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Article

Investigating the Material Composition and Degradation of Wall Paintings at Müstair Monastery Using a Mobile Multi-Spectroscopic System

by
Xueshi Bai
1,*,
Patrick Cassitti
2,
Aude Brebant
3,
Didier Brissaud
4,
Duixiong Sun
5,
Yaopeng Yin
6 and
Vincent Detalle
1,7
1
Centre de Recherche et de Restauration des Musées de France (C2RMF), 14 Quai François-Mitterrand, 75001 Paris, France
2
Archaeological Service, Canton of Grisons, CH-7000 Chur, Switzerland
3
Département Génie Electrique, Institut Sciences et Technique, CY Cergy-Paris Université, 5 Mail Gay Lussac, 95031 Neuville sur Oise, France
4
Laboratoire de Recherche des Monuments Historiques, 29 Rue de Paris, 77420 Champs-sur-Marne, France
5
Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
6
National Research Center for Conservation of Ancient Wall Paintings and Earthen Sites, Conservation Institute, Dunhuang Academy, Dunhuang 736200, China
7
Systèmes et Applications des Technologies de l’Information et de l’Energie (SATIE), CY Cergy-Paris Université, CNRS UMR 8029, 5 Mail Gay Lussac, 95031 Neuville sur Oise, France
*
Author to whom correspondence should be addressed.
Photonics 2025, 12(5), 489; https://doi.org/10.3390/photonics12050489
Submission received: 29 March 2025 / Revised: 7 May 2025 / Accepted: 9 May 2025 / Published: 15 May 2025
(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)
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Versions Notes

Abstract

:
The conservation of cultural heritage requires advanced analytical tools to assess historic materials. In the context of the IPERION-CH project, a mobile multi-spectroscopic characterisation system for the analysis of cultural heritage materials, designated SYSPECTRAL, has been developed. This system integrates Laser-Induced Breakdown Spectroscopy (LIBS), Laser-Induced Fluorescence, Raman spectroscopy, and reflectance spectroscopy. The first application of SYSPECTRAL in a real-world setting was carried out at Müstair Monastery (UNESCO World Heritage Site since 1983) for wall paintings. In this study, stratigraphic analysis using LIBS revealed lead- and iron-based pigments in black and red hues, suggesting pigment degradation and restoration interventions. The presence of titanium in white hues indicated possible retouching. Furthermore, the presence of Egyptian blue in blue hues was identified through a combination of elemental and reflectance spectral analysis, underscoring the potential of SYSPECTRAL for heritage conservation. This approach offers comprehensive material characterization with minimal impact, a finding that is of particular significance in the context of heritage conservation. The subsequent phase of research will extend the application of SYSPECTRAL to a wider range of heritage sites, with the objective of enhancing the spectral databases and refining the analytical techniques for the purpose of improving cultural heritage conservation.

