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Review

Application and Challenges of Plant Oil Detection Techniques in the Conservation of Polychrome Cultural Relics

by
Peng Zhu
1,*,
Chang Shu
1,
Wei Wang
2 and
Xinyou Liu
2
1
School of Marxism, Nanjing Agricultural University, Nanjing 210014, China
2
College of Furnishing and Industrial Design, Nanjing Forestry University Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(9), 1049; https://doi.org/10.3390/coatings15091049
Submission received: 15 July 2025 / Revised: 28 August 2025 / Accepted: 5 September 2025 / Published: 8 September 2025
(This article belongs to the Section Environmental Aspects in Colloid and Interface Science)
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Abstract

The identification of plant oils in polychrome cultural relics is crucial for understanding historical craftsmanship and for developing appropriate conservation strategies. Historically, plant oils were used as binders, protective coatings, and plasticizers, directly influencing the stability and appearance of artifacts. Their degradation—through oxidation, hydrolysis, and environmental exposure—makes accurate detection challenging. Recent advances in spectroscopic methods (Fourier-Transform Infrared Spectroscopy, Raman), chromatographic techniques (Gas Chromatography–Mass Spectrometry, High-Performance Liquid Chromatography), and mass spectrometry imaging (Desorption Electrospray Ionization—Mass Spectrometry Imaging) enable non-invasive or minimally invasive analysis of oils, even within complex matrices. Case studies, including the Meiwu ceiling of the Palace Museum and resin–oil varnishes, illustrate how multi-method approaches improve reliability. Ongoing challenges include interference from degradation products, limited sampling due to ethical concerns, and the absence of comprehensive reference libraries. Future research should prioritize non-destructive techniques, standardized protocols, and interdisciplinary collaboration to enhance the precision and applicability of plant oil identification in cultural heritage conservation.

1. Introduction

Polychrome cultural relics are key witnesses to historical craftsmanship and artistic practice. Their preservation is often threatened by fluctuations in temperature and humidity, light exposure, microbial attack, and the inherent instability of materials, which can lead to pigment loss, binder failure, and surface degradation [1,2]. Among traditional organic materials, plant oils are particularly significant. They functioned as binders, coatings, and plasticizers, and their chemical transformations strongly affect the long-term stability of artifacts [3].
In conservation science, a distinction is made between detection—confirming the presence of oils—and identification, which specifies the type of oil (e.g., linseed, walnut, tung). Accurate identification provides essential information about historical techniques and materials. Fatty acid profiling can help trace the source oil, while evaluating oxidative and polymerization states reveals the extent of aging and degradation, which directly informs restoration strategies [4]. Traditional techniques for plant oil identification, such as microchemical spot tests and light microscopy, are limited by their high sample consumption, invasiveness, and lack of molecular specificity [5]. Modern platforms now offer improved precision: spectroscopic methods (FTIR, Raman), chromatographic techniques (GC–MS, HPLC), and imaging mass spectrometry (DESI-MSI) enable molecular-level analysis with reduced sampling requirements [6,7].
It is also important to contextualize oil usage in different cultural settings. In Western oil painting traditions, particularly from the Renaissance onward, drying oils such as linseed were commonly used as binders in multi-layered canvas and wooden panel paintings. These were often prepared with lead-based grounds to promote drying and adhesion. By contrast, Far Eastern polychrome artworks, such as those in Chinese and Japanese temple murals, were typically created on earthen, lime-based, or clay-coated walls, with silk, paper, or lacquered wood substrates. In these contexts, plant oils like tung or perilla oil were used selectively—often mixed with natural resins for gloss or waterproofing [8,9]. Cross-cultural exchanges further shaped practices: Japanese export furniture incorporated linseed oil finishes under Western influence [9], and Chinese murals frequently combined oils with proteinaceous or resinous binders to improve durability [10]. Comparative pigment studies also suggest that European oil-based methods gradually influenced East Asian applied arts during the early modern period [11].
Despite technological progress, major challenges remain. Complex substrate matrices and interactions among aged materials can obscure diagnostic signals [12]. In addition, ethical restrictions on destructive sampling have encouraged a shift toward non-contact and micro-destructive methodologies [13]. This review systematically examines current strategies for detecting and identifying plant oils, compares their respective strengths and limitations, and proposes integrated approaches for the conservation of polychrome cultural relics.

2. The Role and Impact of Plant Oils in Polychrome Cultural Relics

2.1. Plant Oils as Binders

Plant oils have been extensively employed as traditional organic binders in historically significant polychrome relics, including wooden artifacts, murals, religious paintings, and lacquerware [14]. Their primary function is to disperse pigment particles uniformly and secure them to the substrate, ensuring effective adhesion between the paint layer and base material. Owing to their favorable fluidity and moderate viscosity, plant oils offer excellent workability, enabling control over pigment consistency and modulation of color transparency and saturation—thereby enhancing visual richness and dimensionality [15]. Historically, drying oils such as linseed oil (from flax), walnut oil, and tung oil were most commonly used. Linseed oil, rich in α-linolenic acid, was prized in European painting for its strong film-forming ability and was often combined with resins or metallic dryers to accelerate curing [16]. Walnut oil, valued for its paler color and slower yellowing, was frequently found in Northern Renaissance works, while tung oil was extensively used in East Asian art and furniture coatings, often blended with lacquer to improve gloss and water resistance [9]. Early Chinese treatises such as the Tiangong Kaiwu and Xuanhe Painting Manual describe recipes involving oil-resin mixtures and multiple-layered applications for durability and finish [10].
During drying, autoxidation drives the formation of a cross-linked polymer network that solidifies into a flexible film [17]. Though slow-curing, this film provides strength and elasticity, reinforcing paint layer durability and structural stability [18]. Thus, plant oils serve not only as binding agents but also as multifunctional components critical to both aesthetic expression and artifact preservation.

