Identification, deterioration, and protection of organic cultural heritages from a modern perspective

AdMaPlace March 16, 2025

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Identification, deterioration, and protection of organic cultural heritages from a modern perspective

Introduction

Cultural heritages are non-renewable and irreplaceable resources with immense historical, artistic, and scientific values¹. Natural organic substances are involved in many aspects of the cultural heritages, serving a multitude of utilitarian, decorative, and artistic purposes. These substances could reflect the local customs, economies, and even cultures of different eras², making them invaluable heritage material.

Organic cultural heritages encompass both protein-based heritages (silk and collagen) and cellulose-based heritages (paper and wood). Specifically, the main component of silk, fibroin, consists of 18 amino acids linked by peptide bonds. Type I collagen is a triple helix structure consisting of three interlocking polypeptide chains. Cellulose is composed of D-glucopyranosyl-β-1,4-D-glucopyranose chains. In contrast, as the main non-cellulose components of wood: hemicellulose has a polycrystalline structure, and lignin is composed of amorphous polymers of phenylpropane units connected by carbon-carbon and ether bonds. Despite their diverse molecular structures, fibroin, collagen, and cellulose exhibit common features at the higher structural level. The composition and structure of these biological materials confer them with valuable physical and mechanical properties, thereby facilitating their prolonged existence as heritages. The rich organic matter content and unique structures confer them with biological or chemical interaction, which might lead to a specific deterioration process of organic cultural heritages³. During prolonged periods of use and storage, organic cultural heritages are affected by various environmental factors, including light, temperature, humidity, and microorganisms, resulting in deterioration in structure and performance. To achieve effective conservation, it is crucial to understand the physical and chemical status of these organic cultural heritages, as well as the deterioration progress. The research on the deterioration mechanism of organic cultural heritages is superficial, and the internal deterioration mechanism is still unclear. Traditional analytical techniques are unable to achieve non-destructive or micro-destructive analysis. In addition, multiple analytical techniques cannot support each other, resulting in single test results. Protective approaches focus on physical restoration, such as: simple coating on the surface of heritages to supplement fading, surface cleaning, reinforcement with the help of adhesives, and internal filling with chemical substances. Modern analytical tools are extensively applied to analyze the aged organic cultural heritages, offering insights into their current state and deterioration pathways. An understanding of the deterioration mechanisms under diverse factors could facilitate the advancement of conservation technologies for these precious organic cultural heritages. The identification of their micro-structure and chemical composition is usually accurate but destructive, thus minimally invasive, non-destructive methods are needed⁴. Research on the deterioration of organic cultural heritages is a means to comprehend its mechanisms, which could provide a theoretical guide for restoration and conservation. In recent years, innovative restorative materials such as inorganic nanoparticles, microemulsions and gels, synthetic adhesives, protective coatings, and films^5,6,7,8 have been reported as protective materials. To preserve the historical value of cultural heritages, restoration interventions should be minimized to prevent secondary damage.

In the review, the organic cultural heritages based on the biopolymers of sericin proteins, collagen, and cellulose are described, with the aim of highlighting the recent progress in the identification of organic cultural heritages using modern analytical techniques. Besides, the deterioration mechanisms induced by environmental variables are included, with a view to describing the design and fabrication of eco-friendly conservation materials. In conclusion, the broader challenges for the future development of identification and restoration of organic cultural heritages are presented. This review is expected to provide guidance and inspiration from a modern perspective for characterization methods and the synthesis of green conservation materials for organic cultural heritages.

