Spectroscopic Techniques for Identifying Pigments in Polychrome Cultural Relics

Abstract

This article explores recent advancements in non-destructive spectroscopic techniques for identifying pigments in polychrome cultural relics, emphasizing their significance for sustainability in cultural heritage preservation. Techniques such as infrared spectroscopy, Raman spectroscopy, and X-ray fluorescence spectroscopy facilitate the accurate identification of pigment compositions while ensuring the integrity of the artifacts is maintained. Our findings indicate that integrating multiple spectroscopic methods enhances the accuracy of pigment identification and deepens our understanding of the structural and preservation status of historical artifacts. We also outline future directions for spectroscopic analysis in the field of cultural heritage, including micro-area analysis, data fusion, and intelligent data processing, aimed at improving the efficiency and effectiveness of pigment identification, ultimately contributing to the sustainable preservation of cultural assets.

1. Introduction

The pigments employed in painted artifacts are pivotal for understanding polychrome cultural relics. The types, sources, production techniques, and application methods of pigments directly influence the overall artistic effect and preservation state of the artifacts [1]. By conducting an in-depth analysis of these pigments, researchers can uncover the technological advancements, trade networks, and cultural exchanges underlying the creation of these relics [2]. For instance, certain pigments sourced from distant regions may illustrate ancient trade routes [3]. Hence, identifying the pigment composition is crucial for deciphering the manufacturing techniques and historical context of these artifacts while also offering a scientific foundation for their preservation and restoration [4].

Accurate identification of pigment composition is vital in both the study and preservation of artifacts. Over time, pigments may alter or degrade, leading to color distortion or fading, which complicates restoration efforts and makes it challenging to recover the artifact’s original appearance [5]. Precise pigment analysis allows researchers to ascertain the true colors used in the artifact’s creation, providing a basis for scientific restoration and aiding in returning the artifact to its original state [6]. Additionally, this analysis supports preventive measures to protect artifacts from further damage [7].

Spectroscopy is a valuable analytical method in the preservation of cultural heritage [8]. Traditional pigment analysis often requires sampling, which can damage artifacts and may not fully represent the pigment composition due to sampling limitations [9]. Spectroscopy, by examining the interaction between matter and electromagnetic radiation, enables detailed analysis of pigment composition, structure, and distribution without direct contact with the sample [10]. The primary advantage of this technique lies in its ability to accurately identify the chemical composition of pigments, which is essential for the scientific and safe preservation of cultural heritage [11]. However, it is important to note that bombarding a sample with electromagnetic radiation is not entirely non-destructive, as it may impact certain properties of the sample, such as its luminescence signal [12]. Despite this, spectroscopy remains a valuable tool for non-invasive analysis when used carefully.

With continuous technological advancements, the application of spectroscopy in pigment identification has become increasingly prevalent and sophisticated [13]. Current spectroscopic techniques include, but are not limited to, infrared spectroscopy, Raman spectroscopy, and X-ray fluorescence spectroscopy, each offering unique advantages [14]. For example, infrared spectroscopy is effective in detecting organic components in pigments, while X-ray fluorescence spectroscopy excels in analyzing inorganic pigments [15]. By combining multiple spectroscopic techniques, researchers can obtain a comprehensive understanding of the complex composition and distribution of pigments in artifacts, providing robust data support for further historical research and preservation efforts [16].

2. Overview of Spectroscopic Techniques

Spectral analysis technology leverages the unique characteristics of the absorption, reflection, emission, or scattering spectra of various chemical substances to determine their properties, structure, or content (Figure 1) [17]. Based on these spectral characteristics, spectral analysis can be divided into absorption spectroscopy, scattering spectroscopy, emission spectroscopy, and reflectance spectroscopy. As a scientific study of the interactions between matter and electromagnetic radiation, spectroscopy has become an indispensable tool in the preservation and analysis of cultural heritage. This non-invasive, highly sensitive technique allows for the detailed examination of the composition, structure, and physic-chemical properties of artifacts, making it invaluable to both researchers and conservators.

1. Absorption Spectroscopy: When light passes through a substance, specific wavelengths are absorbed, creating distinct absorption bands. By analyzing these bands, scientists can gain insights into the composition and structure of the substance. For instance, ultraviolet–visible (UV-Vis) absorption spectroscopy is widely used to investigate electronic transitions in molecules, particularly those with conjugated double bonds [18]. A prominent example is the UV-Vis spectrum of beta-carotene, a carotenoid pigment, which shows strong absorption in the visible region due to electronic transitions in its conjugated π electrons. This absorption accounts for the pigment’s characteristic orange color [19].

2. Scattering Spectroscopy: This technique relies on the scattering of light by matter, such as in Raman scattering, to provide molecular vibrational information. In Raman spectroscopy, incident photons undergo inelastic collisions with matter, resulting in energy shifts that correspond to molecular vibrational modes. These vibrational modes are directly related to the molecular structure and composition of pigments, enabling the identification of specific chemical bonds and crystalline structures [20]. For example, Raman spectroscopy can distinguish between different forms of carbon-based pigments, such as graphite and amorphous carbon, or detect specific mineral components in pigments like cinnabar (HgS) or hematite (Fe₂O₃) [21].

3. Emission Spectroscopy: When a substance absorbs energy, it can emit light as it returns to a lower energy state, producing a characteristic emission spectrum. Emission spectroscopy is crucial for elemental analysis. For example, in X-ray fluorescence spectroscopy (XRF), high-energy X-rays excite a substance, causing it to emit photons at specific energies. These emitted photons are characteristic of the elements present in the sample and can be used to identify and quantify the elements, such as detecting metals like copper or lead in archeological artifacts [22].

4. Reflectance Spectroscopy: The reflective properties of light provide detailed information about a substance’s surface and structure. Spectral reflectance analysis is particularly useful for studying surface coatings and optical properties of pigments. By measuring how light is reflected at different wavelengths, it is possible to analyze the composition and condition of pigment layers, helping to identify ancient pigments or detect degradation in artworks and archeological artifacts [23].