2.2. Plant Oils’ Role in Pigment Adhesion

As key historical binding media, plant oils significantly influence pigment adhesion and long-term preservation in polychrome relics [19]. Analyses of polychrome sculptures and wall paintings confirm that vegetable oils—used alongside materials like animal glue—enhanced structural integrity and adhesion, notably exemplified in Tang Dynasty adhesives [20]. Experimental evidence indicates that oxidation and cross-linking during curing alter the paint film’s physical properties: while oxidation facilitates film formation, it also introduces vulnerabilities such as embrittlement and pigment detachment, which compromise long-term stability [21]. In addition to their mechanical function, plant oils such as linseed and tung oil exhibit natural antimicrobial properties, primarily due to the presence of polyunsaturated fatty acids and minor bioactive constituents (e.g., phenolics, aldehydes, terpenoids) that inhibit fungal and bacterial growth [22]. For instance, controlled in vitro studies demonstrated that linseed oil suppresses the growth of Aspergillus niger and Penicillium spp., both of which are frequently identified in deteriorated mural samples [23]. Similarly, tung oil’s polymerized surface was shown to resist microbial colonization in laboratory simulations, consistent with its historical use in Chinese architectural coatings [24]. Such protective mechanisms likely contribute indirectly to preserving pigment adhesion by reducing biological damage to the binder-substrate interface. Advances in non- and micro-destructive analytical techniques now enable in-situ examination of oil-pigment interactions, revealing compatibility and adhesion dynamics in relics [25,26]. Ultimately, understanding plant oils’ aging chemistry, pigment interactions, and protective potential is essential for refining cultural heritage conservation strategies.

2.3. Aging Processes of Plant Oils and Their Impact on Preservation

The aging of plant oils in painted relics profoundly affects their preservation, involving complex chemical and physical alterations over time. Photodegradation, particularly photo-oxidation induced by UV and visible light, is a dominant pathway of deterioration in alkyd and oil-based paints [27]. This process is driven by radical-mediated autoxidation kinetics, wherein hydroperoxides initiate chain scissions and form chromophoric degradation products that lead to yellowing, brittleness, and embrittlement of the paint matrix [28]. Long-term monitoring using real-time FTIR and GC–MS has confirmed that factors such as exposure duration, light intensity, and atmospheric oxygen significantly accelerate fragmentation processes, directly correlating with the visible yellowing and cracking observed in aged oil paintings [29].
Moisture interaction further compounds degradation. The sorption–desorption behavior of oil films under fluctuating relative humidity causes swelling, hydrolysis of esters, and disruption of the pigment-binder interface. Though well documented in cellulose-based heritage, oil-based binders also exhibit increased susceptibility to hydrolytic cleavage and microbial colonization under humid conditions, particularly when pre-exposed to UV light [30,31]. These findings belong primarily to the diagnostic study of degradation pathways; in the subsequent section, we therefore turn to conservation operations, such as the use of solvents for cleaning, to illustrate how this knowledge informs practical intervention. These synergistic effects highlight the need for precise microclimate control in exhibition and storage environments.

3. Common Identification Techniques for Plant Oils in Polychrome Cultural Relics

The accurate scientific identification of plant oils in polychrome cultural relics is critical for understanding historical craftsmanship, trade networks, and material usage across regions and periods. More importantly, it forms the technical foundation for the development of appropriate conservation and restoration strategies. In recent years, particularly over the past five years, significant progress has been made in analytical methodologies, allowing for more precise and non-invasive analysis of binding media, including plant-derived oils and waxes.
While diagnostic techniques focus on identifying plant oils and their degradation, subsequent conservation treatments—including solvent cleaning—require careful methodological transition. Hence, this section first reviews diagnostic and analytical methods, followed later by discussions of restoration implications.
In the field, a variety of analytical techniques are routinely employed, each offering distinct advantages and varying levels of resolution. Fourier-transform infrared spectroscopy (FTIR) enables rapid, non-destructive identification of functional groups associated with lipids and esters, while gas chromatography–mass spectrometry (GC–MS) is considered the gold standard for fatty acid profiling and oxidation studies, and liquid chromatography–mass spectrometry (LC–MS) is more suitable for polar lipids and additives. For complex organic matrices like lacquer or aged oil mixtures where traditional extraction is insufficient, pyrolysis–GC–MS (Py–GC–MS) proves especially useful. Liquid chromatography–mass spectrometry (LC–MS) is suitable for detecting polar lipid compounds, natural additives, and trace binding components, whereas optical microscopy (OM) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS) are applied for morphological assessment and elemental mapping, particularly in layered or degraded samples. Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) allow in-situ, micro-scale identification of organic compounds, reducing the need for destructive sampling, and stable isotope ratio analysis is used to trace the geographic origin or authenticity of natural oils, especially in archaeological contexts.
Each technique presents unique strengths and limitations. For example, FTIR is favored for its speed and minimal invasiveness but lacks the molecular specificity of GC–MS. Conversely, while GC–MS delivers high-resolution molecular information, it typically requires destructive sampling and solvent extraction. Therefore, the choice of technique must be tailored to the characteristics of the sample, conservation requirements, and the specific analytical questions posed. In complex or stratified polychrome systems, the integration of multiple techniques is often necessary to yield reliable and comprehensive results. To illustrate recent applications of these techniques, Table 1 presents a summary of plant oil identification in various types of polychrome artifacts from studies conducted.
This table underscores the diversity of both analytical techniques and oil types identified across different polychrome contexts. Linseed oil remains the most frequently encountered medium, particularly in easel and wall paintings, while specialized uses of tung oil or perilla oil are commonly associated with lacquerware. The inclusion of proteinaceous components in painted manuscripts and icons also reflects regional and chronological variation in media recipes.