Organic cultural heritages

Silk fabrics

The development of the silk process is an indicative of the ongoing enhancement of human production. Silk cultural heritages have promoted the development of culture and art through the provision of distinctive luster, rich color, and mechanical strength. In ancient times, silk was extensively employed to produce textiles for clothing, shoes, and blankets (in Fig. 1a, b)⁹. Most of the silk cultural heritage was made from degummed Bombyx mori silk. Silk is a semi-crystalline biopolymer comprising highly ordered nanocrystals encased in an amorphous matrix. There are 18 amino acids linked by peptide bonds in fibroin, the primary component of silk (in Fig. 1c)^10,11. The composition of Bombyx mori silk is 75% fibroin and 25% sericin, which underlies its structural and mechanical attributes¹². At the macroscopic level (10–20 μm), sericin is present in the silk fibers that wrap around the fibroin, forming a core-shell structure. In the textile industry, sericin is removed from the thread, along with impurities during the degumming process. Thus, the deterioration of silk cultural heritage is essentially the decay of fibroin. Molecularly, the structural proteins in silk can be classified into three main categories: H-fibroin (Heavy Chain), L-fibroin (Light Chain), and P25, with the proportion of 6:6:1¹³. H-fibroin consists mainly of highly repetitive (Gly-Ala)n amino acid sequences, whereas L-fibroin contains disordered domains¹⁴. The repeated amino acids fold into anti-parallel β-sheets and undergo aggregation to form crystallites. These β crystals are embedded in an amorphous matrix formed by the remainder of the protein, making it rich in polar side chains¹⁵. P25 is connected to H-fibroin by a non-covalent bond. The highly organized microcrystalline structure of fibroin contributes to the remarkable mechanical properties of silk¹¹. In summary, silk exhibits outstanding mechanical properties, and its lightweight and smooth texture renders it an ideal fabric material.

Leather and parchment

Collagen-based cultural heritages possess complex structures and properties, which could be subcategorized as tanned leather and untanned raw material (such as parchment, shadow figures, and drums). Among these, the cultural heritages of leather and parchment are the most common archeological collagen-based heritages (in Fig. 2a, b)^16,17. Parchment was a popular writing material from the 2nd century B.C. to the end of Medieval times. Parchment, composed of approximately 95% collagen, is an untanned skin/hide that was directly impregnated with ash, scraped, and dried¹⁸. Most excavated leather cultural heritages were identified as vegetable-tanned leather, comprising approximately 67% collagen¹⁹. In contrast to parchment, leather is mainly tanned by tanning agents, which stabilize the collagen matrix by forming extra hydrogen and covalent bonding between the amino acid chains. The collagen-based heritages were sourced from animal skins/hides, predominantly cattle, sheep, and pigs. The main ingredient is the fibrous type I collagen, whose fundamental structural unit is a triple helix extending from the middle. The type I collagen has a complete four-level structure (in Fig. 2c)²⁰. The primary structure comprises nearly 1050 amino acids, which form a repeating glycine-x-y tripeptide sequence. In this sequence, the x and y sites are usually occupied by proline and 4-hydroxyproline, respectively^20,21. And 4-hydroxyproline accounts for approximately 13% of collagen. The secondary structure of collagen is α-helical as a consequence of the electrostatic interaction of proline and 4-hydroxyproline. Post α-helix formation, all the amino acid residues on the side chain turn outward to form hydrogen bonds within the helical chain, thereby stabilizing the conformation. The tertiary structure is constituted by three left-handed helical polypeptide chains that intertwine to form a right-handed three-stranded helix or superhelix²². The quaternary structure denotes five collagen molecular chains arranged in parallel, with the head and tail staggered by 1/4. These chains are covalently linked to form stable collagen microfibrils, which then aggregate into fibers.

**Fig. 2: The cultural heritages of parchments and leathers and the multi-hierarchical structure of type I collagen.**

Paper and wood

For centuries, paper-based materials, including books, paintings, calligraphies, and archives, have served as significant carriers of cultural information. The information on buildings and decorations made of wood could convey essential esthetic values (in Fig. 3a, b)^23,24. A variety of plants were employed in the production of paper, including hemp, jute, flax, ramie, rattan, mulberry, paper mulberry, and bamboo²⁵. Handmade paper was produced through a series of complex processes, aiming to minimize chemical damage to plant fibers. In the process of paper making, most lignin and hemicellulose are eliminated, leaving cellulose as the primary component (in Fig. 3c)^26,27. In essence, paper deterioration is related to cellulose degradation. With the progress of civilization, wood, a complex natural composite with orderly arranged cell walls, has been used for various purposes such as construction, tools, and ornaments, owing to its practicality and esthetics. Wood is composed of three primary components cellulose, hemicellulose, and lignin. Most archeological woods preferentially lost hemicellulose. Contrary to hemicellulose, cellulose is less susceptible to deterioration due to its superior stability²⁸. The relative content of residual lignin is the highest because of its unique phenolic, three-dimensional cross-linked structure, which confers resistance to deterioration²⁹. The strength and dimensional stability of wood are significantly influenced by cellulose, yet research at the molecular and crystalline level remains limited. Cellulose is a well-organized biopolymer composed of D-glucopyranosyl-β-1,4-D-glucopyranose chains, in which there is a crystalline or semi-crystalline structure through intra- and inter-chain hydrogen bonding and van der Waals forces, conferring the high tensile strength of cellulose. The elementary fibers consisting of 36 cellulose chains represent the fundamental structural units. These fibers, both crystalline and amorphous, can be arranged into nanofibers (<35 nm diameter), nanofiber bundles (<1 µm diameter), and microfibers (<10–50 µm diameter), forming a stable polymer with high axial stiffness³⁰. Unlike cellulose, hemicellulose exhibits a complicated polycrystalline structure in which branched chains lead to an uncompact structure³¹. Lignin is an amorphous polymer consisting of phenylpropane units linked by carbon-carbon and ether bonds.