5. A method that combines all four of these techniques—absorption, transmission, reflection, and attenuated total reflection (ATR)—is Fourier Transform Infrared (FTIR) Spectroscopy. The choice of technique depends on the instrument configuration and sample preparation [24]. FTIR is a powerful tool for identifying and characterizing both organic and inorganic materials by measuring how they absorb infrared light at different wavelengths. In absorption mode, FTIR measures how the sample absorbs infrared light as it passes through, providing detailed information about its molecular structure and functional groups. Transmission mode involves passing infrared light through thinner samples or those with minimal surface coatings, allowing for the analysis of transmitted signals. In reflection mode, FTIR examines how infrared light is reflected from the surface of the material, offering valuable insights into surface composition. Attenuated Total Reflection (ATR) is particularly useful for analyzing the surface of solid samples without extensive preparation, as it measures the interaction of infrared light with the sample’s surface at the interface. The versatility of FTIR across these modes makes it especially valuable for analyzing cultural heritage objects, ranging from pigments to varnishes and coatings. The selection of the appropriate mode depends on the nature of the sample, with ATR being particularly effective for non-invasive analysis of delicate and layered artifacts [25]. However, it is crucial to consider the specific characteristics and limitations of each mode, as they can influence the interpretation of results, especially when working with heterogeneous or highly degraded materials.

The advancement of spectroscopic techniques allows researchers to study the chemical composition and physical properties of various materials with high sensitivity and without damaging the samples. The combined use of different spectroscopic methods further enhances the ability to analyze complex samples. Spectroscopy, in particular, has become a crucial tool for uncovering history and interpreting cultural artifacts.

3. Applications of Spectroscopy in Cultural Heritage Preservation

Spectroscopy has been successfully applied in numerous cultural heritage preservation projects worldwide. These cases not only demonstrate the powerful capabilities of spectroscopy but also highlight its unique advantages in the analysis and preservation of cultural artifacts. Pigments are a crucial component of painted artifacts. Spectroscopic techniques in pigment identification primarily focus on elemental analysis, structural analysis, and micro-area morphology analysis.

3.1. Elemental Analysis

3.1.1. X-Ray Fluorescence Spectroscopy (XRF)

Many of the materials used in ancient painted artifacts are mineral-based pigments, often containing metallic elements. XRF is a well-established elemental analysis technique capable of detecting most elements in the periodic table [26]. When the atoms in a material absorb X-rays, they emit the absorbed energy as secondary X-rays (also known as X-ray fluorescence). Due to differences in internal atomic structure, each element emits X-rays at unique wavelengths, enabling qualitative analysis of the material’s elemental composition [27]. The intensity of the X-ray fluorescence is related to the element’s concentration, forming the basis for quantitative analysis. XRF can be categorized into Wavelength Dispersive XRF (WDXRF) and Energy Dispersive XRF (EDXRF), depending on the detection method used. WDXRF offers higher resolution and better precision for detecting trace elements, making it ideal for analyzing complex materials [28]. However, it generally requires more expensive equipment and longer measurement times. In contrast, EDXRF is faster and more cost-effective, suitable for bulk elemental analysis, but it has lower resolution and sensitivity compared to WDXRF [29]. Both methods can analyze materials to a depth of several micrometers with minimal damage to the sample. However, it has limitations, such as its inability to analyze material structures and its reduced effectiveness in detecting elements with atomic numbers lower than sodium.

For instance, XRF analysis of Nasca ceramics not only identified the major elements in the pigments but also revealed the presence of iron oxides in the red, orange, and gray pigments [30]. A study by Han and colleagues, which combined XRF with X-ray diffraction analysis, uncovered the widespread use of the synthetic green pigment Paris Green (Cu(CH₃COO)₂·3Cu(AsO₂)₂) in ancient architectural paintings across Beijing, Shanxi, and Gansu, China [31]. Interestingly, the study did not detect the commonly used ancient green pigments, malachite, and atacamite.

Today, XRF, when coupled with Raman spectroscopy, is frequently employed in pigment analysis [32], and principal component analysis of combined data offers more reliable results than individual analyses [33]. For instance, Castro used portable XRF and Raman spectroscopy to study the degradation mechanism of green pigment malachite on ancient artifacts, discovering the formation of the rare compound copper oxalate [34]. Using MEDUSA software for computer simulation, it was demonstrated that copper oxalate forms from malachite degradation via intermediate sulfate and chloride compounds, with microorganisms such as lichens playing a crucial role in the degradation process [35]. Vandenabeele employed XRF, Raman, and other techniques to analyze wall paintings in the Tomb of Menna in Egypt, marking the first extensive in situ non-destructive investigation of cultural heritage within Egypt. This study laid the groundwork for understanding the materials and techniques used in wall paintings during the reigns of Thutmose IV to Amenhotep III [36].

3.1.2. Laser-Induced Breakdown Spectroscopy (LIBS)

LIBS is an analytical technique based on atomic emission spectroscopy and laser-induced plasma emission. The technique involves using high-energy pulsed lasers to ablate the sample surface, creating a plasma that emits characteristic spectra containing elemental information, which can be used for both qualitative and quantitative analysis [37]. Compared to other elemental analysis methods, LIBS allows for comprehensive elemental analysis and real-time rapid detection of samples and requires minimal sample preparation and small sample sizes. The sampling area ranges from tens to hundreds of micrometers, causing minimal damage to artifacts. LIBS is now widely applied in the trace elemental analysis of materials [38]. For instance, Osticioli combined LIBS with Raman spectroscopy to differentiate between natural lapis lazuli and synthetic ultramarine blue pigments, finding that the calcium content in natural lapis lazuli was significantly higher than in the synthetic counterpart [39].

3.2. Structural Analysis

3.2.1. Fiber Optics Reflectance Spectroscopy (FORS)

The principle of FORS involves using optical fibers to direct light onto a sample and measuring the reflected light to determine the pigment composition and structural features [40]. FORS is highly user-friendly and requires no sample preparation, making it a truly non-destructive method for analyzing cultural heritage artifacts. While it is particularly effective for analyzing the pigment composition, it can also provide information on the surface structure of the sample, such as identifying variations in surface coatings or detecting changes in texture. However, FORS typically requires a large analysis area, and it struggles with accurately analyzing mixed pigments [41]. Aceto’s research using FORS on simulated samples provided empirical conclusions: blue and green pigments can be identified by their reflectance peaks, while yellow, red, and pink pigments are more easily detected by the slope of their first derivative peaks [42]. This reduces the workload for subsequent analyses. The research team used FORS to study the color change behavior of lead-based pigments under different humidity conditions and concluded that a dry environment favors the preservation of these pigments, with 55% humidity being the optimal condition for minimal color change [43].