3.1. Spectroscopic Analysis

Spectroscopic techniques have emerged as indispensable tools in the analysis of plant oils within polychrome cultural relics, owing to their non-destructive nature, high sensitivity, and capability to provide molecular-level insights. These techniques facilitate the characterization of both organic binders and inorganic pigments, supporting multi-layered analysis of complex paint structures without damaging the original artifacts.
One of the most widely used tools is Fourier-transform infrared spectroscopy (FTIR)—particularly Attenuated Total Reflectance FTIR (ATR-FTIR). This technique is effective in detecting functional groups associated with triglycerides, fatty acids, and esters in drying oils. However, it is important to note that FTIR is better suited for preliminary detection rather than precise compound-level identification. As highlighted by Liu and Kazarian (2021), while FTIR spectra can confirm the presence of lipid-like materials via characteristic absorption bands (e.g., C=O, CH2), it lacks the specificity to distinguish between similar oil sources such as linseed vs. walnut oil without complementary data [38]. Thus, FTIR should ideally be used in conjunction with chromatographic or mass spectrometric techniques for conclusive results.
Raman spectroscopy, including surface-enhanced Raman spectroscopy (SERS), offers highly localized, non-invasive analysis suitable for in-situ detection of plant oils, even within pigment-rich matrices. Ma et al. (2025) reviewed recent advancements in portable Raman systems and emphasized their capacity to identify minor organic components, including aged oil films and binder-pigment interactions, particularly when paired with microscopic mapping [39].
A significant recent advancement is the incorporation of hyperspectral imaging (HSI) in cultural heritage diagnostics. This method, as demonstrated by Picollo et al. (2020), enables the collection of continuous spectral information across the visible to near-infrared range, combining spatial and spectral resolution [40]. HSI has proven capable of identifying organic binders such as oils and waxes based on their distinct spectral fingerprints, especially in stratified or degraded samples [41,42]. Its non-contact nature and ability to cover large surface areas make it ideal for mural paintings and architectural decorations.

3.2. Chromatographic Analysis

Before detailing chromatographic approaches, it is worth noting the role of advanced mass spectrometry techniques. Desorption electrospray ionization (DESI) allows direct surface analysis with minimal preparation, making it highly suitable for fragile artworks. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) provides rapid molecular profiling of lipids and additives, while quadrupole time-of-flight mass spectrometry (QTOF-MS) combines high mass accuracy with structural elucidation. These methods bridge the gap between traditional chromatography and non-invasive imaging, and they are particularly valuable when integrated with GC or LC workflows.
Chromatographic analysis has proven highly effective for identifying plant oils in polychrome cultural relics, particularly in profiling complex lipid compositions. This technique enables both qualitative and quantitative assessment of oil components, such as triacylglycerols (TAGs), fatty acids, and sterols [43,44,45,46]. Gas chromatography (GC), often coupled with MS, is optimal for volatile fatty acids and oxidation products, while liquid chromatography (LC) is better for non-volatile or thermally labile lipids.
High-performance liquid chromatography (HPLC) has been widely applied to distinguish plant oils based on their TAG composition. For instance, Jakab et al. (2002) [45] demonstrated the use of HPLC-APCI-MS in differentiating oils from peanuts, pumpkin seeds, sesame seeds, soybeans, and wheat germ using gradient elution on monolithic silica columns. Further, Hu et al. (2014) [44] and Jakab et al. (2002) [47] highlighted the utility of HPLC-MS in generating distinctive TAG “barcodes,” serving as visual tools for oil authentication.
Such chromatographic profiles not only reveal the botanical origin of oils but also assist in evaluating their quality and authenticity—factors crucial when analyzing heritage artifacts where preserving original materials is essential [43]. Additionally, the application of Principal Component Analysis (PCA) to chromatographic data enhances the ability to distinguish between different plant oils [44].
The integration of chromatographic and spectrometric methods, particularly MS, further expands analytical capability by enabling precise determination of molecular mass and fatty acid composition. This is especially valuable for cultural relics, where accurate identification of original plant oils supports targeted conservation and authentication efforts.
In practice, the reliability of chromatographic and mass spectrometric analyses depends heavily on sampling protocols. Only micro-destructive sampling (e.g., <50 μm cross-sections) is ethically permissible in most cases. Pre-treatment may include solvent extraction, derivatization (e.g., methylation of fatty acids for GC-MS), or embedding in transparent media for sectioning. Best practice requires balancing analytical sensitivity with preservation ethics, making sample minimization a key principle.