1. Introduction

Cultural heritage conservation requires advanced analytical tools to assess the composition and condition of historic materials with minimal impact. While there remains a significant difference between laboratory measurements and field implementation under ambient light conditions, the latter is increasingly required. During recent years, several laser-based mobile instruments have been developed for cultural heritage [1,2,3,4,5,6,7] suitable for conducting in situ analysis. In this context, within the framework of the IPERION-CH (IPERION-CH: Integrated Platform for the European Research Infrastructure ON Cultural Heritage) program, significant efforts have been made in developing new mobile instrumentation that combines multiple spectroscopic techniques: Laser-Induced Breakdown Spectroscopy (LIBS), Laser-Induced Fluorescence, Raman spectroscopy, and diffuse reflectance spectroscopy [8,9,10]; this has become a multi-spectroscopic characterization system for cultural heritage material analysis (SYSPECTRAL). This system can provide complementary elemental and molecular information from the same analysis point, offering comprehensive material characterization. This mobile SYSPECTRAL has been successfully applied in cultural heritage contexts, with its first deployment at the Monastery of St. John in Müstair, a UNESCO World Heritage Site since 1983 [11], under the European transnational access of the mobile laboratory (MOLAB) in IPERION-HS (IPERION-HS: Integrated Platform for the European Research Infrastructure ON Heritage Science) program. This interdisciplinary project also involved the use of different techniques from several European research groups, such as visible to near-infrared (VIS/NIR) large area spectral imaging survey, SWIR hyperspectral imaging, remote VIS/NIR hyperspectral imaging, Raman spectroscopy and LIBS.
The monastery of St. John in Müstair is well known, among other things, for the wall paintings inside the church, which date back to the late 8th/early 9th and late 12th centuries. The 8th/9th century paintings are very rare examples of Carolingian art, forming the largest preserved cycle of wall paintings from the period. Due to their fame, the wall paintings have been the object of research, but also of invasive conservation measures, from the early 19th century onwards [12]. At present, after the last significant restoration in 1947–1951, the interior of the church is characterized by different decorative phases from a variety of periods, which have undergone several conservative treatments, not all of which are precisely documented.
The abbey church was originally constructed as a single-nave church during the time of Charlemagne, and decorated with rich wall paintings. In the late 12th century, the Carolingian paintings were partially covered by new wall paintings, which are also of very high quality. The church underwent significant modifications during the late Gothic period (c. 1500) to become a three-nave structure, and the older paintings were covered by limewash or with new frescoes [13]. The last, massive interventions were the restorations of 1947–1951, during which the Carolingian and Romanesque wall paintings were brought to light, treated with different substances, and partly covered with overpainting and heavy retouching. Together with the alterations and degradations of the surfaces brought about by the passing of time, this creates a very complex situation [14,15,16,17,18]. A clear understanding of the character of the paint layers on the church walls, the materials used, their alteration, and the impact of materials deriving from the 1947–1951 restoration is crucial in adequately interpreting the paintings and formulating suitable conservation strategies.
For this reason, a project proposal was formulated and submitted to the IPERION-HS consortium. IPERION HS integrates national institutions from 23 countries in the field of cultural heritage research, providing access to a wide range of scientific instruments, methods, data, and tools. The proposal was approved in 2021 and the research was carried out in 2022. The research group consisted of nine teams, coordinated by Haida Liang of Nottingham Trent University. The study analyzed a 20 m2 section of the northern wall to assess painting conditions using remote sensing, imaging and point-analysis techniques. It identified materials and overpaintings, correlated findings with spectral data, and developed protocols to guide future conservation and monitoring.
Within this broader project, the French team was able to analyze both 15th-century lime-coated plasterwork and 8th-century Carolingian wall paintings in October 2022. Special attention was given to the identification of blue pigments, particularly Egyptian blue, as well as to understanding the causes of blackening on the painted surfaces [14,19]. The present paper is concerned with the inaugural implementation of this analytical instrumentation in a real-world context, with a view to evaluating its performance and discussing the advantages and limitations that were observed during the course of the study.