3.2.2. Laser Raman Spectroscopy (Raman Microscopy)

Laser Raman Spectroscopy is a molecular spectroscopy technique based on Raman scattering [44]. When laser photons collide with the molecules of a material, most interactions are elastic, but a small fraction undergoes inelastic scattering, where both the direction and energy of the photons change—this phenomenon is known as Raman scattering, and the resulting spectrum is the Raman spectrum [44]. Different molecular structures produce characteristic Raman shifts, providing detailed information about the material’s microstructure. Raman spectroscopy can detect scattered light in the range of 100–3500 cm⁻¹, with a detection spot size of only a few micrometers and requiring less than 1 µg of pigment for rapid identification without prior sample preparation [45]. The application of fiber optics and microscopy techniques has made portable Raman technology widely applicable for cultural heritage analysis [46]. The use of confocal devices has improved the resolution of Raman measurements, allowing for more detailed analysis of small areas. Additionally, the integration of chemometric methods, such as Principal Component Analysis (PCA), has enhanced the processing and interpretation of Raman data, providing more accurate results in complex samples [47]. For example, Kock used Raman spectroscopy to analyze pigments on a multicolored tile from Algiers. The study found that the yellow pigments (including bright yellow, orange-yellow, and brownish-yellow) were composed of oxides of Pb, Sn, and Sb, with the presence of a peak at 132 cm⁻¹ suggesting the possibility of mixed pigments [48]. In another case, Frausto-Reyes used Raman spectroscopy to study the watercolors of 19th-century Mexican zoologist Alfredo Dugès paintings and discovered the artist’s specific pigment usage patterns, such as the consistent use of lead white, lapis lazuli, carbon black, and chrome yellow [49]. Notably, lead white was used for the eyes of all birds in the paintings, providing crucial evidence for authenticating the artworks [50]. The Raman spectral analysis results of Dugès’ watercolor, where the characteristic peaks of cellulose at 1096 and 1120 cm⁻¹ in Figure 2a correspond to paper fibers, and the strong vibration peak at 1049 cm⁻¹ in Figure 2b indicates the presence of lead white [51].

3.2.3. X-Ray Diffraction (XRD)

Most ancient Chinese pigments are naturally occurring mineral pigments with crystalline structures [52]. The ordered and symmetrical structure of these crystal cells acts like a diffraction grating, diffracting X-rays to reveal the material’s microstructure, such as crystal cell structure, parameters, and defects. XRD is widely used for the qualitative analysis of cultural heritage materials, although it requires a relatively large sample size, with a detection limit of around 2% [53]. Factors such as multi-phase crystals, structural defects, and impurities can significantly impact the results. XRD can also provide insight into the structural characteristics of pigments, such as crystal symmetry and lattice parameters, which are essential for understanding the pigment’s stability and preservation. For example, XRD was used to analyze green pigments from the Wadi El Natrun site in Egypt, revealing structural details of the mixture of basic copper carbonate (malachite) and basic lead carbonate (lead white). At the Petrie Museum of Egyptian Archaeology, University College London, XRD was employed to examine mummy coffins from different periods. The analysis uncovered the structural properties of the green pigment “green earth”, which was widely used in the Middle Ages [54]. This study not only confirmed the extensive use of the pigment during the Greco-Roman period but also provided new structural insights into the methods of pigment creation by mixing yellow and blue pigments, offering physical evidence for archeological research.

3.3. Micro-Area and Morphological Analysis

The microscopic structure and morphological characteristics of pigments are intricately linked to their chemical composition, crystal structure, and production techniques. Analyzing these microstructures and morphologies provides valuable insights into the manufacturing processes and properties of pigments, offering a deeper understanding of their historical and cultural significance.

Multispectral imaging leverages the principles of broad-spectrum imaging across the visible and non-visible light spectra [55]. This technique involves capturing a series of images of painted artifacts under multiple spectral bands using a specialized camera and controlled illumination. By utilizing light sources such as visible light (400–700 nm), ultraviolet (UV) light (<400 nm), and infrared (IR) light (>700 nm up to 3000 nm), multispectral imaging collects data on how pigments and surfaces interact with different wavelengths of light. The reflected, absorbed, or emitted light provides information about the material’s spectral characteristics. These spectral fingerprints help identify pigments, detect overpainting or retouches, and map degradation patterns that are invisible to the naked eye. For instance, UV light can reveal the fluorescence properties of organic materials, while IR imaging penetrates surface layers to uncover underdrawings or subsurface features. This non-invasive technology offers a deeper, clearer understanding of pigments’ current states, deterioration, composition, and application techniques, often revealing details missed by other methods [56].

For example, the Dunhuang Academy used multispectral imaging to analyze the pigments in the murals of Cave 285, discovering that some red pigments might be organic dyes. Additionally, in UV fluorescence images of some blue pigments, certain areas emitted a blue-white fluorescence while others did not, leading to the conclusion that these blue pigments were mixtures of different particles [57]. Marengo used multispectral imaging to record various spectral images of the Dead Sea Scrolls and applied multivariate analysis to detect subtle aging processes that are invisible to the naked eye, offering a new method for detecting surface aging in artifacts [58]. Infrared reflectography and UV fluorescence photography were used to examine the 12th-century manuscript Liber Floridus, revealing significant information about the underdrawings and pigments, especially showing that compasses were used to draw circles on multiple pages through infrared reflectography [59].

Polarized Light Microscopy (PLM) PLM is a method that analyzes pigments by observing the crystalline morphology, color, impurities, and other features of pigment particles under an optical microscope. This method is characterized by its rapid analysis, cost-effectiveness, and minimal sample requirement (only a few picograms or nanograms), providing clear visual information on the pigment’s morphology [60]. For instance, Trab-ska used PLM to examine the black substance on the surface of a Bronze Age pottery vessel from Denmark, revealing that this opaque layer might have been intentionally applied to create a shiny, leather-like hard surface [61]. PLM was also used to discover traces of Han Blue in the pigments on painted pottery from the Han Dynasty tombs in Linzi, Shandong Province (Figure 3), and Han Purple on painted pottery from the Han Dynasty tombs in Jinan, Shandong Province [62]. Han Blue (BaCuSi₂O₁₀) and Han Purple (BaCuSi₂O₆) are two rare pigments, and their discovery in these regions was the first of its kind, providing significant insights into the production and use of these ancient Chinese pigments.