3.3. Microscopic Examination

Microscopic examination techniques are indispensable in the multi-scale characterization of polychrome cultural relics. These methods provide direct visual and morphological evidence of binding media distribution, stratigraphy, and degradation phenomena. In particular, optical microscopy (OM) and zoom microscopy are frequently applied to observe microcracking, layer transitions, and the dispersion of materials [48]. For example, a study of Roman frescoes from Rapoltu employed these techniques to distinguish plant oil residues from protein-based binders based on microstructure and surface sheen [49].
A more advanced technique, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), allows high-resolution imaging of cross-sections while simultaneously providing elemental composition data. This dual capability enhances the identification of oil-based residues by correlating organic matrices with mineral pigments and filler phases, especially in aged or contaminated samples [50].
Importantly, UV-induced fluorescence microscopy (UV-FM) has emerged as a sensitive tool for detecting plant oils based on their characteristic fluorescence emission under ultraviolet illumination. This method allows for rapid, non-destructive mapping of oil-rich domains in complex paint layers. As noted in recent studies [51], plant oils often exhibit blue-green autofluorescence that can be distinguished from proteins or synthetic polymers. UV-FM thus enhances pre-screening workflows before sampling or further chemical analysis. Together, these microscopic techniques provide vital complementary information to chemical analyses, enabling accurate spatial localization and physical characterization of plant oil-based materials.

3.4. Molecular-Level Biochemical Analysis

A detailed understanding of plant lipid biosynthesis pathways provides an essential biochemical foundation for identifying plant oils in cultural heritage contexts. At the center of this regulatory network is the transcription factor WRINKLED1 (WRI1), a master regulator of fatty acid biosynthesis in higher plants [52]. Initially characterized in Arabidopsis thaliana, WRI1 governs the expression of genes involved in glycolysis and triacylglycerol (TAG) synthesis, including BCCP2, PKP-β1, and DGAT1 [53].
The functional activity of WRI1 is modulated by upstream regulators such as LEC1, LEC2, FUS3, and ABI3, which coordinate lipid accumulation during seed maturation. These transcriptional cascades are highly conserved but also exhibit species-specific variation, directly influencing the fatty acid composition of plant oils [54]. The TAGs synthesized via these pathways—comprising three fatty acids esterified to a glycerol backbone—represent the primary lipid form detected in oil-based binders.
In the context of cultural relic analysis, such molecular insights support the identification of lipid biomarkers, including monoacylglycerols, dicarboxylic acids, and hydroxy fatty acids, which are diagnostic of specific plant sources. These compounds serve as molecular fingerprints that can differentiate linseed oil from tung oil, or identify degraded drying oils via their oxidation profiles [55,56].
Recent advances in plant metabolic engineering have also shed light on lipid pathway manipulation. For example, overexpression of WRI1 or downstream genes has been used to alter oil yield and stability, offering insight into how natural or modified oils might behave in historical materials. Such knowledge not only enhances the biochemical resolution of oil analysis but also aids in predicting degradation behavior.
In summary, WRI1 serves as a biochemical nexus linking gene regulation to fatty acid biosynthesis, providing a mechanistic basis for understanding and tracing plant oils in cultural artifacts. Molecular-level biochemical approaches, when paired with spectroscopy and microscopy, substantially enhance the accuracy of binding medium identification and support data-driven conservation strategies.

4. Application Cases of Plant Oil Identification Techniques in Polychrome Relics Conservation

Plant oil identification techniques are best understood through concrete case studies that demonstrate both methodological strengths and analytical challenges. The following three cases illustrate how multi-analytical strategies, spectroscopic microanalysis, and advanced chromatographic–mass spectrometric platforms have been deployed in increasingly complex contexts. Together, they reveal a methodological trajectory: from ambiguous signals and overlapping binders, through micro-scale visualization of degraded oils, to the molecular resolution of hybrid resin–oil coatings.

