanwhile, it can also be observed with the naked eye that the coarse layer contains wheat straw. The hemp fibres in this wall painting are distinct from the cotton fibres4 reported in the Ming dynasty Shanxi Xianqing Temple wall paintings, which may be related to local agricultural products.

Fig. 4: Micrographs and ATR-FTIR spectrum of fine fibre.
figure 4

a1 The fine layer of the SP-3 surface; a2 optical microscopic image of fibres in wall paintings; (b1, b2) SEM images of fibres; c ATR-FTIR spectrum of fibres; d optical microscopic image of dyed fibre.

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In sample SP-3, observation of delaminated pigment areas (Fig. 5a) revealed a whitewash layer underlying the painting layer. Major elements, Si (39.52 wt%), Ca (35.86 wt%), Al (9.68 wt%), Fe (6.77 wt%), and K (6.12 wt%), were detected using EDX spectroscope. XRD was further used to characterise this layer. The corresponding PDF card information shows that the soil grime is mainly composed of aragonite (CaCO3), and quartz (SiO2). The strong diffraction peaks of the former (2θ = 26.2°, 33.1°) correspond to the typical crystalline surfaces of calcium carbonate, which constitute the white cemented matrix of the lamellae. The latter (2θ = 26.2°) acts as an inert aggregate and enhances the mechanical properties of the material17. It is worth noting that EDX detected the presence of Al and K in the sample, whereas the two minerals mentioned above do not contain such elements. The presence of layered silicate minerals such as illite (KAl₂(AlSi₃O₁₀)(OH)₂) in the sample is assumed to be due to the traditional use of clays (e.g., loess) as fine aggregates in the process. The lack of significant XRD signals may be related to the fine particle size, low crystallinity, or amorphous structure of the clay minerals. Despite the presence of multiple mineral components, the dominant cementing role of calcium carbonate is consistent with a macroscopic white appearance (Fig. 5b), confirming that the whitewash layer of the wall painting is a whitewash layer with calcium carbonate as the core cementing phase.

Fig. 5: Photographs and XRD pattern of the whitewash layer.
figure 5

a The whitewash layer of the SP-3 surface; b optical microscopic image of the whitewash layer; c XRD pattern of the whitewash layer.

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For five colour areas in four samples, the chromatic coordinates were tested three times at points and then averaged out, and the values were shown in Table 1 and represented in the CIE Lab colorimetric space (Fig. 6). The green shades on SP-1 shown values with b* higher than a* and a lightness L* nearing 60, this is consistent with the difference observed in tones. The black area in SP-1 shows characteristic black chromatic coordinates, with an observed low lightness L* of about 40 and a slightly higher b* value. The red of SP-2 and the reddish orange of SP-3 show typical colour coordinates with a lightness L* close to 603. The white zones in SP-4 present b* values greater than a* ones with lightness L* of about 75, reflecting the white’s observed yellowish hue.

Table 1 Measured chromatic coordinates L*, a*, and b*
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Fig. 6
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CIE Lab colourimetric coordinates of pigment samples (SP-1 to SP-4), showing distinct clusters for green (a*<0), red (a*>10), and white (b*>8) hues.

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Elemental composition analysis via EDX spectroscopy and molecular structural analysis using micro-Raman spectroscopy (Fig. 7, Table 2) provided complementary insights into pigment identification. The green pigment was confirmed as atacamite (Cu₂(OH)₃Cl)8, a copper-based mineral also known as copper green. This compound exists in polymorphic forms including atacamite, paratacamite, and clinoatacamite, and has been reported as a green pigment in Dunhuang wall painting9. The black pigment was identified as lead carbon black (C)10, consistent with Chinese historical production of carbon black, the earliest recorded in the world and for use in architectural pigmentation18,19. The red pigment corresponded to cinnabar (HgS)20,21, which a widely found in Dunhuang cave and imperial temple wall paintings19,22. The reddish-orange pigment was characterised as minium (Pb₃O₄)23, a lead oxide mineral employed as a red pigment since the Han Dynasty and limited to imperial commissions and major monasteries24. Lastly, the white pigment was identified as gypsum (CaSO₄), a common component in Chinese painted cultural relics25.

Fig. 7: Raman spectra of pigment samples.
figure 7

a SP-1 green; b SP-1 black; c SP-2 red; d SP-3 reddish orange; e SP-4 white.