**Fig. 3: The paper, wood, and book artifacts, and the chemical composition and multi-level structure of paper.**

Other organic cultural heritages

Organic cultural heritages encompass common silks, leathers, parchments, papers, and woods, as well as cotton and linen textiles, and lacquerware. Cotton and linen textiles primarily consist of cellulose, colored with vegetable or/and insect dyes⁹. Lacquerware is crafted from lacquer tree sap, colored with pigments or metal fillers. The refined sap acts as a protective coating for a variety of objects, including artwork and daily utensils. Natural raw lacquer, collected from the lacquer tree, is a water-in-oil (W/O) emulsion. The aqueous phase is consisted of water, saccharides, and enzymes, responsible for converting the sap into a cross-linked network. There are substituted catechols and phenolic compounds, with a minor amount of water-insoluble glycoproteins in the oily portion phase³².

Identification of organic cultural heritages from diverse perspectives

The identification of ancient cultural artifacts is vital in archeology, which could provide the foundation for the design of preservation and restoration strategies. Owing to the rapid development of modern analytical techniques, a range of characterization approaches (in Table 1) can be utilized to assess the transformations of organic cultural heritage in macro- and micro-structure, thermal stability, and chemical composition.

Table 1 Various characterization methods for organic cultural heritages

Full size table

Qualitative analysis

The changes of organic cultural heritages in morphology and color are instantly correlated to their physical state, whose features might be observed via optical microscopy (OM), scanning electron microscope (SEM), polarization microscopy (PLM), and transmission electron microscopy (TEM). Electron microscopes are widely employed in a variety of scientific and research applications, using electromagnetism to generate high-resolution images of the internal structure of materials. It can be used to determine the category of organic cultural heritages, analyze deterioration, and research on processing traces. The examination of both contemporary and historical silk samples using SEM revealed a notable difference in the surface characteristics³³. Fresh silk fibers displayed a smooth and pristine surface, whereas historic silk samples exhibited a range of surface roughness, depending on their burial time. Microscopic analysis revealed the characteristic tubular fibers in silk. Moreover, the iridescent colors of fibers under polarized light indicated the possible presence of microcrystalline structures³⁴. SEM analysis revealed the fungus-infested leather with irreversible damage to rough surfaces and torn fibers³⁵. For some archeological wood inside the bronze parts of the excavated chariots samples, TEM images showed the multilayer structure of the cell walls.³⁶. In contrast, two other samples were severely decayed with the cell walls completely decomposed. The residues of these cell walls were needle-shaped, which was related to the crystalline and amorphous forms of iron hydroxide produced by the Fenton reaction of the iron that was embedded in the bronze. Microscopic analysis revealed the microscopic morphology of the samples, thus contributing to assessing the degree of deterioration and the extant state of the organic cultural heritages. However, only a small portion of the target can be analyzed through a microscope. Therefore, the sample analyzed under a microscope cannot represent the entire heritage analyzed. Nevertheless, they are limited by micro-destructive sampling in the form of chunks, pellets, or ultrathin slices.

For the color change, colorimetric analysis²¹ was performed on deteriorated parchment. Both the corium and grain side of samples displayed a considerable overall color variation (ΔE), compared to the initial state (undeteriorated sample). This change became more pronounced with increasing the exposure temperature of the parchment³⁷. The relationship between color and molecular structure of handcrafted paper was further investigated by color difference analysis, indicating that the color change mainly depended on the number of carbonyl groups of aldehydes and conjugated diketones³⁸. As an early indicator of deterioration, the colorimetric analysis could connect the microstructural evolution to the macroscopic deterioration.