4.1. Case Study 1: Multi-Analytical Characterization of Organic Binding Media in the Meiwu Ceiling, Palace Museum

An exemplary application of integrated analytical strategies for identifying and characterizing organic binders—particularly plant oils—is demonstrated in the 2022 study of the Meiwu ceiling in the Hall of Mental Cultivation at Beijing’s Palace Museum [55]. This investigation focused on two cultural relic samples from the ceiling: one comprising the complete multi-layered structure of paper and fabric composites (outermost layer shown in Figure 1d; innermost layer in Figure 1e), and another consisting of an ink-rich fabric fragment isolated from these layers (Figure 1f). As a historically significant architectural feature, the Meiwu ceiling presents complex stratigraphy in its decorative layers. In contrast to single-medium Western easel paintings, the coexistence of starch, proteins, and potentially oils makes this ceiling a valuable test case for integrated methodologies in Chinese heritage [57,58]. The layered structure underscores the analytical challenge of distinguishing plant oils from starch, proteins, and resins when multiple organic binders co-exist, highlighting the need for molecularly specific approaches.
Although the study ultimately identified starch as the primary organic adhesive through potassium iodide staining and characteristic IR absorption (~930 cm−1 for polysaccharide glycosidic linkages), micro-FTIR spectroscopy also revealed carbonyl stretch signals around 1717 cm−1, which are diagnostic of oxidized fatty acids [59,60]. Such bands are classic indicators of drying oil oxidation, suggesting the possible co-application of oils or overlapping degradation processes with starch. Comparable FTIR-based ambiguities have been reported in Ming and Qing dynasty wall paintings, where starch and oil degradation products overlap in the carbonyl region [61,62]. This highlights the importance of careful spectral interpretation, as aged starches and drying oils may exhibit similar functional groups due to oxidative polymerization and hydrolytic cleavage (Figure 2).
Mass spectrometry imaging using DESI-MSI provided a powerful, non-destructive means of mapping molecular distributions directly on the artifact surface. Although the technique successfully localized dyes such as eosin Y (m/z 279) and malachite green, as well as cellulose derivatives (m/z 259), no definitive evidence of plant oils was detected—likely due to their low abundance or interference from the complex matrix [63]. Nonetheless, DESI demonstrated strong potential for future oil diagnostics, particularly for detecting oxidized triglycerides or metal soaps, provided that expanded spectral libraries become available (Figure 3). This is supported by studies confirming the high sensitivity of DESI-MSI to copper carboxylates resulting from oil–pigment interactions, indicating its applicability to the analysis of traditional East Asian mural and architectural surfaces [64].
Complementary elemental mapping (MA-XRF) revealed the presence of chromium and titanium in pigment-bearing layers, both historically associated with oil-based media. SEM-EDS (SIRION-100, FEI (Hillsboro, OR, USA)) and XRD (AXS D8 ADVANCE (Karlsruhe, Germany))further confirmed mineral extenders such as calcite, quartz, and mica—materials known to chemically interact with fatty acids during the drying and aging of oils. These inorganic–organic interactions are consistent with observations from Chinese architectural mortars prepared with tung oil–lime mixtures, where fatty acids form stable complexes with Ca-based compounds through the formation of calcium carboxylates, as identified via FT-IR and XRD analysis [65]. These findings reinforce the plausibility of oil use despite the absence of conclusive direct evidence (Figure 4).
The combined use of microstructural, vibrational, chromatographic, and imaging techniques enabled a detailed reconstruction of the ceiling’s material history, revealing at least four distinct construction phases, each associated with different technologies. Such stratigraphic resolution would have been unattainable without the synergy of these methods. Importantly, this case demonstrates that even negative or ambiguous results in oil detection are methodologically valuable, as they help refine diagnostic strategies and highlight analytical limitations.
Beyond technical insights, the Meiwu ceiling study contributes to broader discussions of binder selection in Qing dynasty architecture. By showing that starch was dominant but with potential oil admixtures, the case complements recent findings from Kizil Grottoes murals, where tung oil was deliberately blended with proteins to enhance water resistance [10]. These comparative results indicate a regional adaptation of oil–starch or oil–protein composites, reflecting practical responses to environmental challenges such as humidity control.
From a conservation perspective, the study underscores the necessity of multi-method analysis in distinguishing between visually similar but chemically distinct binders. Determining whether a degraded layer results from starch, protein, or oil-based media is critical for selecting appropriate solvents, consolidants, and intervention techniques. Moreover, the potential of non-invasive imaging technologies to monitor oil degradation products offers conservators a minimally invasive decision-making framework for fragile painted heritage.

4.2. Case Study 2: FTIR-Based Microanalysis for Plant Oil Binder Identification in Multilayered Paint Systems

A pivotal contribution to advancing plant oil identification in heritage materials is provided by Prati et al. (2010) [56], whose work at the Microchemistry and Microscopy Art Diagnostic Laboratory (M2ADL) at the University of Bologna significantly enhanced the capabilities of Fourier Transform Infrared (FTIR) microscopy and imaging. Their study refined the spatial resolution and analytical depth of FTIR spectroscopy while directly addressing a primary challenge in oil-based binder identification: detecting aged, oxidized plant oils embedded within complex paint stratigraphies. This is especially relevant to polychrome relics where binders are often multi-sourced and chemically altered, such as European panel paintings, Himalayan murals, and Chinese lacquerware.
In Figure 5, the detection of copper carboxylates indicates direct chemical interaction between copper pigments and fatty acid degradation products, serving as a diagnostic marker of oil-based binders. Such pigment–binder interactions have also been observed in Ming dynasty wall paintings, where verdigris and drying oils formed stable metal carboxylates that preserved green tonalities [32]. Methodological insight: Such spectral markers highlight how pigment–binder reactions can be exploited to confirm the presence of oils even when they are highly degraded. This research examined a variety of polychrome artifacts, including mural paintings and panel works from European and Himalayan contexts where plant oils were frequently used as binding media. A particularly revealing application was the analysis of a green paint layer from the 15th-century Thubchen Lhakhang temple mural in Lo Manthang, Nepal. There, FTIR imaging—combined with ATR mapping and complementary Raman spectroscopy—enabled the identification of highly degraded siccative oils alongside malachite and copper-based pigments [66].
Crucially, the study employed an integrated FTIR microscope (Thermo iN10MX, Thermo Fisher Scientific, Madison, WI, USA), which allowed raster scanning with enhanced spatial resolution down to 6 µm. This resolution permitted the clear visualization of pigment-oil interactions in thin layers that would otherwise evade detection, as demonstrated in reconstructed paint samples where lead white and calcite layers as thin as 10–14 µm were successfully differentiated (Figure 6). The enhanced resolution confirmed that oils persist even in microlayers, identifiable via carbonyl and ester peaks. Methodological insight: This precision underscores the importance of instrument resolution when sampling must remain minimal, offering a pathway to detect oils in fragile or limited contexts [67].
To further address interference from embedding resins during cross-section preparation, the research introduced IR-transparent embedding salts like KBr and BaF2. This innovation significantly improved the detection of organic compounds such as oil residues by minimizing spectral contamination and enabling direct binder identification. The authors demonstrated that the signal-to-noise ratio (S/N) in the spectral range relevant to oil oxidation products (e.g., 1710–1740 cm−1 for C=O stretch) is substantially enhanced using KBr-embedded cross sections (Figure 7) [68]. Subsequent conservation studies have adopted this strategy for fragile mural fragments from the Silk Road, confirming its utility in detecting weak oil signals otherwise masked by resin [69]. Embedding with IR-transparent salts clarified oil-related bands that would otherwise be obscured. Methodological insight: Optimized embedding is not a mere preparatory step but a decisive factor for whether oil signatures can be retrieved from fragile cross-sections.
In broader conservation terms, this study demonstrates a transferable framework: the integrated use of transmission and ATR-FTIR, far-IR spectroscopy, Raman mapping, and optimized embedding enables the detection of oils that are partially saponified, chemically transformed, or present in trace amounts within inorganic matrices. The methodological innovations pioneered here directly informed later applications in Chinese polychrome relics, where similar challenges of starch–oil–protein overlap demand equally high-resolution approaches. Together, these refinements show how tailored sampling, embedding, and resolution can unlock oil detection where conventional FTIR fails, thereby providing conservators with a more reliable diagnostic workflow for multilayered or degraded systems [70].