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Table 2 Elements compositions in the pigment particles
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Locally sourced loess-based materials were utilized during preparation of this wall painting, resulting in cross-sectional delamination patterns where coarse or fine layers frequently detach along with the whitewash layer, leaving isolated pigment layers (Fig. 8). Microscopic examination of cross-sections revealed distinct pigment application patterns, green, red, reddish orange, and white pigments exhibit monolayer application. Black pigment overlies green in SP-1 (Fig. 8c), indicating sequential layering without overpainting events. This evidence supports the hypothesis of original single-phase execution, consistent with traditional wall painting techniques where black was often applied as a final contour layer4.

Fig. 8
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Microscopic images of sample cross-sections, where different painting layers are observed and labelled with white dashed lines.

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Mineral and organic pigment particles pose challenges when directly applied to coloured paintings, necessitating the use of adhesives26,27. In the same painting, it is common to employ the same adhesives for different pigments22. To characterise the adhesives in sample SP-4, Py-GC/MS analysis was performed. The total ion chromatogram and the identified compounds are presented in Fig. 9 and Table 3, respectively. In the sample, several compounds were detected, including allocholesterol (peak 7), cholesta-3,5-diene (peak 16), sulphurous acid, cyclohexylmethyl heptadecyl ester (peak 18), and cholest-5-en-3-ol (3.beta.)-, propanoate (peak 23). Peaks 7, 16, and 23 are derivatives of cholesterol, which is a strong indication of the presence of egg yolks28,29. Meanwhile, peak 18 belongs to sulphur-containing compounds, suggesting the use of egg white28,30. These results demonstrate that the pigment binder used in this wall painting might be egg-based adhesive. Eggs are a well-known and commonly used heritage protein-based binder material in wall painting28,31.

Fig. 9
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The total ion chromatogram of SP-4.

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Table 3 Compounds identified by Py-GC/MS in the total ion chromatogram of SP-4
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Characterisation and synergy mechanisms of three types of degradation

Calcareous soil encrustation represents a major conservation threat to wall paintings32. Analysis of SP-1 sample (Fig. 10a) revealed complex soil deposits under optical microscope (OM), indicating sandstone aggregates and white crystalline phases (Fig. 10b). Energy X-ray fluorescence spectrometry (EDX) results demonstrated major elements, such as Ca (46.53 wt%), Si (40.65 wt%), Fe (4.88 wt%), K (4.34 wt%), and S (2.38 wt%) in the encrustation matrix. The corresponding PDF card information shows that the soil grime is mainly composed of potassium oxide K2O, haematite Fe2O3, calcium carbonate CaCO3, quartz SiO2, and calcium sulphite CaSO3 are shown in Fig. 12c.

Fig. 10: Photographs and XRD pattern of soil grime.
figure 10

a Soil grime on SP-1 sample surface; b soil grime under OM; c XRD pattern of soil grime composition.

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Water-soluble salts in the wall painting soil were analysed via ultrasonic extraction. 24.97 g of soil that had dropped from the four fragment wall paintings was collected and mixed with 124.85 g deionized water and sonicated for 15 min. The electrical conductivity (EC) of the supernatant was measured as 1159 μS/cm, indicating moderate salinity. Within low concentration ranges, EC correlates linearly with total dissolved solids33,34, confirming significant soluble salt presence. Following filtration, the aqueous extract was evaporated to dryness, yielding approximately 0.05 g of crystalline salts, and the calculated soil salinity was 2.00 g·kg−1. Elemental analysis via EDX revealed the presence of major components Ca, S, and Cl. Subsequent XRD analysis (Fig. 11) identified crystalline phases as NaCl and calcium sulphate dihydrate (CaSO₄·2H₂O). The latter’s hydrated form reflects the hygroscopic nature of sulphate minerals under semi-arid conditions35.

Fig. 11
figure 11

XRD pattern of crystalline salt precipitated.

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Extensive surface colonisation by microbial hyphae and spores was observed on the wall paintings (Fig. 12a1-a2), posing aesthetic and structural threats. Two distinct microbial colonies were isolated and purified from the wall painting surface. Cultivation of PDA followed standard morphological characterisation protocols. Internal Transcribed Spacer (ITS) sequencing identified the isolates as Aspergillus sp. (black colony) and Trichoderma harzianum (green colony)23,36. On PDA medium, Aspergillus sp. formed flat, olive-black colonies with smooth surfaces (Fig. 12b1), while T. harzianum displayed initially white-flocculent colonies that darkened to green with age (Fig. 12b2). Aspergillus sp. exhibited unbranched conidiophores terminating in globose conidial heads observed by the naked eye (Fig. 12c1), whereas T. harzianum featured penicillate conidiophores with bottle-shaped phialides producing clustered conidia (Fig. 12c2). These morphological features align with taxonomic descriptions of these common wall painting-inhabiting fungi37. SEM images further support these morphological observations and ITS identification. Both species belong to the fungal kingdom, with Aspergillu sp. displaying rapid growth typical of soil or plant-associated strains.