The molecular structure of organic cultural heritages could be characterized using spectral analysis, primarily employing the analytical techniques such as FTIR, Raman, and UV-Vis spectroscopy in non-invasive ways. Spectrum can be contributed to analyze the composition and properties of substances by measuring their absorption, scattering, and emission of light at different wavelengths. Spectral analysis can provide extensive information about the substances found in heritages and the degree of deterioration. The non-invasive methods are crucial for the evaluation of rare heritages because of their sample limitations. ATR-FTIR technique, highly sensitive to the changes in the secondary structure of the protein, could be utilized to evaluate the chemical and structural changes in protein samples. The spectral regions of amide I (1590–1700 cm^–1), amide II (1460-1590 cm^-1), and amide III (1190–1300 cm^–1) are considered suitable for qualitative analysis³⁹. It was revealed that the β-sheet structure of historical and hydrolytically aged silk differed from fresh silk, implying that historical silk might have undergone hydrolytic aging³³. ATR-FTIR spectroscopic imaging, combined with macroscopic ATR attachment, could be used in the examination of historic leather book covers⁴⁰. This non-destructive, high spatial resolution methodology could be used to detect the collagen gelation and the initial formation of calcium stearate. Comprehensive data could be provided by the punctual reflectance spectroscopy in the UV–Vis-NIR, also known as Fiber Optics Reflectance Spectroscopy (FORS)²³. Specifically, qualitative insights into dyes and pigments on paper could be provided. Additionally, it is suitable for quantitative analysis, such as the concentration of oxidized chromophores, through appropriate illumination and sampling techniques. The Raman spectral bands were used for the definition of cellulose molecular conformation⁴¹. NIR μ-Raman technique, involving non-destructive measurements directly in situ on the heritages, successfully correlated the parameters to cellulose crystallinity with Raman spectra⁴². In addition, based on the NIR band changes of hydroxyl groups of crystalline cellulose, acetyl groups of hemicellulose, and C-H bands of lignin, a combination of NIR and stoichiometry successfully assessed the preservation status of archeological wood subjected to water immersion⁴³. Therefore, spectral analysis is an excellent method for collecting structural data on solid samples, by which information on substances found in heritages could be provided, as well as the state of deterioration to properly plan conservation interventions. High-performance spectrometers like the ATR-FTIR could offer higher data quality in the laboratory. Some portable, low-cost instruments, such as FORS, are more commonly available because they could provide rapid qualitative diagnostic information. Nevertheless, there remain some limitations due to the required tested sample space of the instrument for larger cultural heritages, while obtained spectra are hard to recognize occasionally.

Thermal analysis provides an accurate record of the physical and chemical properties of organic cultural heritages as a function of temperature change. However, the samples are non-renewable. Macroscopic variations in thermal stability could reflect the relationship between molecular structure and stability. Also, thermal analysis is suitable for samples in different states. Consequently, thermal analysis methodologies could provide insights into the influence of molecular structure alterations of organic heritage on the mechanical properties and hydrothermal stability, primarily thermogravimetry/derivative thermogravimetry (TG/DTG), differential scanning calorimetry (DSC), and micro hot table methods (MHT). By DSC analysis, it was indicated that the thermal stability of fibroin protein was linked to the secondary structure and microcrystal orientation³⁴. Micro-DSC and MHT analyses of 10 historical leather samples confirmed that partial de-tanning happened in leather during long-term natural aging⁴⁴. Ultimately, gelation and irreversible denaturation take place in collagen molecules. Characterization of thermo-oxidative damage in leather was derived from TG/DTG curves analysis⁴⁵, and the results suggested that leather underwent two major complicated processes during natural deterioration: cross-link bond disruption between collagen and tanning agents, and collagen macromolecule fragmentation. The thermal degradation process of wood is determined by the composition and concentration of the main compounds of cellulose, hemicellulose, and lignin. Therefore, DTG curves were applicable to categorize different types of wood⁴⁶. The pyrolysis of asphalt wood exhibited unique behavior, with the two peaks in the DTG curve indicating its presence in traditional Canary buildings⁴⁷. In summary, thermal analysis techniques are widely adopted because there are no limitations on the state of samples and can be used in conjunction with other technologies. While thermal analysis offers a fairly reliable determination of the molecular stability and chemical composition information, it is not ideally desirable for the valuable heritages because of destructive sampling. More valuable applications of thermal analysis methods depend on improving its destructive sampling limitations.