Scanning Electron Microscopy (SEM): The principle of SEM involves using a high-energy electron beam to bombard the surface of the sample, where elastic and inelastic collisions occur under the influence of atomic Coulomb forces. The resulting secondary electrons and scattered electrons provide information about the material’s microstructure, composition, and morphology [63]. SEM is widely used to observe the microphysical morphology and composition of various artifacts, with the following advantages: high magnification (up to hundreds of thousands of times), high resolution (capable of resolving nanoscale information), and large depth of field. SEM is often combined with energy-dispersive X-ray spectroscopy (EDS or EDX), which allows for the analysis of micro-area elements alongside microstructure information, with a detection limit as low as 0.1% [64]. SEM-EDX analysis of the Thousand Buddha Grottoes in Guangyuan identified a light green pigment layer beneath the black paint on some artifacts. While SEM itself does not detect color, the elemental composition obtained through EDX analysis allowed researchers to infer the presence of a light green pigment [65].

SEM is instrumental in acquiring detailed information on the microstructure of cultural heritage artifacts and understanding their production techniques. For example, Mangone used SEM as the primary method to study painted pottery from several sites in the Apulian region of southern Italy [66]. The analysis revealed that the white, yellow, and reddish-brown pigments shared similar raw materials and production techniques, suggesting that these artifacts were likely made by the same group of artisans. Pascual used SEM to investigate the production techniques of murals in Maya tombs, finding that some green pigments were created by mixing blue and yellow pigments [67]. Moreover, the blue pigments were found to have a surface layer of palygorskite crystals with 2–5 nm sized pits (Figure 4) [68].

3.4. The Synergistic Application of FTIR and Hybrid Spectroscopic Techniques in Cultural Heritage Analysis

Recent advancements in hybrid or combined spectroscopic techniques have significantly enhanced pigment identification in cultural heritage studies, particularly for polychrome artworks. One notable example is the combination of Fourier Transform Infrared (FTIR) Spectroscopy with techniques like Raman Spectroscopy or X-ray Fluorescence (XRF), which improves the accuracy and reliability of pigment analysis. FTIR is a versatile, non-invasive technique that analyzes both organic and inorganic materials by detecting their absorption of infrared light. It can operate in several modes—absorption, transmission, reflection, and Attenuated Total Reflection (ATR)—depending on the sample and analysis needs. ATR-FTIR is particularly effective for delicate or layered artifacts, requiring minimal sample preparation, while transmission mode is ideal for thin samples, and reflection mode works well for reflective surfaces, such as varnishes. However, FTIR alone may struggle with complex or degraded pigments, necessitating the use of complementary techniques.

For instance, Raman Spectroscopy provides detailed molecular vibrational data, distinguishing between different pigment types, such as carbon-based pigments like graphite and amorphous carbon or inorganic pigments like cinnabar (HgS) and hematite (Fe₂O₃). This combination offers a more complete analysis of both molecular and elemental composition. Additionally, FTIR coupled with XRF is effective in analyzing complex pigment mixtures, with FTIR detecting organic pigments and binders and XRF identifying and quantifying metals like lead, copper, or iron. Examples of FTIR applications in pigment identification include the detection of alizarin in madder pigments through characteristic C=O stretching vibrations around 1600 cm⁻¹ [69], the identification of lead white using a carbonate stretching vibration at 1370 cm⁻¹ [70], and the detection of Prussian blue through a distinctive band around 2100 cm⁻¹, linked to C≡N stretching vibrations. FTIR is also crucial for identifying varnishes and binders in artworks, as it detects ester functional groups in natural resin-based coatings [71]. The integration of statistical methods like Principal Component Analysis (PCA) or machine learning algorithms further enhances spectral data analysis, improving pigment identification, especially in complex datasets [72]. This hybrid approach, combining FTIR with Raman Spectroscopy and XRF, provides a comprehensive and reliable means of analyzing historical pigments and materials, advancing conservation and understanding of cultural heritage.

4. Challenges in Using Spectroscopy for Identifying Pigments

Although spectroscopy has greatly advanced the analysis and identification of pigments in painted cultural artifacts, several technical and operational limitations still hinder its full potential and widespread application in heritage conservation. Addressing these challenges is crucial for optimizing the role of spectroscopy in the identification and preservation of historical pigments. Key issues include the following:

1. Limitations in Detection Sensitivity and Precision: While spectroscopy methods, such as infrared and X-ray fluorescence spectroscopy, enable non-contact and non-destructive pigment analysis, they often struggle to precisely detect low-concentration elements or complex organic compounds present in certain pigments. These detection limits can result in partial or incomplete data, especially when dealing with layered or aged pigments. Furthermore, each spectroscopic technique has a unique detection range, which may miss important pigment components or trace elements critical to accurate pigment identification.

2. Surface and Internal Composition Variability: Painted artifacts often exhibit non-uniform pigment distribution due to layering, aging effects, or environmental contamination, requiring analysis at multiple spots to obtain accurate results. However, analyzing multiple locations can be time-consuming and costly, especially when using techniques like Laser-Induced Breakdown Spectroscopy (LIBS), which allows for high-resolution, micro-scale analysis. Even minimal sample damage can risk compromising the integrity of rare or fragile artifacts”.

3. Complexity in Data Analysis and Interpretation: The vast, intricate data produced by spectroscopic methods demand expert interpretation, with advanced algorithms required to ensure accuracy. While machine learning approaches are emerging to streamline data analysis, substantial algorithm development and standardization are still needed to enhance reliability. Additionally, data fusion across multiple spectroscopy methods remains a challenge, making it difficult to establish unified models for pigment identification.

4. Instrument Portability and Cost Constraints: Field analysis in heritage conservation benefits from compact, user-friendly equipment; however, high-resolution spectroscopic instruments are often bulky, expensive, and difficult to use outside controlled laboratory environments. Although portable devices have been introduced, they typically lack the sensitivity and resolution of laboratory-grade equipment, which can limit the accuracy and scope of in situ pigment analysis.

5. Future Directions in Spectroscopic Analysis

As technology progresses, spectroscopy’s applications in analyzing pigments in cultural heritage artifacts have great potential to expand. Key future directions include:

1. Development of High-Sensitivity, Non-Destructive Micro-Area Analysis Techniques: Enhancing high-sensitivity spectroscopic methods for micro-area analysis, such as Surface-Enhanced Raman Spectroscopy (SERS) and Soft X-ray Spectroscopy (STXM), will improve the ability to detect minute pigment variations at a high spatial resolution. These methods promise to support more comprehensive analyses of pigments, especially for rare elements or components within complex mixtures, furthering the understanding of historical artifact composition.

