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Review

Applications and Challenges of Modern Analytical Techniques for the Identification of Plant Gum in the Polychrome Cultural Heritage

by
Liang Xu
1,
Weijia Zhu
1,
Xi Chen
1,* and
Xinyou Liu
2
1
School of Arts, Jiangsu University, Zhenjiang 212013, China
2
Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(9), 1042; https://doi.org/10.3390/coatings15091042
Submission received: 14 July 2025 / Revised: 27 August 2025 / Accepted: 4 September 2025 / Published: 5 September 2025
(This article belongs to the Section Surface Characterization, Deposition and Modification)
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Abstract

Plant gums have long served as essential binding media in polychrome cultural heritage, contributing to pigment adhesion, surface cohesion, and long-term stability. This review evaluates recent advances in analytical technologies, including FTIR, Raman spectroscopy, GC-MS, LC-MS/MS, MALDI-TOF MS, hyperspectral imaging, and immunological assays, for the identification of gums such as gum arabic, peach gum, and tragacanth in diverse cultural contexts. Drawing on case studies from 19th-century watercolours, ancient Egyptian coffins, and Maya murals, the paper demonstrates how these methods enable precise chemical characterization even in complex, aged, and mineral-rich matrices. Such information directly aids conservators in selecting compatible restoration materials, tailoring treatment protocols, and assessing deterioration mechanisms. Persistent challenges remain, including gum degradation, spectral interference from pigments and restoration materials, sample heterogeneity, and limited reference libraries, particularly for non-European species. Future research directions emphasize multi-modal, non-invasive workflows that integrate hyperspectral imaging with spectroscopic and chromatographic methods, drone-assisted micro-Raman for inaccessible surfaces, machine learning-assisted spectral databases, and bio-inspired adhesives replicating historical rheology. By linking molecular identification to conservation decision-making, plant gum analysis not only deepens our understanding of historical material practices but also strengthens the scientific basis for sustainable heritage preservation strategies.

1. Introduction

Polychrome cultural artifacts represent invaluable legacies of human civilization, bearing significant historical, artistic, and scientific value. These artifacts are often composed of complex composites of organic and inorganic materials. Due to their intricate structure, sensitive composition, and long-term exposure to fluctuating environmental conditions, their pigment layers are prone to ageing, powdering, or delamination [1]. Among the various organic binders, plant gums, such as gum arabic, peach gum, and tragacanth, have been widely used in ancient paintings artistic production but also in restoration practices, especially in watercolour-based inpainting. Their natural flexibility, ready availability, and low toxicity make them particularly valuable for both historical use and conservation efforts [2]. Gum arabic, for instance, remains a key component in watercolours used by restorers from the 19th century to the present. It plays a vital role in conservation interventions such as retouching wall paintings, canvases, and polychrome wooden sculptures [3,4]. Consequently, plant gums are not only remnants of ancient craftsmanship but also reflect material continuities into modern conservation science.
Recent advancements in analytical technologies have greatly enhanced the precision of plant gum identification. Techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS), Liquid Chromatography–Tandem Mass Spectrometry (LC-MS/MS), and micro-Raman spectroscopy allow detailed characterization even from microsamples [5,6]. Notably, Derrick (1989) demonstrated the capability of FTIR in detecting polysaccharide-based binders in complex mixtures [7], while Granzotto pioneered MALDI-MS methodologies that enabled the differentiation of saccharide profiles in aged gums [8]. Similarly, Sotiropoulou et al. (2018) developed robust protocols for assessing degradation markers and distinguishing between historical and restoration binders [9].
Despite these advances, several challenges persist in practical applications. These include sampling limitations, interference from pigments or restoration materials, signal confusion caused by degradation by-products, and the lack of standardized reference spectral databases, especially for non-European gum species [5]. Moreover, plant gums’ susceptibility to hydrolysis and oxidation adds further complexity to their reliable identification in aged and stratified artifact surfaces. In recent years, researchers have explored multi-technique integration, non-destructive imaging approaches, and database expansion to broaden the analytical capabilities for gum detection. For instance, studies on murals and polychrome sculptures from Egypt, Dunhuang, and Central Asia have revealed the use of mixed plant gums, identified through combinations of GC-MS, immunoassays, and spectroscopic profiling. Studies on watercolour binding media by Lluveras-Tenorio et al. have further demonstrated how degradation patterns influence spectral signatures and conservation decision-making [10].
Nonetheless, critical bottlenecks such as inconsistent methodologies, low in-field efficiency, and the high cost of advanced instrumentation continue to hinder widespread adoption. Therefore, this review aims to provide a comprehensive overview of modern analytical techniques used to identify plant gums in polychrome cultural heritage. It evaluates their applications, limitations, and potential improvements, drawing upon key case studies, including watercolour-based restoration materials. By bridging technical insight with historical context, the review offers practical recommendations for future research directions and highlights the evolving role of plant gum identification in the field of heritage science.