2. Materials and Methods

The mobile instrument utilized for this analysis was developed for heritage science by combining multi-spectroscopic techniques: LIBS, LIF, Raman Spectroscopy and diffuse reflectance spectroscopy. It is implemented on the scaffolding at a height of about 4.5 m from the floor, as illustrated in Figure 1.
The analysis was carried out with the mobile SYSPECTRAL, installed on a modular scaffolding structure, carefully positioned to ensure mechanical stability and optimal access to the targeted areas of the mural. The instrument platform was mounted at a working height of approximately 4.5 m from the floor, as shown in Figure 1. The scaffolding not only supported the weight and operation of the SYSPECTRAL system but also provided safe working conditions for the technical team conducting the measurements. The mobility of SYSPECTRAL, combined with its remote operation capabilities, made it possible to conduct detailed spectroscopic scans across selected regions of the painting with high spatial resolution. The system’s articulated mount and adjustable optical head facilitated the precise targeting of individual features such as pigment layers, overpaintings, and altered zones, even on curved or uneven plaster surfaces.
As described in our previous work [8,9], a compact laser diode pumped nanosecond Nd:YAG laser (VIRON, Lumibird, Villejust, France) with a wavelength of 355 nm was used to carry out this multi-spectroscopic analysis. The optical pathway of the mobile device is illustrated in Figure 2. The laser beam is focused onto the object surface by a lens of 20 cm focus, confining the interaction region to a diameter of approximately 200 µm. The working distance is about 12 cm from the laser beam exit of the prototype and the painting surface. The signal collection pathway is based on an optical fibre positioned at the image plane of a 4-f imaging system comprising two lenses. This fibre is subsequently divided into five individual fibres, three of which are coupled to spectrometer modules (HR2000+, Ocean Insight, Orlando, FL, USA). The integrated time of the detection from these spectrometer modules is 1 ms; as such, all the emission from the plasma can be collected, because a plasma lifetime is about a few microseconds.
Four primary color categories were identified—black (Point 1–9), red (Point 10–12), white (Point 13–16) and blue (Point 17–19), as shown in Figure 3—each potentially corresponding to different pigments or application techniques. At each point, 20 laser pulses were delivered to the same location and at different depths to allow comparative analysis in identifying pigment composition, potential degradation phenomena and residues from previous restoration treatments.
The time-integrated spectrometer modules were used for LIBS measurement (Ocean Insight HR2000+) with three spectral ranges: 190–353 nm (Resolution: ~0.21 nm); 357–559 nm (Resolution: ~0.21 nm); and 556–1085 nm (Resolution: ~0.4 nm). Each spectrum was recorded for one plasma’s emission, which corresponds to the depth of laser ablation. Figure 4a shows the typical spectra of Point 7 within the 190–353 nm spectral range. In this spectral range, the emissions from Fe, Pb, Si, Ca, Al and Ti were observed [20]. In order to better identify the stratigraphic structure of the pictural layers, the emission line intensity is then classified as a function of number of laser shots according to the different depths inside the sample, as shown in Figure 4b. The variation in the peak intensity or the presence/absence of the elements reveals that layer structures can be distinguished at this analyzed point [21,22].

3. Experimental Results

To determine the original coloration of areas that had undergone degradation, the painted surface was carefully examined to assess whether discoloration had resulted in darkening (blackening) or lightening (whitening). Stratigraphic analysis was conducted to identify any evidence of repainting or restoration interventions. Previous research has suggested the presence of Egyptian blue in the artwork [17,18,19]. Therefore, the analysis aimed to confirm the use of this pigment through the application of various spectroscopic techniques [23]. The results of these investigations are presented in this section, and organized according to hue classification.