2. Integration of Multiple Techniques and Data Fusion: Integrating a variety of spectroscopy techniques, including X-ray fluorescence, LIBS, and Raman spectroscopy, can provide a more complete analysis of pigments in painted artifacts. Moreover, advancements in data fusion techniques could enhance the accuracy and reliability of pigment identification by combining complementary datasets across these methods, leading to more holistic insights into artifact composition.

3. Intelligent Data Processing and Automated Identification: Artificial intelligence (AI) and machine learning algorithms can significantly improve the efficiency of spectroscopic data analysis by automating the interpretation of complex datasets. Future research could focus on developing AI-based software for pigment classification, reducing reliance on manual data interpretation. This automation would streamline the analysis process and provide consistent, accurate results.

4. Design of Lightweight, Portable Field Spectroscopy Devices: Advances in miniaturization and sensitivity optimization can enable the development of highly portable, user-friendly spectroscopic instruments for field use. Enhanced portability will facilitate on-site analyses, allowing conservationists to assess pigments without transferring fragile artifacts to laboratory settings, thereby reducing handling risks and speeding up conservation processes.

5. Establishment of Standardized Pigment Databases: Creating extensive reference databases that catalog pigment spectra across different historical periods, geographic regions, and artistic methods would provide invaluable resources for future pigment identification efforts. Such databases would serve as comparative tools, enabling researchers to quickly and accurately identify pigment compositions, streamline conservation processes, and provide a standardized framework for data sharing within the cultural heritage community.

6. Conclusions

Spectroscopy has shown remarkable promise in pigment identification for painted artifacts, providing a non-invasive means to uncover details about historical materials. As non-destructive micro-analysis, multi-technology integration, and data processing technology advance, spectroscopic analysis will continue to play an increasingly vital role in heritage conservation. By addressing current challenges related to precision, portability, and data processing, spectroscopy can achieve broader applications in pigment analysis, fostering deeper insights into historical artifacts and supporting the preservation of cultural heritage for future generations.

Author Contributions

Conceptualization, M.L. and Z.W.; methodology, Z.W.; software, M.L.; validation, X.L.; formal analysis, X.L.; investigation, X.L.; writing—original draft preparation, M.L.; writing—review and editing, Z.W. and X.L. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 1. Schematic diagram of spectroscopic techniques classification.

Figure 2. Raman spectra of white zones (a) paper and (b) lead white pigment [49].

Figure 3. Micrograph obtained using PLM showing very small Chinese blue pigment particles (at the location of the arrows) surrounded by red pigment particles [62].

Figure 4. SEM image showing palygorskite crystals from Maya Blue pigment [62].

References

1. Riu, J.; Giussani, B. Analytical Chemistry Meets Art: The Transformative Role of Chemometrics in Cultural Heritage Preservation. Chemom. Intell. Lab. Syst.; 2024; 247, 105095. [DOI: https://dx.doi.org/10.1016/j.chemolab.2024.105095]

2. Harth, A. X-ray Fluorescence (XRF) on Painted Heritage Objects: A Review Using Topic Modeling. Heritage; 2024; 12, 17. [DOI: https://dx.doi.org/10.1186/s40494-024-01135-2]

3. Nabais, P.; Brøns, C.; Wozniak, M.M. Investigating Organic Colorants Across Time: Interdisciplinary Insights into the Use of Madder, Indigo/Woad, and Weld in Historical Written Sources, Archaeological Textiles, and Ancient Polychromy. Textile Crossroads: Exploring European Clothing, Identity, and Culture Across Millennia. Anthology of COST Action “CA 19131–EuroWeb”; University of Nebraska: Lincoln, NE, USA, 2024.

4. Tamburini, D.; Sabatini, F.; Berbers, S.; van Bommel, M.R.; Degano, I. An Introduction and Recent Advances in the Analytical Study of Early Synthetic Dyes and Organic Pigments in Cultural Heritage. Heritage; 2024; 7, pp. 1969-2010. [DOI: https://dx.doi.org/10.3390/heritage7040094]

5. Angelin, E.M. The Fate of Colors in the 20th–21st Centuries: Preserving the Organic Colorants in Plastic Artifacts. Ph.D. Dissertation; Universidade NOVA de Lisboa: Lisbon, Portugal, 2021.

6. Faber, K.T.; Casadio, F.; Masic, A.; Robbiola, L.; Walton, M. Looking Back, Looking Forward: Materials Science in Art, Archaeology, and Art Conservation. Annu. Rev. Mater. Res.; 2021; 51, pp. 435-460. [DOI: https://dx.doi.org/10.1146/annurev-matsci-080819-013103]

7. Lucchi, E. Review of Preventive Conservation in Museum Buildings. J. Cult. Herit.; 2018; 29, pp. 180-193. [DOI: https://dx.doi.org/10.1016/j.culher.2017.09.003]

8. Madariaga, J.M. Analytical Chemistry in the Field of Cultural Heritage. Anal. Methods; 2015; 7, pp. 4848-4876. [DOI: https://dx.doi.org/10.1039/C5AY00072F]

9. Doménech-Carbó, M.T.; Osete-Cortina, L. Another Beauty of Analytical Chemistry: Chemical Analysis of Inorganic Pigments of Art and Archaeological Objects. ChemTexts; 2016; 2, 14. [DOI: https://dx.doi.org/10.1007/s40828-016-0033-5]

10. Ustin, S.L.; Roberts, D.A.; Gamon, J.A.; Asner, G.P.; Green, R.O. Using Imaging Spectroscopy to Study Ecosystem Processes and Properties. BioScience; 2004; 54, pp. 523-534. [DOI: https://dx.doi.org/10.1641/0006-3568(2004)054[0523:UISTSE]2.0.CO;2]

11. Coccato, A.; Caggiani, M.C. An Overview of Principal Components Analysis Approaches in Raman Studies of Cultural Heritage Materials. J. Raman Spectrosc.; 2024; 55, pp. 125-147. [DOI: https://dx.doi.org/10.1002/jrs.6621]

12. Sharma, S.; Kumar, S.; Chauhan, V.S.; Sinapius, M. A Review on Deformation-Induced Electromagnetic Radiation Detection: History and Current Status of the Technique. J. Mater. Sci.; 2021; 56, pp. 4500-4551. [DOI: https://dx.doi.org/10.1007/s10853-020-05538-x]