2. Overview of Plant Gums in Polychrome Cultural Heritage

2.1. Common Types and Chemical Composition of Plant Gums

Plant gums have been widely used as natural adhesives in historical polychrome artworks. Among the most common are gum arabic, peach gum, tragacanth gum, and tamarind gum. These substances are composed primarily of water-soluble polysaccharides, including galactose, arabinose, rhamnose, and glucuronic acid. Their molecular structures are typically highly branched and hydrophilic, granting them excellent film-forming ability and redispersibility in water-properties that make them particularly suitable for use in polychrome layers [11].
These gums were not only chosen for their functional characteristics but also reflected regional botanical resources and traditions, with varying applications across cultures and periods. Table 1 summarizes the primary components, plant sources, and typical uses of major plant gums identified in cultural heritage materials.

2.2. Historical Periods and Geographical Distribution of Plant Gum Use

The application of plant gums in polychrome artifacts exhibits both deep historical continuity and clear cultural regionality. As binding agents and pigment media, plant gums have played indispensable roles in the painted heritage of numerous ancient civilizations. Due to ecological diversity and the availability of local flora, artisans across regions adopted indigenous botanical materials, leading to distinct cultural formulations of plant gums.
Both the archaeological literature and instrumental analysis confirm the extensive use of gum Arabic during the New Kingdom (16th–11th centuries BCE). Historical and technical sources describe its role in coffins, cartonnage panels, and wooden sculptures [18,19], while modern FTIR and GC-MS analyses of painted fragments from Luxor and Abydos provide direct chemical evidence for its presence [7,8]. The identification of Acacia senegal as its botanical source also suggests active botanical exchange between Egypt and sub-Saharan Africa.
In the Western Han to Tang dynasties (2nd century BCE–10th century CE), historical records of painting recipes [20] align with GC-MS and immunoassay evidence showing the use of peach gum, often blended with animal glue or beeswax, in painted pottery figurines, tomb murals, and polychrome sculptures from Shaanxi, Henan, and Gansu [21]. These blends improved flexibility and pigment adhesion stability.
Technical art-historical studies document the high-viscosity gums used in Buddhist statues and cave temples, while chemical analyses from Dunhuang’s Mogao Caves and Kizil Grottoes identified tragacanth gum and tamarind gum [22]. Their translucency and adhesive properties were ideal for multilayered surfaces and gold leaf application. Botanical sourcing suggests origins in Iran, Afghanistan, and neighbouring regions [23].
Historical accounts of mural production and restoration [24] note the use of wood apple gum (Limonia acidissima) and lac tree gum, while chemical and microscopic analyses of samples from Ajanta and Sigiriya murals [25] have confirmed their presence. These gums were also employed as dye vehicles to enhance colour fastness, reflecting the influence of Ayurvedic medicinal practices and ritual crafts. From ancient Egypt to China, Central Asia, and South Asia, this diversity of plant gums-evidenced through both historical/technical literature and modern scientific analysis-not only demonstrates ancient artisans’ ingenuity and ecological adaptation but also provides valuable insight into technological lineages and regional identities. Modern conservation science increasingly relies on such geographically and temporally contextualised data to reconstruct material traditions and heritage networks.

2.3. Ageing and Degradation Mechanisms of Plant Gums

As natural polysaccharide-based macromolecules, plant gums provide excellent film-forming and adhesive functions in polychrome heritage objects. However, they are vulnerable to environmental stressors. Prolonged exposure to high humidity, ultraviolet radiation, acidic gases (e.g., SO2, NOx), and microbial activity can initiate complex degradation pathways involving hydrolysis, oxidation, and acid-catalyzed cleavage [10]. These reactions break down the polymer backbone, significantly reducing the adhesive’s performance and jeopardizing the integrity of polychrome layers.
The primary consequences of gum degradation include molecular weight reduction and viscosity loss, which impair adhesion and result in pigment detachment. In addition, dehydration and shrinkage of the gum film often led to powdering, cracking, and delamination—issues frequently observed in wall paintings and painted ceramics. More critically, degradation generates by-products such as formaldehyde, acetic acid, and glucuronic acid, which can chemically interact with pigments or substrates, causing darkening, chromatic distortion, or irreversible fading [26].
Notably, the rate and pattern of degradation are influenced by both external conditions and intrinsic structural factors of the gum. For instance, tragacanth gum, enriched with branched polysaccharides and uronic acids, exhibits higher structural stability and slower ageing compared to gum arabic, whose linear backbone is prone to rapid hydrolysis under heat and moisture [27].
Advanced analytical technologies, particularly thermally assisted hydrolysis and methylation gas chromatography–mass spectrometry (THM-GC/MS) and matrix-assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI-TOF MS), now enable in-depth molecular tracking of ageing markers. Granzotto and Sutherland (2017), for example, utilized MALDI-TOF MS to fingerprint Acacia gum in microsamples, identifying specific oligosaccharide markers essential for degraded gum identification in heritage diagnostics [28]. Moreover, the geographical origin and extraction method of plant gums significantly influence their ageing behavior. Variations in impurities, native pH, and non-saccharide content can alter stability profiles, meaning that gums from different regions or prepared using different techniques may exhibit markedly different durability. Understanding these intrinsic factors is therefore essential for interpreting analytical results and for selecting appropriate conservation treatments.