3.1. Identification of Black and Red Hues

As a case for Point 7, Figure 5a,b show the emission intensity as a function of the depth profile of the LIBS on the spectral ranges, 200–357 nm and 356–559 nm. Four layers can be distinguished according to the variation in the presence of different elements and the evolution of emission intensity: in the first layer (first to second laser shot), a strong Ti emission implied that one layer from a restoration intervention is present. From the sixth laser shot, the emission from calcium (i.e., Ca I 318.0 nm) becomes more intense, and other elements’ emissions become lower. From the 10th laser shot, the emissions from silicium (Si I 288.2 nm) and aluminum (Al I 309.3 nm) define the transition from the third layer to the fourth layer.
In this black hue, layers 1 through 4 contain Pb, Fe, Al, and calcium oxide (CaO) [24]. The repeated detection of these elements suggests that the mural was retouched over time, incorporating different restoration materials. This kind of presentation in depth can offer good stratigraphy vision, but the averaged spectra from the same layer can be a better way of identifying the composition of different layers. Therefore, the averaged spectra were obtained from the layer information (number of laser shots) offered by the stratigraphic structure. The emission spectra from the second layer of the third to fifth laser shot are shown in Figure 5c. Distinct emissions of carbon (C I 247.8 nm) and lead (Pb I 405.8 nm) can be noted and their intensities are stronger than those in other layers, where calcium (Ca II 315.9 nm and 318.0 nm) is found to be less intense than in the support layer; this implies that a material consisting of carbon and lead, such as lead carbonate, was used in this layer. In this case, the black hue could be a blackening of lead white [25,26]. However, this second layer also contains iron, and iron oxides can offer a black hue. Therefore, conclusions are uncertain for the original color at this point. The black hue may come from the mixture of iron oxide black and lead white blackening. The Si and CaO emissions became stronger in a deeper layer; these are elements from the support or preparation layers, like the sand (SiO2) in the wall or calcite (CaCO3).
Other structural configurations were also observed, as illustrated in Figure 6; an example is that from Point 1, characterized by variations in peak intensity. As indicated by the arrows, the emission intensity is observed to be diminished in comparison to the adjacent depth zones, which are characterized by a deficiency or paucity of material. Additionally, it displays a similar composition across all layers, on the contrary to Point 7. A constant presence of iron can be observed at this location, and lead emissions are also identified, which may imply that a mixture of pigments consisting of Fe and Pb was used. Therefore, this black hue is difficult to determine as degradation from a lead-based pigment because of the possibility of using black iron oxide pigment, but the blackening of the pigment may be an explanation for this black hue. The same is true of the red hue, which is why the black and red hues are presented together. The lead-based red pigment, possibly minium, and the red ochre consisting of Fe can be found at the same point.
The same analytical approach was applied to other areas characterized by black and red hues. The main elements identified at different depths, which help to determine the nature of the pigments, are summarized in Table 1. This table presents the elemental composition detected in successive layers (from surface to deeper levels) for each analyzed point.
At Point 2, the composition is characterized by silica in the first layer and by calcium oxide, calcium, and aluminum in the underlying layers. This profile resembles the deeper layers observed at other points, suggesting that the laser beam reached a substrate where no pigment remains, likely due to degradation or loss of the original painted surface.
At Points 3, 5, 9, and 11, only emissions related to iron were detected, with no lead signal. This absence of lead indicates the use of iron-based pigments—possibly iron oxides such as hematite (red) or magnetite (black)—to produce the observed hues.
Other analyzed points show mixed compositions, with both lead and iron present. These findings complicate interpretation: the resulting color could derive from altered lead pigments, intentional mixtures of lead- and iron-based pigments, or stratified application techniques.
In the case of the red hues (Points 10 to 12), a similarly diverse composition is observed. Point 10 contains lead, iron, titanium, and calcium oxide across its layers, suggesting a complex pigment mixture. Points 11 and 12 are dominated by iron, with variable contributions from titanium and aluminum, indicating the use of stable iron-based pigments in these areas.

3.2. Indices of Retouching in White Hues

The white hue observed in the analyzed areas may originate from the application of a white pigment, the alteration of pre-existing-colored layers, or the presence of a preparatory layer without any added pictorial material. Table 2 presents the main elements detected at different depths for all points associated with white hues, along with photographic documentation of the sampled locations.
Points 13 and 15 have identical elemental compositions and stratigraphic structures. Photographic analysis of these sites shows a visible white layer overlaying a red layer in Point 13. Similarly, at Point 15, the underlying presence of iron-based elements suggests the existence of a red pigment beneath the observed white or grey surface, supporting the hypothesis of an overlying white layer applied on a red background.
Point 14, located on the face of the figure, is interpreted as a case of discoloration due to pigment alteration. The detection of lead in the upper layers suggests a whitening effect commonly associated with the degradation of lead-based pigments, such as minium (Pb3O4), a phenomenon that is well documented in previous studies [27].
At Point 16, the absence of any pigment-related elements indicates that the pictorial layer has been entirely lost. Only components corresponding to the underlying wall substrate were identified at this site.
All analyzed white areas contain titanium (Ti), often in the surface layers. This consistent presence may point to either restoration interventions or residues of cleaning agents applied across the wall surface [28].