4.3. Case Study 3: GC-QToF-MS Analysis Reveals Plant Oil Usage in Ancient Resinous Coatings

A cutting-edge demonstration of how high-resolution analytical chemistry can uncover plant oils in complex archaeological mixtures is presented in Tamburini et al.’s 2024 study [71]. This research pioneered the application of gas chromatography coupled to quadrupole time-of-flight mass spectrometry (GC-QToF-MS) for characterizing mastic resin (Pistacia sp.) in ancient artifacts. Unlike earlier GC–MS protocols limited to fatty acid profiling [72], the QToF-MS platform enabled simultaneous resolution of triterpenoid and lipid markers, offering a more integrated molecular perspective.
The study investigated diverse samples: resin lumps from the Late Bronze Age Uluburun shipwreck (ca. 1320 BCE) [73], burnt residues from 18th Dynasty ceramic incense burners on Sai Island (Sudan), and varnish layers on ancient Egyptian coffins and cartonnage from the British Museum and Fitzwilliam Museum. Across these materials, key triterpenoid markers (e.g., oleanonic acid, masticadienonic acid, moronic acid) confirmed mastic resin use [74]. Importantly, fatty acid esters and oxidized lipid markers were also detected, confirming the co-use of linseed or walnut oil with resins and thus providing chemical evidence for composite “oil–resin varnishes” widely hypothesized in historical practice. Similar findings have recently been reported in Chinese architectural decorations, where THM-Py-GC/MS revealed tung oil mixed with resinous components, suggesting a convergent technological tradition in East and West [75].
This conclusion was supported by two complementary analytical strategies. First, chemical profiling via GC-QToF-MS, under optimized chromatographic conditions, enabled the separation and structural elucidation of over 25 triterpenoid compounds, often as derivatized TMS esters; crucially, it also revealed degradation products and molecular features consistent with lipidic matrices, including fatty acid esters characteristic of aged plant oils. The co-distribution of triterpenoids and fatty acids strongly suggests that oils acted as solvents or carriers for resin application, aligning with historical accounts of resin–oil varnishes. This interpretation resonates with European Renaissance treatises on “vernice liquida,” where mastic resin was cooked with linseed oil to produce glossy, durable varnishes [76]. Second, thermal degradation experiments demonstrated that controlled heating of reference Pistacia resin produces the compound 28-norolean-17-en-3-one, which forms upon heating but degrades over time [71]; this compound served as a diagnostic marker for ancient thermal processing of resin-oil mixtures, helping distinguish them from resin-only layers. Importantly, the absence of this marker in some archaeological samples, despite other signs of heating, underscored the need for caution in marker-based interpretations.
These molecular findings align with known historical practices: Mastic resin, solid at room temperature, requires a solvent or oily carrier for application as a coating. Given the lack of evidence for distillation in ancient Egypt [77], the researchers propose that boiling the resin in drying oils was the likely application method—a technique mirroring medieval and Renaissance “oil varnish” formulations. This hypothesis is further supported by the identification of co-distributed lipid-resin matrices in coffin varnishes, observable even in visually blackened and aged materials. Such evidence of intentional oil–resin blending underscores a continuity of technical knowledge across millennia, paralleling East Asian practices of combining tung oil with lacquer for hydrophobicity and gloss [78].
From a methodological perspective, this case illustrates the strength of high-resolution GC-QToF-MS: it can simultaneously resolve triterpenoid resin markers and oil degradation products, enabling the study of hybrid organic coatings that were previously difficult to characterize. Such integration not only advances molecular archaeology but also provides conservationists with tools to reconstruct ancient recipes and to design historically informed conservation materials.

4.4. Synthesis of Cases

Taken together, the three case studies form a progressive methodological narrative. Case 1 highlights the ambiguity of overlapping signals and the value of multi-analytical corroboration. Case 2 demonstrates how micro-scale spectroscopic refinements can retrieve oil signatures in stratified contexts. Case 3 extends this trajectory into molecular archaeology, where advanced chromatography–mass spectrometry resolves hybrid resin–oil varnishes.
For conservation practice, this synthesis underscores the necessity of multi-method, resolution-aware, and cross-culturally comparative approaches. Whether diagnosing ambiguous FTIR bands, detecting microlayered oils, or characterizing resin–oil composites, the key lies in tailoring methods to both the material complexity and the ethical constraints of heritage science.