3. Overview of Plant Gum Identification Techniques in Polychrome Cultural Relics

3.1. Traditional Methods

Before the widespread adoption of modern instrumental analysis, the identification of plant gums in the field of cultural heritage conservation primarily relied on a range of experience-based traditional techniques. These methods are simple to perform and low in cost, making them useful for preliminary investigations and on-site screening. However, their limited accuracy and specificity often require them to be used in conjunction with other analytical approaches. The paper reagent test is one of the earliest on-site detection methods applied in cultural relic analysis. It typically involves applying acidic or basic dyes to micro-samples and observing colour changes to preliminarily determine the presence of organic adhesives. For example, dyes such as alizarin red, silver nitrate, and aromatic aldehydes produce colour reactions in the presence of polysaccharides, indicating the possible presence of plant gums [29]. However, this method is prone to interference from pigments and degradation products, which can lead to ambiguous results.
Under polarized light microscopy and fluorescence microscopy, plant gums usually appear as colourless, semi-transparent films with a glassy luster. Cross-sectional observation can reveal the distribution patterns at the interface between the adhesive and mineral pigments, aiding in the identification of binding media [30]. Although this method cannot definitively distinguish between different types of gums, it is still useful for preliminary screening to determine whether a plant-based material is present.
Taking advantage of the water solubility of plant gums, researchers have attempted to dissolve micro-samples in distilled water or ethanol and analyze the components through filter paper or precipitation methods [31]. This approach is fast and straightforward, but due to overlapping solubility among various binders (e.g., Arabic gum and gelatin), it often leads to misidentification.
Despite their limitations in precision, these traditional methods still hold value in practical applications, particularly when instrumental resources are limited or during preliminary surveys (Table 2). However, with increasing demands for refined conservation and the advancement of scientific techniques, these methods are gradually being supplemented or replaced by multi-spectral and mass spectrometry-based technologies [32].

3.2. Modern Analytical Chemistry Techniques

With the increasing demand for precision in cultural heritage conservation, modern analytical chemistry techniques have rapidly expanded their application in the identification of plant gums. Compared to traditional empirical methods, spectroscopic and chromatographic techniques offer high sensitivity, strong molecular recognition ability, and require minimal sample amounts, making them especially suitable for studying complex, trace, and mixed organic-inorganic systems in painted artifacts.

3.2.1. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR and its variant Attenuated Total Reflection FTIR (ATR-FTIR) are widely used non-destructive techniques in cultural heritage research, particularly suitable for identifying organics like plant gums. The principle is based on molecular functional groups absorbing infrared radiation at characteristic frequencies. Common absorption peaks for hydroxyl (–OH), ether bonds (C–O–C), and carboxyl groups (–COOH) appear, respectively, in the 3200–3400, 1000–1150, and 1700–1740 cm−1 regions [33]. Although different plant gums exhibit similar spectra, subtle differences such as the 1020 cm−1 peak in Arabic gum and the broad 1060 cm−1 peak in tragacanth gum provide clues for differentiation [26]. ATR-FTIR collects surface spectra directly via contact between a crystal and the sample, greatly reducing sample damage, and is suitable for micro or fragile cultural relics in situ. For example, Casoli (2021) used non-invasive ATR-FTIR to analyze 15th-century religious frescoes in Parma, Italy, detecting key polysaccharide signals confirming Arabic gum as the main binder [34]. Despite FTIR challenges in multi-component systems due to overlapping signals, coupling with Raman spectroscopy or GC-MS often improves identification accuracy [35].

3.2.2. Raman Spectroscopy

Raman spectroscopy is based on energy shifts in scattered light from molecular vibrations and offers high spatial resolution, non-destructiveness, and minimal sample preparation, making it widely adopted in cultural heritage analysis [36]. It is insensitive to water and suitable for moist samples. Raman is commonly used to identify pigments, binders, and coatings and is ideal for analyzing complex stratified or micro-area samples. Micro-Raman (μ-Raman) employs microscopic focusing for point scanning and 3D imaging, revealing spatial distribution of plant gums and pigments, especially at coating interfaces. Although plant gum signals are weak and susceptible to fluorescence interference, longer integration times, using low-background excitation lasers (e.g., 785 nm), and multi-spectrum averaging can improve signal-to-noise ratios [37]. Combining multispectral imaging and fluorescence suppression techniques allows μ-Raman to achieve precise localization under challenging conditions [38].
Tamburini (2019) used μ-Raman scanning on 19th-century Burmese lacquerware, revealing alternating distributions of red ochre and organic binders. Weak C–H vibrational peaks in the 1200–1500 cm−1 range enabled reconstruction of the plant gum spatial distribution, illustrating its function as a topcoat binder and the multilayer lacquering process. Despite challenges from weak signals and fluorescence, advances in lasers and detectors continue to enhance Raman detection capabilities, making μ-Raman indispensable for micro-area non-destructive analysis [1].