3.3. Blue Hues: The Search for Egyptian Blue

Egyptian blue is a mineral pigment composed primarily of calcium copper silicate, with the chemical formula CaCuSi4O10 or, as alternatively expressed, CaO·CuO·(SiO2)4. In considering this composition, the presence of copper (Cu) is a pivotal indicator for identifying this pigment compared with other pigments used in this wall painting; therefore, because calcium (Ca) and silicon (Si) are ubiquitous, they cannot be considered conclusive evidence on their own.
As demonstrated in Figure 7(a1–a3), the stratigraphic profiles indicate the potential presence of copper within the second layer, situated beneath an aluminum-rich preparation layer. In order to investigate this possibility, spectra from the corresponding regions were analyzed in order to detect copper emissions. These regions are highlighted with yellow squares in the images.
The mean spectra collected from each analyzed point are displayed in Figure 7b. Of the three locations under investigation, Point 18 exhibited a significant copper emission; this evidence supports the presence of Egyptian blue in this area. But, the other two points demonstrate an absence of a significant copper signal, suggesting the probable absence of this pigment in those regions.
The microscopic image displayed in Figure 7c demonstrates that the blue hue observed at Point 18 comprises multiple distinct blue crystals, interspersed with uncolored materials. This microstructural composition provides substantial evidence for the use of Egyptian blue, a pigment distinguished by the presence of rectangular blue crystals, accompanied by unreacted quartz and traces of a glassy matrix.
In this particular context, the capacity to undertake stratigraphic analysis is of paramount importance, as it facilitates the selective removal of upper layers, thereby enabling direct access to and analysis of the pigment layer, which shows that LIBS is suitable for depth-resolved explorations. However, it should be noted that the laser beam has a diameter of approximately 200 µm. Given the visibly sparse and heterogeneous distribution of Egyptian blue particles within the matrix, it can be deduced that copper (Cu) is likely not uniformly distributed but rather concentrated within discrete crystalline grains. Consequently, the constrained dimensions of the laser beam may potentially result in the complete absence of these grains, should the beam not intersect with a sufficient number of pigment particles. It is possible that the absence of detectable copper emission at Points 17 and 19 can be explained by the presence of Egyptian blue grains, which may be either absent or not intercepted by the laser during analysis.

4. Discussion

The implementation of the SYSPECTRAL mobile multi-spectroscopic instrument in the analysis of the mural paintings at the Monastery of St. John in Müstair has yielded significant results, underscoring both the capabilities and current limitations of this approach.
The depth-resolved LIBS measurements were particularly effective in identifying stratigraphic structures, revealing sequences of pigment application and alterations across different historical periods. For example, the detection of lead and iron in various configurations suggested the use of both original and restoration materials, while the identification of copper at Point 18 confirmed the presence of Egyptian blue, an ancient pigment with diagnostic spectral features. White areas provided clear signs of overpainting and degradation, particularly the whitening of lead-based pigments like Minium.
Crucially, the mobile nature of SYSPECTRAL allowed these complex analyses to be performed directly on-site, at elevations and on uneven surfaces, without requiring sample removal. This represents a major advancement in heritage science, bringing laboratory-grade analytical capacity into the field.
However, the results also highlight limitations inherent to this configuration. While the combination of multiple spectroscopic techniques in a single mobile setup is innovative, not all techniques performed equally well under field conditions. Raman spectroscopy, in particular, was not effective in this implementation due to strong background fluorescence from the calcite. The simultaneous operation of all techniques proved challenging, especially when targeting micro-heterogeneous materials like Egyptian blue, where pigment grains are sparse and unevenly distributed. The limited beam size also risked missing key features unless carefully aligned.
In summary, SYSPECTRAL has proven to be a valuable tool for mobile, multi-layer material analysis in cultural heritage contexts, offering a robust LIBS-driven platform for elemental mapping. While the integration of additional techniques shows promise, further refinement is needed to optimize their use under real-world constraints. This study provides a foundation for future improvements in mobile multi-spectroscopic instrumentation and demonstrates the practical potential of bringing advanced analytics into conservation environments.

5. Conclusions

The implementation of the SYSPECTRAL mobile multi-spectroscopic instrument in the analysis of the mural paintings at the Monastery of St. John in Müstair has yielded significant results, underscoring both the capabilities and current limitations of this approach. The LIBS stratigraphic analysis revealed significant insights into the materials and degradation processes of the historic wall paintings. The stratigraphic analysis of black and red hues indicated the presence of lead- and iron-based pigments, with some areas exhibiting signs of blackening due to alterations in lead pigments. The identification of titanium in white hues suggested traces of previous restorations. Furthermore, the presence of Egyptian blue, as indicated by the presence of copper, confirmed its historical use in the paintings, a conclusion further supported by microscopic observations of its crystalline structure. This study demonstrates the merits of SYSPECTRAL, which provides significant data for conservation strategies while minimizing impact on the artwork.
In forthcoming studies, the potential integration of LIBS spectral data with reflectance spectroscopy will also be explored, with the aim of extracting more comprehensive information regarding the composition and properties of materials. Future research will focus on broadening the application of this multi-spectroscopic approach to additional heritage sites, incorporating other integrated spectroscopic techniques, and improving spectral databases to facilitate more accurate material identification.