5. Challenges and Future Directions in Plant Oil Identification for Polychrome Relics

The precise identification of plant oils within polychrome cultural relics remains fraught with significant challenges, necessitating ongoing innovation. Key obstacles and promising research avenues are outlined below:

5.1. Persisting Challenges

The analysis of plant oils in aged polychrome artifacts is complicated by the complex composition of the artifact matrix and the extensive degradation of the oils themselves. These materials typically include a mix of pigments, binders, coatings, and degradation products that interfere with analytical signals. Aged oils undergo significant chemical changes—hydrolysis, oxidation, polymerization—that obscure or eliminate key molecular markers, often interacting with pigments to form metal soaps and overlapping signals (e.g., carbonyl bands or chromatographic peaks). While non-invasive and micro-invasive tools such as FTIR, Raman, and reflectance spectroscopy are preferred for ethical reasons, they frequently lack the sensitivity and specificity needed to detect or differentiate trace, degraded oils. In contrast, destructive techniques such as GC-MS or ambient mass spectrometry offer deeper insight but are limited by the ethics of sample collection in heritage contexts.
Another persisting challenge concerns the balance between chromatographic methods: gas chromatography (GC) remains the gold standard for separating volatile and thermally stable lipid derivatives, whereas liquid chromatography (LC) is more suitable for non-volatile or thermally labile components. However, their complementary nature is often underutilized in cultural heritage studies, leading to incomplete interpretation.
Furthermore, a major limitation lies in the interpretation of data: the absence of comprehensive aged reference libraries and the complexity of multi-modal spectral data require advanced chemometric expertise, which is often inaccessible due to institutional, technical, or financial constraints.

5.2. Future Directions

To overcome these challenges, future work must prioritize the development of portable and high-resolution non-invasive analytical tools capable of in situ molecular identification with minimal or no sampling. Advancements in ambient spectroscopy, hyperspectral imaging, and mass spectrometry—such as DESI-MSI or MALDI-MSI—have the potential to revolutionize this field, especially if optimized for use in heritage environments.
Equally critical is the establishment of comprehensive aged reference libraries representing multiple plant oils, pigments, and degradation pathways under realistic conditions. The integration of artificial intelligence and machine learning for spectral pattern recognition and automated interpretation will be essential to handle the complexity of multi-modal data, reduce operator bias, and accelerate diagnostic workflows.
In addition, comparative research should strengthen the discussion of GC versus LC in cultural heritage contexts, clarifying their respective strengths: GC for volatile fatty acid methyl esters and oxidation products, LC for intact triglycerides, polar lipids, or thermally fragile molecules. Such distinctions will help refine protocols for different artifact types.
Parallel to this, standardized analytical protocols must be developed to harmonize sampling strategies, instrumental settings, and data processing across research teams, ensuring reproducibility and comparability of results. Finally, as noted earlier, plant oil research in cultural relics must be understood not only as a technical challenge but also as a bridge between scientific inquiry and conservation practice. Developing methodologies that both enhance analytical precision and respect the ethics of heritage protection will be the cornerstone of progress.

6. Conclusions

The identification of plant oils in polychrome relics sits at the intersection of science and heritage preservation. These oils, historically employed as binders and surface treatments, contributed to both the function and aesthetics of artworks across cultures. Modern techniques—spectroscopic imaging, chromatographic mass spectrometry, and ambient MSI—have enhanced our ability to detect these components, even in minimally sampled contexts.
Yet, persistent gaps remain. Degradation obscures markers critical for oil identification, while analytical trade-offs between non-invasiveness and sensitivity continue to limit outcomes. Equally important is the methodological balance between gas chromatography (GC), which excels in detecting volatile fatty acid derivatives, and liquid chromatography (LC), which is more suitable for intact or thermally labile lipid molecules. The lack of systematic integration of these complementary methods remains a bottleneck. Regional research biases, particularly between Eastern and Western traditions, hinder broader comparative understanding. Ethical concerns over sampling and a lack of standardized protocols or aged material databases further complicate analysis.
Moving forward, progress hinges on interdisciplinary collaboration. Priority should be given to developing portable, high-resolution, non-invasive tools, constructing comprehensive aged reference libraries, and integrating AI-driven chemo-metric interpretation to manage multi-modal data complexity and reduce subjective bias. Coordinated frameworks that unify conservation science, materials analysis, and data modeling are essential.
Ultimately, advancing plant oil identification requires not only technical refinement but also a conscious effort to link methodology with conservation practice. By aligning analytical precision with ethical stewardship, future research can reconstruct historical material recipes while guiding preservation strategies. Unlocking the material stories of polychrome artifacts thus depends on a shared, globally inclusive vision that combines scientific rigor with cultural responsibility.

Author Contributions

Conceptualization, P.Z. and W.W.; Methodology, W.W. and X.L.; Validation, W.W., X.L. and P.Z.; Formal Analysis, W.W. and X.L.; Investigation, W.W. and X.L.; Resources, W.W. and X.L.; Data Curation, W.W.; Writing—Original Draft Preparation, P.Z. and C.S.; Writing—Review & Editing, P.Z., C.S. and W.W.; Visualization, X.L.; Supervision, P.Z.; Project Administration, P.Z.; Funding Acquisition, P.Z. 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.