3.2.3. Gas Chromatography–Mass Spectrometry (GC-MS)

GC-MS is highly sensitive for analyzing low-molecular-weight organic compounds, and is capable of identifying natural plant gums, including those that have undergone ageing, by first hydrolyzing the sample to release component monosaccharides. This enables the specific and reproducible detection of characteristic saccharides and degradation products such as uronic acids, organic acids, and alcohols through chromatographic separation and mass spectral fragmentation [35].Conventional GC-MS analysis typically involves sample pretreatment steps such as acid hydrolysis and derivatization. Importantly, derivatization is performed on the extracted microsample, not directly on the artifact, and therefore does not pose physical risks to the object itself. Pyrolysis GC-MS (Py-GC/MS) thermally degrades polymers into volatile fragments, eliminating the need for derivatization and making it especially suitable for micro-amount samples and aged materials [39].
Py-GC/MS provides excellent discrimination between plant gum types. For example, gum Arabic yields pyrolysis products rich in arabinose, rhamnose, and galactose, whereas tragacanth gum produces uronic acid-containing polymers that act as structural markers for accurate identification [40]. Together, GC-MS and Py-GC/MS can reveal binder sources, micro-distributions, ageing mechanisms, and restoration history. With the adoption of automated derivatization systems, micro-pyrolysis chips, and portable GC-MS units, these approaches are progressing toward minimally destructive, in situ, and high-throughput detection capabilities [41,42].

3.2.4. Liquid Chromatography–Tandem Mass Spectrometry (LC-MS/MS)

LC-MS/MS is suitable for analyzing polar, thermally unstable, non-volatile compounds without derivatization, directly detecting monosaccharides, oligosaccharide degradation products, and organic acids, especially useful for severely degraded samples [43]. Plant gums, mainly complex polysaccharides, degrade into oligosaccharides and monosaccharide residues over time, with LC-MS/MS reconstructing their structures through targeted ion detection. Ultra-high-performance liquid chromatography (UPLC-MS) enhances separation speed and resolution, widely applied to complex organics in cultural heritage.
LC-MS/MS’s advantages include high-precision quantification and monitoring of sugar acid or glycosidic degradation progress to assess ageing. Colombini et al. (2002) developed an ion chromatography method for polysaccharide characterization in ancient wall paintings, enabling identification and quantification of uronic acids such as galacturonic and glucuronic acids, and successfully distinguishing different plant gums including tragacanth, fruit tree, and guar/locust bean gums [44]. LC-MS/MS also supports building plant gum fingerprint databases and machine learning-assisted identification. Coupling UV detection and fluorescent labeling extends analysis to glycoprotein complexes, opening new directions in composite coating research.

3.2.5. X-Ray Fluorescence Analysis (XRF)

XRF cannot directly detect organic plant gum components but, in cultural heritage studies, it provides important auxiliary data by analyzing elemental co-distribution in pigments and binders. XRF excites sample elements to emit characteristic X-rays, enabling rapid, non-destructive multi-point detection of metals and non-metals like potassium, calcium, iron, lead, and chlorine [45].
Plant gums contain elements such as potassium, calcium, and magnesium or form composites with mineral pigments. Vila and Centeno (2013) demonstrated that potassium and chromium enrichment detect-ed by XRF, in combination with FTIR and Raman confirmation of polysaccharide and pigment phases, enables inference of gum distribution and its role in coating or colloid formation [46]. XRF also distinguishes pigment stratigraphy, separating original materials from restoration layers. High potassium content often indicates potassium-rich gums like Arabic or peach gum used in restoration [47].
Dighe (2024) applied portable XRF on frescoes in India’s Ellora Cave 16, detecting multiple potassium-rich areas; ATR-FTIR identified tragacanth gum [48]. The literature indicates potassium aids colloid stability and adhesion in humid environments. Coupled with μ-Raman identification of red ochre and gum layers, it is inferred that gums not only bind but also promote pigment dispersion and coating protection. XRF combined with FTIR and Raman greatly enhances spatial resolution and material analysis, ideal for non-sampling contexts like murals, wood carvings, and temple interiors. In practice, portable XRF integrated with GIS and 3D scanning enables elemental and material spatial visualization, assisting stratigraphic diagnostics and restoration planning.