Analytical techniques such as X-ray diffraction (XRD), wide-angle X-ray diffraction (WAXD), and selected-area electron diffraction (SAED) are based on the diffraction effect of X-rays or electron beams in crystalline substances for the applications of phase analysis, crystal crystallinity determination, and orientation analysis of crystals. XRD analysis was conducted on original and heat/UV-treated samples, by which significant silk damage was revealed at 150°C of thermal radiation⁴⁸. To evaluate the crystal structure changes in detail, SAED was carried out on silks that artificially deteriorated in different ways. Remarkably, the variance in their diffraction patterns underscored the profound influence of aging history on the crystal structure³³. Historical silk was investigated by WAXD and it was demonstrated that the destruction of the fibrillar-oriented structure of the β-sheets was irreversible⁴⁹. Comparatively, the XRD patterns of both old and new parchment and leather were similar to those of pure collagen, exhibiting principal diffraction peaks notably at the 2θ values of 14.1, 16.9, and 25.5°⁵⁰. The presence of crystalline regions in the tested materials was confirmed. XRD was deployed to evaluate the crystallinity of lignocellulosic materials, as only cellulose exhibits crystallinity in wood. After deconvolution of the diffraction patterns of lime wood (Tilia cordata Miller) samples degraded by fungal, it was observed that all crystallographic data varied with fungal exposure duration⁵¹. The crystallinity and cellulose fraction decreased with biodeterioration progress. However, the crystallinity of cellulose cannot be directly determined by XRD, but rather by the mass fraction of crystalline cellulose in the sample⁵². XRD diffraction pattern of artificially deteriorated paper demonstrated that dry heat weathering affected the destruction of amorphous regions in cellulose fibers, which led to an increase in crystallinity⁵³. The variation in crystallinity and crystal structure could precisely reflect the pathway and deterioration degree of the organic cultural heritages. It is impossible to reflect the overall compositional structure for complex, multi-component samples due to the limited test area, even not suitable for the analysis of organic pigments and binders.

Organic cultural heritages were initially analyzed via nuclear magnetic resonance (NMR) by dissolving them in solution and identifying the soluble organic remnants in them. Unfortunately, the dissolution process might change or even completely destroy the molecular structure of the heritages. Microprobes minimized the required sample for the characterization to the microgram levels, making NMR analysis a feasible method for evaluating cultural heritages⁵⁴. Liquid and solid-state nuclear magnetic resonance (NMR) probes the molecular composition and structure of a given substance by capturing subtle changes in the magnetic behavior of specific nuclei by the interactions between neighboring electrons and atoms, which can be influenced by the surrounding environment. Currently, solid-state NMR spectroscopy, an extensively employing non-destructive analytical tool, could provide information about the chemical structure and molecular dynamics at the molecular levels. The molecular secondary structure of the ancient silk was characterized by ¹³C CPMAS NMR spectroscopy⁵⁵. The intensity ratios of the β-sheet and the random coil of the sample were revealed, supporting preferential changes of the amorphous regions during silk deterioration. NMR signals relevant to collagen and tannins were not consistent in evaluating the deterioration of archeological leathers. Therefore, the non-destructive ¹³C solid-state NMR was used to identify the tanning agents and lipid compounds⁵⁶. More importantly, the structural breakdown of collagen and its conversion to gelatin was revealed. Eight types of archeological submerged leathers were investigated by solid-state ¹³C CP/MAS NMR, revealing minor preservation of cellulose, primarily in an amorphous form⁵⁷. Low carbohydrate and lignin levels, coupled with substantial hemicellulose loss, were also observed. In situ investigation of paper book covers was conducted by two-dimensional NMR relaxation (2D¹H-NMR-R) to characterize the water molecules functioning as hydrated and void-filling in amorphous cellulose⁵⁸. The results indicated that the amorphous structure was solely responsible for the internal humidity of cellulose. The development of modern analytical techniques rendered NMR analysis a non-destructive in situ method for analyzing complex polymers, which is very suitable for organic cultural heritages with diverse compositions. So, the non-invasive testing furnishes comprehensive insight into the preservation of precious organic cultural heritages. Moreover, the constrained movement of molecules results in a reduction in the resolution of NMR techniques for solids in comparison to those employed for liquids. Breaking through the limitations of the low resolution of solid-state NMR, this technology will provide more valuable results.