13. Allakhverdiev, E.S.; Kossalbayev, B.D.; Sadvakasova, A.K.; Bauenova, M.O.; Belkozhayev, A.M.; Rodnenkov, O.V.; Allakhverdiev, S.I. Spectral Insights: Navigating the Frontiers of Biomedical and Microbiological Exploration with Raman Spectroscopy. J. Photochem. Photobiol. B; 2024; 252, 112870. [DOI: https://dx.doi.org/10.1016/j.jphotobiol.2024.112870] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38368635]

14. Dassanayake, R.S.; Dissanayake, N.; Fierro, J.S.; Abidi, N.; Quitevis, E.L.; Boggavarappu, K.; Thalangamaarachchige, V.D. Characterization of cellulose nanocrystals by current spectroscopic techniques. Appl. Spectrosc. Rev.; 2023; 58, pp. 180-205. [DOI: https://dx.doi.org/10.1080/05704928.2021.1951283]

15. Castellá, F.; Pérez-Estebanez, M.; Reinoso, M.; Schilling, M.; Khanjian, H.; Learner, T.; Marte, F. Comprehensive Characterization of Modern Industrial Argentinian Paints for Improved Chronological Painting Attribution. J. Cult. Herit.; 2024; 67, pp. 203-213. [DOI: https://dx.doi.org/10.1016/j.culher.2024.03.001]

16. Shad, R.; Egon, A.; Potter, K. Digital Approaches to Artifact Provenance Studies and Authentication. EasyChair; 2024; 14270.

17. Nemeș, N.S.; Negrea, A. Infrared and Visible Spectroscopy: Fourier Transform Infrared Spectroscopy and Ultraviolet–Visible Spectroscopy. Microb. Electrochem. Technol.; 2023; 1, pp. 163-200.

18. Singh, A.; Ahmad, S.; Ahmad, A. Green Extraction Methods and Environmental Applications of Carotenoids—A Review. RSC Adv.; 2015; 5, pp. 62358-62393. [DOI: https://dx.doi.org/10.1039/C5RA10243J]

19. Fried, A.; Richter, D. Infrared Absorption Spectroscopy. Analytical Techniques for Atmospheric Measurement; Wiley: Oxford, UK, 2006; pp. 72-146.

20. Inaba, H. Detection of Atoms and Molecules by Raman Scattering and Resonance Fluorescence. Laser Monitoring of the Atmosphere; Springer: Berlin/Heidelberg, Germany, 2005; pp. 153-236.

21. Petrova, O.I.; Pankin, D.V.; Povolotckaia, A.V.; Borisov, E.V.; Beznosova, M.O.; Krivul’ko, T.A.; Kurochkin, A.V. Identification of pigments in colored layers of a painting by Raman spectroscopy. Opt. Spectrosc.; 2017; 123, pp. 965-969. [DOI: https://dx.doi.org/10.1134/S0030400X17120128]

22. Janssens, K. X-ray Based Methods of Analysis. Comprehensive Analytical Chemistry; Elsevier: Amsterdam, The Netherlands, 2004; Volume 42, pp. 129-226.

23. Navas, N.; Romero-Pastor, J.; Manzano, E.; Cardell, C. Benefits of applying combined diffuse reflectance FTIR spectroscopy and principal component analysis for the study of blue tempera historical painting. Anal. Chim. Acta; 2008; 630, pp. 141-149. [DOI: https://dx.doi.org/10.1016/j.aca.2008.10.008]

24. Prasher, S.; Kumar, M. Exploring X-Ray Techniques for Comprehensive Material Characterization and Analysis. Next Generation Materials for Sustainable Engineering; IGI Global: Hershey, PA, USA, 2024; pp. 272-296.

25. Romero-Bastidas, M.; Guacho-Pachacama, K.; Vásquez-Mora, C.; Espinoza-Guerra, F.; Díaz-Benalcázar, R.; Ramírez-Bustamante, J.; Ramos-Guerrero, L. Multi-Analytical Characterization of Illuminated Choirbooks from the Royal Audience of Quito. Heritage; 2024; 7, pp. 6592-6613. [DOI: https://dx.doi.org/10.3390/heritage7120305]

26. Vanhoof, C.; Bacon, J.R.; Fittschen, U.E.; Vincze, L. Atomic spectrometry update–a review of advances in X-ray fluorescence spectrometry and its special applications. J. Anal. At. Spectrom.; 2021; 36, pp. 1797-1812. [DOI: https://dx.doi.org/10.1039/D1JA90033A]

27. Haschke, M. Laboratory Micro-X-Ray Fluorescence Spectroscopy; Springer: Berlin/Heidelberg, Germany, 2014; pp. 978-983.

28. West, M.; Ellis, A.T.; Potts, P.J.; Streli, C.; Vanhoof, C.; Wobrauschek, P. 2015 Atomic Spectrometry Update—A Review of Advances in X-ray Fluorescence Spectrometry and Their Applications. J. Anal. At. Spectrom.; 2015; 30, pp. 1839-1889. [DOI: https://dx.doi.org/10.1039/C5JA90033F]

29. Croffie, M.E.; Williams, P.N.; Fenton, O.; Fenelon, A.; Metzger, K.; Daly, K. Optimising Sample Preparation and Calibrations in EDXRF for Quantitative Soil Analysis. Agronomy; 2020; 10, 1309. [DOI: https://dx.doi.org/10.3390/agronomy10091309]

30. Pappalardo, L.; Masini, N.; Rizzo, F.; Romano, F.P. The polychromy of Nasca pottery: A nondestructive analytical approach for compositional and mineralogical investigation of pigments. The Ancient Nasca World: New Insights from Science and Archaeology; Springer: Cham, Switzerland, 2016; pp. 593-603.