3.2.6. Hyperspectral Imaging (HSI) in the SWIR Region

Hyperspectral Imaging (HSI) in the short-wave infrared (SWIR, 1000–2500 nm) range is an emerging non-destructive and non-invasive analytical method that has been successfully applied to the study of watercolours and other plant gum-based binders. HSI combines spatial imaging with spectral information acquisition, producing a three-dimensional dataset (x, y, λ) that allows for both material classification and mapping. When applied using portable SWIR hyperspectral systems, it enables in situ analysis without sampling, making it highly suitable for fragile polychrome heritage surfaces [49].
In the context of plant gum detection, SWIR HSI can classify characteristic absorption features of gum Arabic and distinguish it from other polysaccharide binders or proteinaceous media based on subtle differences in overtone and combination bands of O–H and C–H vibrations. For example, Ricciardi et al. demonstrated through the analysis of 15th-century illuminated manuscripts that NIR imaging spectroscopy in the 1000–2500 nm range can effectively identify gum Arabic and separate it from proteinaceous binders such as egg white, as well as from lipid-containing binders like egg yolk, even in complex pigment mixtures [50].
An additional advantage of SWIR HSI is its ability to generate binder distribution maps across entire artworks, providing context on binder use, degradation patterns, and restoration interventions. When integrated with μ-Raman, FTIR, or GC-MS results, HSI can offer a macro-to-micro analytical workflow-HSI rapidly surveys the whole surface to locate gum-rich areas, which can then be targeted for confirmatory point analysis by other techniques [51]. This sequential integration not only improves analytical efficiency but also minimizes invasive sampling.
Given its portability, rapid acquisition, and compatibility with large-scale documentation methods such as 3D scanning, SWIR HSI is increasingly regarded as a valuable addition to the non-invasive analytical toolkit for plant gum identification in both original polychrome layers and later conservation materials like watercolours [52].

3.3. Biochemical and Immunological Techniques

Antibody-based biochemical immunoassays have become increasingly important for the identification of protein–polysaccharide composite binders in painted artifacts. Among these, the enzyme-linked immunosorbent assay (ELISA) stands out for its high sensitivity, specificity, quantitative capability, and operational simplicity [53].
ELISA works by exploiting the specific binding between antigen and antibody, combined with enzyme-catalyzed colourimetric or fluorometric amplification, to detect target organic compounds. By calibrating against standard curves prepared from known concentrations of purified plant gums, ELISA can provide semi-quantitative to fully quantitative measurements of gum content in microsamples-something that complements the compositional data from techniques like FTIR or GC-MS. This quantitation is particularly valuable in conservation science, where relative binder ratios can influence restoration strategy. Specific antibodies have been developed against polysaccharide epitopes from peach gum, gum arabic, and tragacanth gum, enabling clear discrimination even in complex mixtures containing proteinaceous binders such as gelatin or fish glue. For example, Wu et al. (2024) applied monoclonal antibodies targeting galactose residues of peach gum to Yuan dynasty Buddhist statue paint layers; quantitative ELISA readings indicated peach gum as the dominant polysaccharide binder. In parallel, SDS-PAGE and bicinchoninic acid (BCA) assays confirmed fish glue content, revealing both the composition and the proportions of composite binders [15].
Sensitivity and spatial resolution have been further enhanced through biotin–streptavidin amplification systems, allowing the detection of plant gums in sub-milligram samples [54]. Immunochip and microplate formats now facilitate high-throughput binder screening, while integration with complementary methods such as FTIR, GC-MS, and Py-GC/MS enables multi-level plant gum identification-from qualitative fingerprinting to quantitative compositional profiling. Additionally, immunofluorescence staining combined with confocal laser scanning microscopy allows three-dimensional visualization of polysaccharide distribution within stratigraphic layers [55,56]. This integration of biochemical specificity, quantitative capability, and spatial mapping offers conservation scientists a powerful toolset for both historical analysis and decision-making in restoration practice.

4. Applications of Plant Gum Identification Technologies in the Conservation of Polychrome Cultural Heritage

Three representative application cases are presented to demonstrate different analytical approaches, namely mass spectrometry (MALDI-TOF MS), chromatographic analysis (GC-MS), and multi-technique integration. Cases 4.1 employ MALDI-TOF MS but in different contexts: one for 19th-century artworks and the other for ancient Egyptian artifacts, allowing a comparative view of how the same core technology adapts to distinct materials, ages, and conservation questions. Case 4.2 illustrates the use of GC-MS for ancient mural and mortar analysis, highlighting the role of plant gums in composite material systems.
For each case, a brief critical commentary is provided, discussing technical strengths, interpretive limitations, and potential improvements through complementary methods.