Quantitative analysis

X-ray photoelectron spectroscopy (XPS) and Energy dispersive spectrometer (EDS) are efficient analytical tools for elemental analysis. The utilization of the distinctive characteristic energies of X-ray photons emitted by different elements facilitates compositional analysis. Regarding to the quantification of deteriorated organic cultural heritages, systematic evaluation is adopted from the perspective of elemental composition and chemical state. EDS was applied to identify the silk pre- and post-artificial aging in terms of elemental composition and content⁴⁸. The proportion of oxygen increased significantly after heat and UV exposures, suggesting oxidative reactions during both aging procedures. The varying chemical compositions of ancient and fresh silk were quantified by XPS⁵⁵. The degree of oxidative deterioration was deduced from the atomic percentages of the elements C, N, and O, along with the O/C values. XRF spectroscopy was performed on the ink-free areas of the parchment⁵⁹. The analysis indicated the predominant residual elements as calcium (Ca), accompanied by minute quantities of chlorine (Cl) and sulfur (S). The Ca element was hypothesized associated with the application of lime for wool removal, the Cl element might be sourced from the use of NaCl to avoid skin putrefaction, and the S element was derived from the proteins of the parchment. This discovery inspired insights into how ancient parchment was prepared. The micro-distribution of bronze corrosion products within the wood cell walls of archeological wood inside the bronze parts of excavated chariots was investigated by EDS spectroscopy³⁶. The results showed that the highest content of Cu²⁺ permeated the middle layer of the cell wall, owing to the reactivity between the phenolic hydroxyl groups of lignin and Cu²⁺. Given an understanding of the deterioration mechanisms, feasible protection methods might be established, such as the development of copper preservatives. In comparison to XPS, EDS is unable to measure the valence state of a substance and is therefore unsuitable for quantitative analysis of the elemental content of a surface. X-ray fluorescence (XRF) is unable to determine light elements, and the precision and accuracy of quantification still require improvement.

Proteins are strongly biologically characterized across species. Therefore, biomass spectrometry is extensively employed for the identification and analysis of protein-containing archeological items. By proteomics, sophisticated liquid chromatography-mass spectrometry (LC-MS) methods, and biological assays were employed to accurately determine the amino acid content, species, and protein abundance. Historically and artificially deteriorated silks were analyzed by proteomics methods at the molecular level⁶⁰. The samples displayed a significant decrease in recognizable peptides and protein components with increasing the burial or aging time, along with a substantial decrease in L-, P25-, and H-chain components. Through the preparation of new materials, trace collagen peptides were extracted, which contributed to proteomics¹⁷. The animal species and composition of the tanning agent of boot leather were determined by LC-MS and pyrolysis mass spectrometry (Py-MS). Due to the obtained results, the traceability of the studied leather might be established. However, adequate knowledge of the relevant persons is needed in the interpretation of these results. The collagen deterioration can be quantitatively analyzed according to the relative content of Hyp, Gly, and Pro. The lower relative content of amino acids indicated a more severe deterioration. Thus, proteomics can be adopted to systematically assess the leather deterioration and subsequent repair. Proteomics, LC-MS, and biological assays represent an exemplary approach for analyzing deterioration mechanisms in organic cultural heritages.