Author Contributions

Conceptualization, P.C and V.D.; methodology, X.B. and V.D.; software, X.B.; validation, X.B., D.B., D.S., Y.Y. and V.D.; formal analysis, X.B., D.S., Y.Y. and V.D.; investigation, X.B., P.C., D.B., and V.D.; resources, X.B., P.C., A.B., and V.D.; data curation, X.B.; writing—original draft preparation, X.B. and V.D.; writing—review and editing, all authors; visualization, X.B., D.S., and V.D.; supervision, P.C. and V.D.; project administration, V.D.; funding acquisition, X.B., D.S., Y.Y. and V.D. All authors have read and agreed to the published version of the manuscript.

Funding

This work was achieved with support through the IPERION-HS research activities funded by the European Commission (H2020-IN-FRAIA-2019-1, Grant No. 871034). This work was also supported by state aid, as managed by the Agence Nationale de la Recherche (French National Research Agency) under the Investment for the Future Program: ESR ESPADON-PATRIMEX (ANR-21-ESRE-00050), as well as under the Investment for the Future Program integrated into France 2030. Under reference ANR-17-EURE-0021, this study also received support from Ecole Universitaire de Recherche Paris Seine–Foundation for Cultural Heritage Sciences. The authors are grateful for the support from the project 2024 “China-French Scientific Research Partnership Exchange Program” PHC CAI YUANPEI 2024 (No. 51127QA) and from the National Natural Science Foundation of China (No. 12474411 and 12204103).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Dataset available on request from the authors.

Acknowledgments

The authors would like to thank the staff of CNRS UAR 3506 Laboratoire de développement instrumental et de méthodologies innovantes pour les Biens Culturels (Lab-BC, former FR3506 NewAGLAE), especially the director, Ina Reiche, for the project and financial management.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
IPERION-CHIntegrated Platform for the European Research Infrastructure ON Cultural Heritage
IPERION-HSIntegrated Platform for the European Research Infrastructure ON Heritage Science
LIBSLaser-Induced Breakdown Spectroscopy
SYSPECTRALMulti-Spectroscopic Characterization System for the Analysis of Cultural Heritage Materials
MOLABMobile Laboratory