4.1. MALDI-TOF MS Identification in 19th-Century Watercolours and Ancient Egyptian Artworks

Enzymatic and partial enzymatic digestion combined with Matrix-Assisted Laser Desorption–Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) has been successfully applied to the identification of plant gums in both 19th-century watercolours and ancient Egyptian artworks.
In the 19th-century case, full enzymatic hydrolysis was employed to break down gum polysaccharides into oligosaccharides, which were then analyzed by MALDI-TOF MS. This method produced unique mass fingerprints for gum arabic, tragacanth, peach gum, and related species [8,57]. It demonstrated robustness with sample sizes as small as a few micrograms and in the presence of complex pigment mixtures. The resulting profiles were stable despite ageing and contamination, enabling accurate identification without extensive sample manipulation. Compared with GC-MS, which may face limitations in distinguishing gums when mixed or degraded, MALDI-TOF MS provided higher specificity, sensitivity, and speed [8,57].
Figure 1 shows the distinct m/z peaks generated after enzymatic digestion, each corresponding to characteristic oligosaccharide fragments of gum arabic. These peaks form a unique “fingerprint” that enables clear differentiation from other plant gums, even when the binder is part of a complex pigment mixture or has undergone significant ageing.
In the ancient Egyptian case, partial enzymatic digestion was combined with MALDI-TOF MS and multivariate Principal Component Analysis (PCA) to enable genus- and species-level discrimination [8,58]. A reference database of Acacieae gums was expanded to 56 samples from 19 species, addressing the limitations of complete hydrolysis and GC-MS in differentiating closely related species. PCA distinguished gums from Vachellia, Acacia, and Senegalia genera, and separated species within the Acacia complex. Notably, Vachellia nilotica and V. tortilis—both native to Egypt—were identified as major binders, aligning with archaeological evidence of local botanical sourcing [58,59]. The plot in Figure 2 shows clear clustering by species, with Vachellia species grouping separately from Senegalia, thus enabling botanical provenance attribution.
Both cases demonstrate that MALDI-TOF MS can generate distinctive oligosaccharide fingerprints for various plant gums, even in aged and complex mixtures. However, the reliance on enzymatic digestion, whether full or partial, may not fully release all structural markers, risking underrepresentation of certain gum fractions. The 19th-century study focused primarily on technical validation without linking identified gums to broader art-historical or trade patterns, while the ancient Egyptian study’s PCA-based botanical attribution could be further strengthened through cross-validation with GC-MS monosaccharide profiling or FTIR spectral mapping to confirm spatial distribution within paint layers. Integrating such complementary approaches would enhance both chemical accuracy and cultural interpretation.

4.2. GC-MS Identification of Plant Gums in Maya Mural Paintings and Mortars

Gas Chromatography–Mass Spectrometry (GC-MS) monosaccharide analysis has been applied to pigment layers and lime mortars from Maya mural sites, including Ek’ Balam, Sta. Rosa Xtampak, and Chichén Itzá [60]. These plant gums, used as binding agents in pigments and additives in mortars, provide essential insights into the technological practices of the Maya [61]. In particular, GC-MS detection of characteristic sugars such as glucose and mannose has proven instrumental in identifying gum-based materials, even within lime-rich matrices [35,61].
The identification of these monosaccharides enables conservators to reconstruct the original composition of the murals and better understand the working properties required by Maya artisans [62]. The consistent detection of glucose and mannose suggests that plant gums were intentionally added to lime mortar to enhance workability and improve pigment cohesion. Such findings are consistent with ethnographic accounts indicating the addition of tree bark to lime hydration mixtures during the Maya period [63].
For analysis, samples underwent hydrolysis to break down glycosidic bonds, releasing monosaccharides and uronic acids, which were subsequently derivatized for chromatographic separation. The resulting chromatograms clearly display peaks corresponding to glucose and mannose in both pigment and mortar layers (Figure 3 and Figure 4).
However, interpretation is complicated by several factors. Over centuries, alkaline degradation in lime mortars can lead to sugar loss, while microbial activity may further alter the chemical composition of the gums [62]. Restoration interventions involving modern materials such as acrylic emulsions also risk obscuring original organic signatures.
Despite these challenges, chromatograms from multiple Maya sites consistently indicate the presence of gum-related monosaccharides in both mural paint and mortar layers [64], reinforcing their integral role in mural preparation.
GC-MS has proven effective for detecting gum-derived sugars in complex lime-based systems, but sugar degradation and the potential contribution of sugars from non-gum sources (e.g., cellulose) limit interpretive certainty. Multi-technique integration-such as FTIR for functional group identification and Hyperspectral Imaging (HSI) for surface mapping-could help distinguish original binder components from later restoration materials, thereby improving both chemical accuracy and cultural interpretation.

4.3. Summary Table of Case Studies

The three case studies presented above illustrate how different analytical strategies can be applied to the identification of plant gums in polychrome cultural heritage, each targeting distinct object types, chronological periods, and conservation questions. Table 3 consolidates key information on object type, analytical method, gum species identified, and a brief assessment of technical strengths and limitations.
This comparative format highlights the methodological diversity, from MALDI-TOF MS workflows adapted for both modern and ancient artworks to GC-MS applications in mineral-rich archaeological contexts, and clarifies how each technique’s capabilities align with specific conservation challenges. Notably, the Egyptian case demonstrates the potential of multivariate MALDI-TOF MS for species-level botanical attribution, while the Maya mural analysis shows the practicality of GC-MS in detecting plant gum signatures within lime-based mortars.
The limitations column underscores common challenges: incomplete release of oligosaccharide markers in enzymatic digestion, sugar degradation in alkaline environments, and the interpretive gap between chemical identification and historical context. The suggested complementary methods aim to address these gaps, for instance, linking gum identification to pigment sourcing records or using FTIR and Hyperspectral Imaging for binder mapping.