31. Huang, D.; Han, K.; Teri, G.; Cheng, C.; Qi, Y.; Li, Y. Material and Microstructure Analysis of Wood Color Paintings from Shaanxi Cangjie Temple, China. Molecules; 2024; 29, 2734. [DOI: https://dx.doi.org/10.3390/molecules29122734]

32. Deneckere, A.; de Vries, L.; Vekemans, B.; Van de Voorde, L.; Ariese, F.; Vincze, L.; Moens, L.; Vandenabeele, P. Identification of inorganic pigments used in porcelain cards based on fusing Raman and X-ray fluorescence (XRF) data. Appl. Spectrosc.; 2011; 65, pp. 1281-1290. [DOI: https://dx.doi.org/10.1366/11-06368]

33. Secchi, M.; Zanatta, M.; Borovin, E.; Bortolotti, M.; Kumar, A.; Giarola, M.; Sanson, A.; Orberger, B.; Daldosso, N.; Gialanella, S. et al. Mineralogical investigations using XRD, XRF, and Raman spectroscopy in a combined approach. J. Raman Spectrosc.; 2018; 49, pp. 1023-1030. [DOI: https://dx.doi.org/10.1002/jrs.5386]

34. de Castro, M.D.L.; Jurado-López, A. The Role of Analytical Chemists in the Research on Cultural Heritage. Talanta; 2019; 205, 120106. [DOI: https://dx.doi.org/10.1016/j.talanta.2019.07.001]

35. Castro, K.; Sarmiento, A.; Martínez-Arkarazo, I.; Madariaga, J.M.; Fernández, L.A. Green copper pigments biodegradation in cultural heritage: From malachite to moolooite, thermodynamic modeling, X-ray fluorescence, and Raman evidence. Anal. Chem.; 2008; 80, pp. 4103-4110. [DOI: https://dx.doi.org/10.1021/ac800255w]

36. Vandenabeele, P.; Garcia-Moreno, R.; Mathis, F.; Leterme, K.; Van Elslande, E.; Hocquet, F.; Rakkaa, S.; Laboury, D.; Moens, L.; Strivay, D. et al. Multi-disciplinary investigation of the tomb of Menna (TT69), Theban Necropolis, Egypt. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2009; 73, pp. 546-552. [DOI: https://dx.doi.org/10.1016/j.saa.2008.07.028]

37. Fortes, F.J.; Moros, J.; Lucena, P.; Cabalín, L.M.; Laserna, J.J. Laser-induced breakdown spectroscopy. Anal. Chem.; 2013; 85, pp. 640-669. [DOI: https://dx.doi.org/10.1021/ac303220r]

38. Carter, S.; Fisher, A.; Garcia, R.; Gibson, B.; Marshall, J.; Whiteside, I. Atomic spectrometry update: Review of advances in the analysis of metals, chemicals and functional materials. J. Anal. At. Spectrom.; 2016; 31, pp. 2114-2164. [DOI: https://dx.doi.org/10.1039/C6JA90044E]

39. Osticioli, I.; Mendes, N.F.C.; Porcinai, S.; Cagnini, A.; Castellucci, E.M. Spectroscopic analysis of works of art using a single LIBS and pulsed Raman setup. Anal. Bioanal. Chem.; 2009; 394, pp. 1033-1041. [DOI: https://dx.doi.org/10.1007/s00216-009-2653-8] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/19214480]

40. Kortüm, G. Reflectance Spectroscopy: Principles, Methods, Applications; Springer Science & Business Media: Berlin/Heidelberg, Germany, 2012.

41. Lauwers, D. Characterisation and Optimisation of Mobile Raman Spectroscopy for Art Analysis. Ph.D. Thesis; Ghent University: Ghent, Belgium, 2017.

42. Aceto, M.; Agostino, A.; Fenoglio, G.; Idone, A.; Gulmini, M.; Picollo, M.; Ricciardi, P.; Delaney, J.K. Characterisation of colourants on illuminated manuscripts by portable fibre optic UV-visible-NIR reflectance spectrophotometry. Anal. Methods; 2014; 6, pp. 1488-1500. [DOI: https://dx.doi.org/10.1039/c3ay41904e]

43. Wang, L.; Dang, G.; Zheng, L.; Wang, X. Studies on the identification of pigments on relics and the analysis of color changes by fiber optics reflectance spectroscopy. Anal. Lett.; 2001; 34, pp. 2403-2414. [DOI: https://dx.doi.org/10.1081/AL-100107304]

44. Keresztury, G.; Chalmers, J.M.; Griffith, P.R. Raman Spectroscopy: Theory. Handbook of Vibrational Spectroscopy; Wiley: Oxford, UK, 2002; Volume 1, pp. 71-87.

45. Schulte, F.; Brzezinka, K.W.; Lutzenberger, K.; Stege, H.; Panne, U. Raman spectroscopy of synthetic organic pigments used in 20th century works of art. J. Raman Spectrosc.; 2008; 39, pp. 1455-1463. [DOI: https://dx.doi.org/10.1002/jrs.2021]

46. Rousaki, A.; Vandenabeele, P. In Situ Raman Spectroscopy for Cultural Heritage Studies. J. Raman Spectrosc.; 2021; 52, pp. 2178-2189. [DOI: https://dx.doi.org/10.1002/jrs.6166]

47. Shinzawa, H.; Awa, K.; Kanematsu, W.; Ozaki, Y. Multivariate data analysis for Raman spectroscopic imaging. J. Raman Spectrosc.; 2009; 40, pp. 1720-1725. [DOI: https://dx.doi.org/10.1002/jrs.2525]

48. Kock, L.D.; De Waal, D. Raman Analysis of Ancient Pigments on a Tile from the Citadel of Algiers. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2008; 71, pp. 1348-1354. [DOI: https://dx.doi.org/10.1016/j.saa.2008.04.024]

49. Frausto-Reyes, C.; Ortiz-Morales, M.; Bujdud-Pérez, J.M.; Magaña-Cota, G.E.; Mejía-Falcón, R. Raman spectroscopy for the identification of pigments and color measurement in Dugès watercolors. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2009; 74, pp. 1275-1279. [DOI: https://dx.doi.org/10.1016/j.saa.2009.09.060]

50. D’Imporzano, P.; Keune, K.; Koornneef, J.M.; Hermens, E.; Noble, P.; Vandivere, A.L.S.; Davies, G.R. Time-dependent variation of lead isotopes of lead white in 17th century Dutch paintings. Sci. Adv.; 2021; 7, eabi5905. [DOI: https://dx.doi.org/10.1126/sciadv.abi5905]

51. Oil and Colour Chemists’ Association Broad, R.; Power, G.; Sonego, A. Extender pigments. In Surface Coatings: Volume 1 Raw Materials and Their Usage; Springer: Berlin/Heidelberg, Germany, 1993; pp. 514-529.

52. Zhao, C.; Zhang, Y.; Wang, C.C.; Hou, M.; Li, A. Recent progress in instrumental techniques for architectural heritage materials. Herit. Sci.; 2019; 7, 36. [DOI: https://dx.doi.org/10.1186/s40494-019-0280-z]

53. Scott, D.A.; Warmlander, S.; Mazurek, J.; Quirke, S. Examination of some pigments, grounds and media from Egyptian cartonnage fragments in the Petrie Museum, University College London. J. Archaeol. Sci.; 2009; 36, pp. 923-932. [DOI: https://dx.doi.org/10.1016/j.jas.2008.12.011]

54. Tran, C.D. Infrared Multispectral Imaging: Principles and Instrumentation. Applied Spectroscopy Reviews; Taylor & Francis: New York, NY, USA, 2003.