Microbiological analysis

The Omics tools are based on high-throughput sequencing and high-throughput analysis platforms, as well as computational biology methods and tools. It offers an in-depth comprehension of the structure and functionality of living organisms through the measurement and analysis of biological system components on a large scale. The Omics tools could not only be used in the characterization of the deterioration of protein-based heritages but also the investigation of microorganisms on other organic cultural heritages. Microbial activity is a key determinant of heritage deterioration, which could thrive in the rich nutrients of organic cultural heritages. By genomics, transcriptomics, proteomics, and metabolomics, crucial knowledge about the type and metabolic role of microorganisms could be provided, as well as environmental restrictions on their growth, which is indispensable for heritage preservation. The combination of proteomics and metabolomics could reveal the interactions between bacteria and archeological silk during biodeterioration. Specifically, different species of bacteria tend to damage different regions of fibroin, and protease inhibitors contained in silk could inhibit bacterial metabolic activities. through this method, information on molecular level changes in proteins and bacterial metabolites could be provided. By this method information on the variations at the molecular level of proteins and bacterial metabolites could be provided⁶¹. The Next-Generation Sequencing (NGS) metagenomic analyses indicated that the prokaryotic communities were different between affected and unaffected regions of the parchment document¹⁶. The unaffected regions contained Pseudonocardiales, while Gamma-Proteobacteria (Vibrio) was prevalent in the affected areas. Illumina sequencing method was employed to identify ancient books⁶². Omic techniques proved the cellulolytic decomposers and the competitors of these organisms. In order to determine the microorganisms and their main mechanisms that result in damage to heritages, it is essential to understand the microbial community and its metabolic pathways. Metabolomics method and Illumina sequencing were performed to examine selected samples of construction wood⁶³. Metabolomics data indicated the presence of multiple metabolic pathways involved in the deterioration process, encompassing regulating the production of primary and secondary metabolites, as well as the DNA sequences identified 15 bacterial phyla representing 99 genera. High-throughput sequencing (HTS) was applied to analyze the microbial communities on the ancient wooden structures, with a special focus on the cellulose deterioration pathways of the microbial communities by bioinformatics methods⁶⁴. The archeological wooden structures were less susceptible to microbial erosion when buried in the soil. This information will assist in the adoption of appropriate measures such as controlling moisture and oxygen in the environment to control microbial growth. The strength of multi-omics studies lies in their capacity to provide a comprehensive and integrated view of biological systems. Notably, an enormous amount of data in omics technology, coupled with intricate analysis stages and automated data processing, might yield erroneous biological information. The prospective advancement of multi-omics technology is to enhance the capability to analyze data and obtain precise biological information.

Advanced modern analytical tools are employed to thoroughly investigate the morphology and structural variations in organic cultural heritages, encompassing non-invasive, micro-invasive, and destructive techniques. Each analytical technique has its own specific limitations. Therefore, obtaining different information about the whole sample requires multiple instruments to complement each other. Quantitative and qualitative assessments make it possible to precisely evaluate the current state and identify the deterioration mechanisms of organic cultural heritages. For example, microscopic technology could be employed to conduct a visual analysis of the extant state of organic cultural heritages, with a particular focus on the local morphological characteristics⁶⁵. The combination of spectral analysis with quantitative and qualitative assessment provided a powerful insight into the deterioration mechanism of organic cultural heritage. This approach enabled the mechanical properties and deterioration degree to be studied in detail^66,67. A microbiological analysis may be employed to clarify the most efficacious means of inhibiting biological deterioration from the metabolism perspective⁶⁸. The requiring improvement in these techniques lie in the utilization of multiple tools for the acquisition of different information on the entire sample. Furthermore, the heritages should be characterized with greater accuracy according to their state of conservation, with minimal damage. The effective application of these modern analytical techniques has significantly facilitated the preparation of targeted protective materials.

Deterioration of organic cultural heritages by environmental factors

Various factors affecting the organic cultural heritages

The organic cultural heritages exhibit severe fragility after excavation, primarily manifested as mechanical strength and brittleness, or even pulverization. Furthermore, appearance deterioration usually takes place in color fading and mottling. Because of their nutrient content, organic materials are vulnerable to deterioration due to burial conditions and biological attacks, resulting in the loss of key components via denaturation, hydrolysis, and oxidation. Typically, organic cultural heritages are subjected to different environmental conditions such as temperature, relative humidity (RH), light, air pollutants, and microorganisms (in Fig. 4), leading to potentially significant changes in their appearance, physical properties, chemical composition, and internal structure. Therefore, understanding the deterioration mechanism of organic cultural heritages could help develop the formulation of targeted preservation strategies.

Distinct deterioration mechanism

The specific deterioration mechanisms vary greatly because of the diverse organic cultural heritage components and distinct production processes. Notably, the universal deteriorations include oxidation and hydrolysis.