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Figure 1. Photographic images of the on-site positioning of SYSPECTRAL mounted on scaffolding to access elevated mural surfaces. (left: the scaffolding, right: zoom in on the position of the SYSPECTRAL at the top of the scaffolding).
Figure 1. Photographic images of the on-site positioning of SYSPECTRAL mounted on scaffolding to access elevated mural surfaces. (left: the scaffolding, right: zoom in on the position of the SYSPECTRAL at the top of the scaffolding).
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Figure 2. Optical path of the mobile SYSPECTRAL. M1–M3: mirrors.
Figure 2. Optical path of the mobile SYSPECTRAL. M1–M3: mirrors.
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Figure 3. Analysed by SYSPECTRAL on different hues: black (1–9), red (10–12), white (13–16) and blue (17–19).
Figure 3. Analysed by SYSPECTRAL on different hues: black (1–9), red (10–12), white (13–16) and blue (17–19).
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Figure 4. (a) Typical spectra as a function of laser shot number from Point 7. (b) Emission intensity as a function of the depth profile of the LIBS spectra.
Figure 4. (a) Typical spectra as a function of laser shot number from Point 7. (b) Emission intensity as a function of the depth profile of the LIBS spectra.
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Figure 5. Emission intensity as a function of the depth profile of the LIBS spectra from Point 7 for the spectral ranges of 2001–357 nm (a) and 3561–559 nm (b); the numbers in the circles show different layers in depth. (c) The averaged spectra showing the emissions of C, Si, Pb and AlO from different layers.
Figure 5. Emission intensity as a function of the depth profile of the LIBS spectra from Point 7 for the spectral ranges of 2001–357 nm (a) and 3561–559 nm (b); the numbers in the circles show different layers in depth. (c) The averaged spectra showing the emissions of C, Si, Pb and AlO from different layers.
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Figure 6. Emission intensity as a function of the depth profile of the LIBS spectra on three spectral ranges from 200 to 357 nm from Point 1. The white arrows point out the position in depth with a lower global emission intensity.
Figure 6. Emission intensity as a function of the depth profile of the LIBS spectra on three spectral ranges from 200 to 357 nm from Point 1. The white arrows point out the position in depth with a lower global emission intensity.
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Figure 7. (a1–a3) Emission intensity as a function of the depth profile of the LIBS spectra for the spectral range of 356–559 nm from the analysed points, Point 17, 18 and 19. (b) The averaged spectra from the second layer zoomed for Cu emissions for Point 17, 18 and 19. (c) Microscopic image of Point 18.
Figure 7. (a1–a3) Emission intensity as a function of the depth profile of the LIBS spectra for the spectral range of 356–559 nm from the analysed points, Point 17, 18 and 19. (b) The averaged spectra from the second layer zoomed for Cu emissions for Point 17, 18 and 19. (c) Microscopic image of Point 18.
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Table 1. Main elements presented at different depths for all analyzed points of black (Point 1–9) and red (Point 10–12) hues.
Table 1. Main elements presented at different depths for all analyzed points of black (Point 1–9) and red (Point 10–12) hues.
Analyzed Point1st Layer2nd Layer3rd Layer4th Layer
1Pb, Fe, AlPb, Fe, AlPb, Fe, AlPb, Fe, Al
2SiCaO, Ca, AlCaO, Ca, AlCaO, Ca, Al
3Ti, FeTi, FeTi, Fe, CaOSi, Ti, Al, CaO
4Pb, FePb, FeCa, AlCa, Al
5TiFe, CaO, Al
6PbFe, Al
7Ti, Pb, FePb, Fe
8Fe, PbFe, PbPb, CaOPb, CaO
9TiTi, FeSiAl, Si
10Ti, Fe, PbTi, FeCaOFe
11Ti, FeTi, FeTi (+), FeFe
12Al, FeAl, FeAl, FeAl, Fe
Table 2. Main elements presented at different depths for all analyzed points of white hues and photographic images of these positions.
Table 2. Main elements presented at different depths for all analyzed points of white hues and photographic images of these positions.
Analyzed PointPhotonics 12 00489 i001Photonics 12 00489 i002Photonics 12 00489 i003Photonics 12 00489 i004
1st LayerTi, Al, SiTi, Pb, AlTi, Al, SiTi, Si, Al
2nd LayerTi, Fe, Al, SiFe, Pb, AlFe, Al, SiSi, Al
3rd LayerAl, SiFe, Pb, Al
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MDPI and ACS Style

Bai, X.; Cassitti, P.; Brebant, A.; Brissaud, D.; Sun, D.; Yin, Y.; Detalle, V. Investigating the Material Composition and Degradation of Wall Paintings at Müstair Monastery Using a Mobile Multi-Spectroscopic System. Photonics 2025, 12, 489. https://doi.org/10.3390/photonics12050489

AMA Style

Bai X, Cassitti P, Brebant A, Brissaud D, Sun D, Yin Y, Detalle V. Investigating the Material Composition and Degradation of Wall Paintings at Müstair Monastery Using a Mobile Multi-Spectroscopic System. Photonics. 2025; 12(5):489. https://doi.org/10.3390/photonics12050489

Chicago/Turabian Style

Bai, Xueshi, Patrick Cassitti, Aude Brebant, Didier Brissaud, Duixiong Sun, Yaopeng Yin, and Vincent Detalle. 2025. "Investigating the Material Composition and Degradation of Wall Paintings at Müstair Monastery Using a Mobile Multi-Spectroscopic System" Photonics 12, no. 5: 489. https://doi.org/10.3390/photonics12050489

APA Style

Bai, X., Cassitti, P., Brebant, A., Brissaud, D., Sun, D., Yin, Y., & Detalle, V. (2025). Investigating the Material Composition and Degradation of Wall Paintings at Müstair Monastery Using a Mobile Multi-Spectroscopic System. Photonics, 12(5), 489. https://doi.org/10.3390/photonics12050489

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