55. Sikander, B. Analysis of Multispectral Imaging of Illuminated Medieval Manuscripts. Ph.D. Thesis; The University of Manchester: Manchester, UK, 2024.

56. Sun, M.; Zhang, D.; Wang, Z.; Ren, J.; Chai, B.; Sun, J. What’s wrong with the murals at the Mogao Grottoes: A near-infrared hyperspectral imaging method. Sci. Rep.; 2015; 5, 14371. [DOI: https://dx.doi.org/10.1038/srep14371] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26394926]

57. Marengo, E.; Manfredi, M.; Zerbinati, O.; Robotti, E.; Mazzucco, E.; Gosetti, F.; Bearman, G.; France, F.; Shor, P. Technique based on LED multispectral imaging and multivariate analysis for monitoring the conservation state of the dead sea scrolls. Anal. Chem.; 2011; 83, pp. 6609-6618. [DOI: https://dx.doi.org/10.1021/ac201068s]

58. Deneckere, A.; De Reu, M.; Martens, M.P.J.; De Coene, K.; Vekemans, B.; Vincze, L.; De Maeyer, P.; Vandenabeele, P.; Moens, L. The use of a multi-method approach to identify the pigments in the 12th century manuscript Liber Floridus. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2011; 80, pp. 125-132. [DOI: https://dx.doi.org/10.1016/j.saa.2011.03.005]

59. McCrone, W.C. Polarized Light Microscopy in Conservation: A Personal Perspective. J. Am. Inst. Conserv.; 1994; 33, pp. 101-114. [DOI: https://dx.doi.org/10.1179/019713694806124757]

60. Trąbska, J.; Wesełucha-Birczyńska, A.; Zięba-Palus, J.; Runge, M.T. Black painted pottery, Kildehuse II, Odense County, Denmark. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2011; 79, pp. 824-830. [DOI: https://dx.doi.org/10.1016/j.saa.2010.08.068]

61. Xia, Y.; Ma, Q.; Zhang, Z.; Liu, Z.; Feng, J.; Shao, A.; Wang, W.; Fu, Q. Development of Chinese barium copper silicate pigments during the Qin Empire based on Raman and polarized light microscopy studies. J. Archaeol. Sci.; 2014; 49, pp. 500-509. [DOI: https://dx.doi.org/10.1016/j.jas.2014.05.035]

62. Han, W.S.; Wang, L.Q. Application of Spectral Technologies in Analyzing Pigments of Colored Relics. Spectrosc. Spectr. Anal.; 2012; 32, pp. 3394-3398.

63. Ebnesajjad, S. Surface and Material Characterization Techniques. Handbook of Adhesives and Surface Preparation; William Andrew Publishing: Norwich, NY, USA, 2011; pp. 31-48.

64. Schreiner, M.; Melcher, M.; Uhlir, K. Scanning Electron Microscopy and Energy Dispersive Analysis: Applications in the Field of Cultural Heritage. Anal. Bioanal. Chem.; 2007; 387, pp. 737-747. [DOI: https://dx.doi.org/10.1007/s00216-006-0718-5]

65. He, L.; Wang, N.; Zhao, X.; Zhou, T.; Xia, Y.; Liang, J.; Rong, B. Polychromic structures and pigments in Guangyuan Thousand-Buddha Grotto of the Tang Dynasty (China). J. Archaeol. Sci.; 2012; 39, pp. 1809-1820. [DOI: https://dx.doi.org/10.1016/j.jas.2012.01.022]

66. Mangone, A.; Giannossa, L.C.; Colafemmina, G.; Laviano, R.; Traini, A. Use of various spectroscopy techniques to investigate raw materials and define processes in the overpainting of Apulian red figured pottery (4th century BC) from southern Italy. Microchem. J.; 2009; 92, pp. 97-102. [DOI: https://dx.doi.org/10.1016/j.microc.2009.02.004]

67. De Ágredos Pascual, M.L.V.; Carbó, M.T.D.; Carbó, A.D. Characterization of Maya Blue Pigment in Pre-classic and Classic Monumental Architecture of the Ancient Pre-Columbian City of Calakmul (Campeche, Mexico). J. Cult. Herit.; 2011; 12, pp. 140-148. [DOI: https://dx.doi.org/10.1016/j.culher.2009.12.002]

68. De Ágredos-Pascual, M.L.V.; Roldán-García, C.; Murcia-Mascarós, S.; Barber, D.J.; Sánchez, M.G.J.; Faugere, B.; Darras, V. Multianalytical characterization of pigments from funerary artefacts belonging to the Chupicuaro Culture (Western Mexico): Oldest Maya blue and cinnabar identified in Pre-Columbian Mesoamerica. Microchem. J.; 2019; 150, 104101. [DOI: https://dx.doi.org/10.1016/j.microc.2019.104101]

69. Kim, K. Study of SERS Library of Natural Dyes in Heritage Using Chelation Bond-Assisted AuNP-on-AuNF Substrates. Ph.D. Thesis; Seoul University Graduate School: Seoul, Republic of Korea, 2024.

70. Rigante, E.C.; Calvano, C.D.; Ventura, G.; Cataldi, T.R. Look but Don’t Touch: Non-Invasive Chemical Analysis of Organic Paint Binders—A Review. Anal. Chim. Acta; 2024; 1335, 343251. [DOI: https://dx.doi.org/10.1016/j.aca.2024.343251]

71. Chen, S.; Sun, D.; Zou, L.; Huang, Y.; Cheng, F.; Shi, L.; Tasleem, A.; Ullah, A.; Daood, S.S.; Yi, Q. Hydrothermal Pre-Anchoring and In-Situ Preparation of Prussian Blue Analogue-Polymeric Carbon Nitride Nanorods Hybrid for Robust and Efficient Cesium Adsorption. Chem. Eng. J.; 2024; 495, 153457. [DOI: https://dx.doi.org/10.1016/j.cej.2024.153457]

72. Faltynkova, A. Advancing Methods for Sampling and Analysis of Microplastics in the Marine Environment. Ph.D. Thesis; Norwegian University of Science and Technology: Trondheim, Norway, 2024.