5. Research Limitations and Future Perspectives

5.1. Current Technical Limitations

Despite significant advancements in plant gum identification technologies, several challenges hinder their broader application in polychrome cultural heritage conservation. The micro-destructive or non-invasive nature of modern techniques often requires minimal sample sizes, which may not adequately represent the heterogeneity of complex, multilayered artifacts. For instance, FTIR and Raman spectroscopy struggle with deeply stratified samples where plant gums are intermixed with pigments or degradation products, leading to signal overlap and ambiguous interpretations. Additionally, aged plant gums undergo chemical transformations such as oxidation and hydrolysis, altering their original spectral or chromatographic fingerprints and complicating identification. The lack of comprehensive reference libraries for regional gum varieties and the high cost and limited portability of advanced instruments like MALDI-TOF MS further restrict field applications. Cross-disciplinary integration of biochemical and physicochemical methods also faces protocol harmonization challenges, particularly for artifacts with mixed binders or restoration materials.

5.2. Future Research Directions

To address the current limitations, future research should advance the integration of multi-modal analytical strategies that combine hyperspectral imaging (HSI), Raman/FTIR, and chromatographic–mass spectrometric techniques (e.g., GC-MS, LC-MS/MS) into unified, cross-platform workflows. Such fusion systems, supported by machine learning algorithms, could enable non-invasive, high-resolution mapping of plant gum distributions across large and complex surfaces, facilitating differentiation between original binders and later restoration materials in situ. Emerging mobile and aerial platforms offer promising solutions for inaccessible or high-altitude heritage surfaces. For example, recent studies have demonstrated the feasibility of drone-mounted micro-Raman spectrometers for high-wall mural inspection, enabling safe, non-contact acquisition of spectral data in environments where scaffolding or direct access is impractical [65]. Miniaturized Raman modules-originally developed for planetary exploration-have been adapted for cultural heritage, showing effective pigment and binder identification from distances of up to several meters [65]. Incorporating such systems with high-precision positioning and HSI pre-screening would allow operators to target specific zones for Raman or FTIR confirmation without excessive scanning time.
Further development of microfluidic Py-GC/MS chips and portable MALDI-TOF MS instruments could enhance on-site chemical specificity while minimizing sampling. Expanding reference libraries of aged gum degradation markers-via accelerated ageing simulations and synchrotron-based micro-spectroscopy-will improve the reliability of identifications in heavily degraded contexts.
Finally, the cross-application of biochemical assays such as ELISA microarrays with spectroscopic mapping could link molecular specificity with spatial resolution. Combined with isotopic provenance tracing, this approach could open new avenues for reconstructing ancient material supply networks. Beyond analysis, translating these findings into bio-inspired conservation materials-for instance, pH-stabilized hybrid adhesives replicating the rheological properties of historical gums-could align conservation practice with both historical authenticity and modern durability requirements.

6. Conclusions

The study of plant gum identification technologies has emerged as a critical frontier in the conservation of polychrome cultural heritage, offering profound insights into historical material practices while informing modern preservation strategies. Over the past decade, analytical advancements-from FTIR and Raman spectroscopy to MALDI-TOF MS and immunological assays-have revolutionized our ability to detect, characterize, and differentiate plant gums in complex artifact matrices. These techniques have not only elucidated the material composition of artworks across civilizations, from Egyptian cartonnages to Maya murals and Dunhuang frescoes, but have also revealed the sophisticated technological choices of ancient artisans, shaped by ecological and cultural contexts.
Such information directly aids restorers in selecting compatible conservation materials, tailoring treatment methods, and evaluating deterioration mechanisms over time. By linking analytical results with the physical and chemical needs of the artifact, conservators can make evidence-based decisions that balance historical authenticity with long-term stability.
However, persistent challenges underscore the need for continued innovation. The degradation of plant gums over time, interference from mixed materials, and limitations in reference databases complicate precise identification, while the high cost and specialization of advanced instruments restrict their accessibility. Future progress hinges on interdisciplinary collaboration, integrating spectroscopy, chromatography, and biomolecular methods with emerging technologies like machine learning and portable micro-analytical devices. The development of standardized protocols and shared global databases will be pivotal in overcoming current bottlenecks.
Ultimately, the conservation of polychrome heritage demands a dual focus: refining analytical precision to decode historical materiality, and applying these insights to develop sustainable, historically informed restoration materials. As this field evolves, it promises to bridge past and present, ensuring that the vibrant legacy of polychrome artifacts endures for future generations while deepening our understanding of humanity’s artistic and technological heritage.