Informed Consent 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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Figure 1. The original condition (a–c) and sampling photos (d,e): outermost and innermost of overall structure; (f): printed fabric surface, of the Meiwu ceiling in the Hall of Mental Cultivation [55].
Figure 1. The original condition (a–c) and sampling photos (d,e): outermost and innermost of overall structure; (f): printed fabric surface, of the Meiwu ceiling in the Hall of Mental Cultivation [55].
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Figure 2. FTIR mapping of stratigraphic layers: (a) cross-section photomicrograph (50×); (b) representative examples of extracted spectra corresponding to the layer locations; (c) chemical images of top half-part (layers No. 3–11); (d) chemical images of bottom half-part (layers No. 1–4). Red and green represent relatively higher and lower intensity, respectively [55].
Figure 2. FTIR mapping of stratigraphic layers: (a) cross-section photomicrograph (50×); (b) representative examples of extracted spectra corresponding to the layer locations; (c) chemical images of top half-part (layers No. 3–11); (d) chemical images of bottom half-part (layers No. 1–4). Red and green represent relatively higher and lower intensity, respectively [55].
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Figure 3. Surface-specific composition imaging of the printed fabric layer with DESI-MSI: (a) selected area containing characteristic molecular ions, (b) m/z 146.9638 (−), (c) m/z 291.0132 and 351.1823 (−), (d) m/z 268.1768 (+), (e) m/z 329.2017 (+), (f) m/z 363.1808 (+), and (g) m/z 644.7028, 646.6999 and 648.7001 (−) [55].
Figure 3. Surface-specific composition imaging of the printed fabric layer with DESI-MSI: (a) selected area containing characteristic molecular ions, (b) m/z 146.9638 (−), (c) m/z 291.0132 and 351.1823 (−), (d) m/z 268.1768 (+), (e) m/z 329.2017 (+), (f) m/z 363.1808 (+), and (g) m/z 644.7028, 646.6999 and 648.7001 (−) [55].
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Figure 4. MA-XRF mapping for element-specific distributions containing Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, and Ba, respectively, corresponding to the top-left visible photograph of the selected area in the printed fabric pattern (element was listed in the top-left corner of each picture). Green and dark blue represent relatively higher and lower intensity, respectively [55].
Figure 4. MA-XRF mapping for element-specific distributions containing Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, and Ba, respectively, corresponding to the top-left visible photograph of the selected area in the printed fabric pattern (element was listed in the top-left corner of each picture). Green and dark blue represent relatively higher and lower intensity, respectively [55].
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Figure 5. (a) Composition of T2 sample as obtained by raster scanning analyses; (b) spectra of a green area indicating the presence of copper carboxylates and of a standard verdigris (spectrum registered in transmission) [56].
Figure 5. (a) Composition of T2 sample as obtained by raster scanning analyses; (b) spectra of a green area indicating the presence of copper carboxylates and of a standard verdigris (spectrum registered in transmission) [56].
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Figure 6. (a) Cross section photomicrograph of a green sample from the Salomone portrait with the indication of the area investigated; (b) same cross section photomicrograph taken at higher magnification indicating the area investigated with FTIR microscopy; (c) composition scheme of the sample obtained by mapping FTIR in ATR with a traditional microscope (aperture 80 µm × 80 µm, scans 64); (d) Composition scheme of the sample obtained by mapping FTIR in ATR with the integrated microscope (aperture 20 µm × 20 µm, scans 16) [56].
Figure 6. (a) Cross section photomicrograph of a green sample from the Salomone portrait with the indication of the area investigated; (b) same cross section photomicrograph taken at higher magnification indicating the area investigated with FTIR microscopy; (c) composition scheme of the sample obtained by mapping FTIR in ATR with a traditional microscope (aperture 80 µm × 80 µm, scans 64); (d) Composition scheme of the sample obtained by mapping FTIR in ATR with the integrated microscope (aperture 20 µm × 20 µm, scans 16) [56].
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Figure 7. Variation of the signal-to-noise ratio (S/N) with time measured in FTIR spectra acquired on the preparation layer of a malachite paint reconstruction embedded in KBr or BaF2 [56].
Figure 7. Variation of the signal-to-noise ratio (S/N) with time measured in FTIR spectra acquired on the preparation layer of a malachite paint reconstruction embedded in KBr or BaF2 [56].
Coatings 15 01049 g007
Table 1. Techniques for Identifying Plant Oils in Polychrome Cultural Relics.
Table 1. Techniques for Identifying Plant Oils in Polychrome Cultural Relics.
Polychromy TypeTechniques UsedIdentified Oils/WaxesReferences
Wall paintingsFTIR, GC–MS, OMLinseed oil, walnut oil[32]
LacquerwarePy–GC–MS, LC–MSTung oil, beeswax, perilla oil [33]
Painted sculptureFTIR, SEM–EDS, GC–MSLinseed oil, drying oils [34]
Painted manuscriptsGC–MS, FTIR, RamanWalnut oil, proteinaceous media[35]
Architectural décorSERS, GC–MS, Isotope analysisWalnut oil, poppyseed oil[36]
Mixed media iconsGC–MS, Protein/lipid ELISA, RamanEgg-oil emulsions, plant oils[37]
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Zhu, P.; Shu, C.; Wang, W.; Liu, X. Application and Challenges of Plant Oil Detection Techniques in the Conservation of Polychrome Cultural Relics. Coatings 2025, 15, 1049. https://doi.org/10.3390/coatings15091049

AMA Style

Zhu P, Shu C, Wang W, Liu X. Application and Challenges of Plant Oil Detection Techniques in the Conservation of Polychrome Cultural Relics. Coatings. 2025; 15(9):1049. https://doi.org/10.3390/coatings15091049

Chicago/Turabian Style

Zhu, Peng, Chang Shu, Wei Wang, and Xinyou Liu. 2025. "Application and Challenges of Plant Oil Detection Techniques in the Conservation of Polychrome Cultural Relics" Coatings 15, no. 9: 1049. https://doi.org/10.3390/coatings15091049

APA Style

Zhu, P., Shu, C., Wang, W., & Liu, X. (2025). Application and Challenges of Plant Oil Detection Techniques in the Conservation of Polychrome Cultural Relics. Coatings, 15(9), 1049. https://doi.org/10.3390/coatings15091049

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