Fig. 12: Photographs of microorganisms.
figure 12

a1, a2 Microorganisms on wall paintings; b1, c1 morphology and b2, c2 SEM images of microorganism colony.

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Located in the Loess Plateau’s hilly-gully region, the wall paintings are situated within a friable Loess matrix characterised by low organic content and high sand/gravel fractions. The semi-arid climate has contributed to their relatively intact preservation38. However, the 2017 extreme rainfall event (247.3 mm precipitation) drove the synergistic effects of soluble salts, soil grime, and microbial activity that led to the degradation of Guanyin Temple wall paintings. Crystallisation and dissolution of salt have been severely damaging problems faced by wall paintings39,40. Floodwater infiltration dissolved NaCl and CaSO₄·2H₂O inherent salts in the wall structure41. Water-soluble salts undergo seasonal dissolution-migration-crystallization cycles, during summer monsoons, salts dissolve in infiltrating rainwater and migrate to the painting layer, while winter drying (<5% RH) drives supersaturation and crystalline expansion42,43, particularly of hygroscopic CaSO₄·2H₂O, which expands volumetrically by ~15% during hydration-dehydration cycles40,44,45. This generated mechanical stress46, resulting in micro-cracks within the painting layer (Fig. 13).

Fig. 13
figure 13

Optical microscopic images of SP-1 green, SP-1 black, SP-2 red, SP-3 reddish orange and SP-4 white.

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Soil grime accumulates in these microcracks, forming hardened crusts composed of atmospheric dust and historical restoration residues. Over time, the painted layer of the adhesive material has undergone a certain degree of degradation, and soil grime and painted layer adhesion is greater than the adhesion of the painted layer and the wall painting substrate32. With increased ambient humidity from flooding, microorganism spores exploit the humid microenvironments within cracks, and mycelium can penetrate to greater depths. The egg gel adhesive in the wall paint layer provides a key source of nutrients for the moulds47.

Metabolically active Aspergillus sp. secreted oxalic acid (C₂H₂O₄), a common metabolite in wall painting biodeterioration48. This acid could react with the white powder layer (H₂C₂O₄ + CaCO₃ → CaC₂O₄ + CO₂ + H₂O)49, reducing Ca content from theoretical 40 wt% to 35.86 wt% as detected by EDX. Ca²⁺ ions combine with sulphate ions (from environmental deposition or gypsum dissolution), regenerating CaSO₄·2H₂O and establishing a self-sustaining degradation loop11. This positive feedback mechanism, where microbial activity accelerates salt formation, which in turn creates more niches for fungal growth, mirrors sulphate and carbonate cycles documented in the Mogao Grottoes11.

Discussion

This study systematically characterises the five-layered wall paintings system at Guanyin Temple in Shaanxi Suide County, revealing a composite structure of rammed earth supports, wheat straw-reinforced coarse layer, hemp-fortified fine layer, whitewash layer composed of calcium carbonate and silicon dioxide, and a pigment layer composed of cinnabar (HgS), minium (Pb₃O₄), atacamite (Cu₂(OH)₃Cl), carbon black (C), gypsum (CaSO4), and egg-based adhesives. Three interdependent processes including soluble salts (NaCl, CaSO₄·2H₂O), soil grime (K₂O, Fe2O₃, CaCO₃, SiO₂, CaSO₃), and fungal colonisation (Aspergillus/Trichoderma harzianum) exhibit synergistic interactions, accelerating wall painting degradation. Water-soluble salts create hygroscopic environments promoting microbial proliferation, while soil encrustations trap metabolites and salts. Fungal colonisation degrades proteinaceous binders through protease secretion and accelerates carbonate dissolution via oxalic acid production.

Using high-grade mineral pigments such as cinnabar and complex egg-based binders, these wall paintings are of great artistic and historical value, demonstrating the content of the social and cultural life of the people at that time, and further reflecting the importance that the Ming Dynasty attached to this frontier region. This research also establishes a multi-analytical framework for characterising wall painting systems in semi-arid environments, providing critical data for conservation strategies that prioritise cleaning of soil grime, reduction of salt transport, and inhibition of microbial growth. For future restoration efforts, a combination of physical pre-cleaning and mild chemical treatments can be employed to remove stains while preserving the stability of mineral pigments and adhesive materials. Maintaining a controlled temperature and humidity environment will help prevent further salt-induced deterioration. Further analysis of microbial community dynamics will be conducted to enhance our understanding of the wall paintings’ degradation mechanisms.