Author Contributions

Conceptualization, L.X. and X.C.; methodology, L.X. and X.L.; software, W.Z.; validation, L.X., W.Z. and X.L.; formal analysis, L.X.; investigation, L.X. and W.Z.; resources, X.L. and X.C.; data curation, L.X. and W.Z.; writing—original draft preparation, L.X.; writing—review and editing, X.L. and X.C.; visualization, W.Z.; supervision, X.C.; project administration, X.L.; funding acquisition, X.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 15 01042 g001
Figure 1. MS profiles of the enzymatically digested gum Arabic [57].
Figure 1. MS profiles of the enzymatically digested gum Arabic [57].
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Figure 2. PCA scores plot (PC1 vs. PC2) of MALDI-TOF MS data: (a) reference samples with legend, and (b) reference samples with samples from artworks [58].
Figure 2. PCA scores plot (PC1 vs. PC2) of MALDI-TOF MS data: (a) reference samples with legend, and (b) reference samples with samples from artworks [58].
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Figure 3. Chromatogram of the paint layer sample from Sta. Rosa Xtampak (SR) [61].
Figure 3. Chromatogram of the paint layer sample from Sta. Rosa Xtampak (SR) [61].
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Figure 4. Chromatogram of the paint layer sample from Chichén Itzá (CI) [61].
Figure 4. Chromatogram of the paint layer sample from Chichén Itzá (CI) [61].
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Table 1. Common types of plant gums—chemical composition and heritage applications.
Table 1. Common types of plant gums—chemical composition and heritage applications.
Type of Plant GumMain ComponentsBotanical SourceTypical ApplicationsReference
Gum ArabicArabinose, Galactose, RhamnoseAcacia senegal (Acacia species)Chinese murals, paper-based paintings, watercolour restoration media[12,13]
Peach GumGalactose, Rhamnose, GlucoseResin exudate from Prunus spp. (peach)Painted pottery figurines, wood sculptures[14,15]
Tragacanth GumGalacturonic acid, GalactoseAstragalus spp. (leguminous shrubs)Tibetan thangkas, Buddhist statue coatings[16]
Tamarind GumPolygalactoseTamarindus indica (tamarind tree)South Asian decorative painting[17]
Table 2. Advantages and Disadvantages of Traditional Plant Gum Identification Methods.
Table 2. Advantages and Disadvantages of Traditional Plant Gum Identification Methods.
Method TypeAdvantagesLimitations/Drawbacks
Paper reagent testFast, low-cost, suitable for on-site screeningNon-specific reactions; easily affected by contaminants or ageing products
Microscopic AnalysisVisualizes coating structure; helps distinguish organic/inorganic bindersCannot definitively identify plant gum type; requires complementary methods
Solubility TestingSimple operation, based on physicochemical propertiesOverlapping solubility among gums; results affected by material ageing
Table 3. Comparative summary of case studies on plant gum identification in cultural heritage objects.
Table 3. Comparative summary of case studies on plant gum identification in cultural heritage objects.
Case StudyObject TypeAnalytical Technique(s)Identified Gum(s)Technical StrengthsLimitationsSuggested Complementary Methods
19th-centuryWatercolours, drawingsEnzymatic hydrolysis+ MALDI-TOF MSGum arabic, othersHigh sensitivity, minimal samplingLimited historical contextLink with pigment sourcing records
EgyptianCoffins, reliefs, cartonnagesPartial enzymatic digestion + MALDI-TOF MS + PCAV. nilotica, V. tortilisSpecies-level botanical attributionDependence on enzymatic digestionGC-MS monosaccharide profiling, FTIR mapping
Maya muralsMurals, mortarsGC-MSMixed gums (glucose, mannose)Works in mineral-rich matrixSugar degradation, ambiguous sourcesFTIR, HSI for binder mapping
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Xu, L.; Zhu, W.; Chen, X.; Liu, X. Applications and Challenges of Modern Analytical Techniques for the Identification of Plant Gum in the Polychrome Cultural Heritage. Coatings 2025, 15, 1042. https://doi.org/10.3390/coatings15091042

AMA Style

Xu L, Zhu W, Chen X, Liu X. Applications and Challenges of Modern Analytical Techniques for the Identification of Plant Gum in the Polychrome Cultural Heritage. Coatings. 2025; 15(9):1042. https://doi.org/10.3390/coatings15091042

Chicago/Turabian Style

Xu, Liang, Weijia Zhu, Xi Chen, and Xinyou Liu. 2025. "Applications and Challenges of Modern Analytical Techniques for the Identification of Plant Gum in the Polychrome Cultural Heritage" Coatings 15, no. 9: 1042. https://doi.org/10.3390/coatings15091042

APA Style

Xu, L., Zhu, W., Chen, X., & Liu, X. (2025). Applications and Challenges of Modern Analytical Techniques for the Identification of Plant Gum in the Polychrome Cultural Heritage. Coatings, 15(9), 1042. https://doi.org/10.3390/coatings15091042

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