Deterioration of silk fabrics

The optimal deterioration temperature for fibroin is 150 °C. During the thermal aging period, the initial degradation of silk fabrics took place in the amorphous region, resulting in a considerable reduction of amino acids and an increase in crystallinity (in Fig. 5a)^48,60. As aging progresses, both the crystalline and amorphous regions are destroyed, which decreases the crystallinity content and increases the random coil content proportionately. The deterioration of both amorphous and crystalline regions could decrease the mechanical strength³³. The fluctuation of environmental humidity is usually related to temperature. In simulating the high-temperature process of silk, high humidity, and oxygen content are the main factors to cause silk deterioration. Briefly, water molecules could catalyze the free radical oxidation of peptides and trigger silk fibroin hydrolysis and oxidation at high temperature, producing new amino and carboxyl groups. In the artificially heat-deteriorated samples, the concentrations of all amino acids, except Gly and Ala decreased³³. The weight loss of the heat-deteriorated samples was 39%, which was attributed to the removal of gaseous products of thermal degradation, such as CO₂ and NH₃. The tendency of silk to age rapidly under UV radiation is because of its superior UV light absorption capacity. Photochemical reactions, including the yellowing and stretching of silk, typically take place from the surface to the interior. Therefore, the color change can be an early warning sign for the preservation of cultural heritage. The yellowing of silk is due to the photooxidation of tyrosine and tryptophan residues, forming yellow chromophores. Tyrosine is the primary contributor to silk yellowing due to its oxidized product of di-tyrosine products, with contents (>10 wt%) higher than tryptophan (<0.5 wt%)⁶⁹. Post UV-induced aging, tyrosine content and crystallinity were decreased with lower size of the β-sheet crystallites, eventually transforming them into random coils. Meanwhile, the surface of UV-irradiated silk fibers turned rough owing to the loss of protein organization⁷⁰. Air pollutants can usually lead to the inside oxidation of fabrics. Ozone is a typical factor as a powerful oxidizing agent in the oxidation of amino acid residues in fibroin, the breakage of the peptide backbone, and the formation of gaseous compounds⁷¹. Amino acids such as glycine, alanine, tyrosine, and serine are oxidized to produce chromogenic compounds containing carbonyl groups, which could enhance the yellowness index in silk fabrics⁷². Moreover, the breakage of the C-N bond in the peptide chain reduced polymer length and strength⁶⁹. The reduction in elongation at break was attributed to the breakage of the peptide backbone. Silk cultural heritages are less susceptible to the influence of microorganisms⁷³. Sericin is the main protein used by microorganisms as nutrient. Therefore, the microbial degradation rate is lower in silk fabrics with sericin removed. Only a few bacteria belonging to Bacillus, Pseudomonas, Serratia, and Streptomyces were proven to deteriorate silk fabrics or proliferate on silk fabrics or proliferation of microorganisms with silk fibroin as the carbon source⁷⁴. Microorganisms can be attached and proliferated on the silk surface, subsequently disrupting the peptide bond by secreting specific enzymes, such as proteases, to compromise the protein structure and increase the brittleness. Urea and organic acids were also found in silk biodeterioration, indicating that there may be other mechanisms for silk deterioration besides proteolysis⁷⁵. The thermal deterioration of silk is typically evidenced by a gradual decline in the crystallinity content, mechanical strength, and weight⁷⁶. A change in temperature is generally accompanied by a change in humidity⁷⁷. It can be observed that water molecules play a role in the oxidation and hydrolysis of silk fibroin at elevated temperatures. Similarly, intense UV light and ozone irradiation have a detrimental effect on silk fibroin, resulting in molecular rearrangements that lead to the oxidation of tyrosine and the production of yellowing, as well as peptide shearing, which ultimately reduces the intensity of silk⁷⁸.

**Fig. 5: Diverse deterioration mechanism of distinct organic cultural heritages.**

Deterioration of leather and parchment

Compared to stable silk fabrics, collagen-based heritages are more susceptible to the deterioration by various environmental factors due to their poor thermal stability and mechanical properties, as well as the complicated chemical composition. Parchments and leathers were mainly decomposed by hydrolysis and oxidation (in Fig. 5b)⁷⁹, and the former was primarily related to acidic pollution. The two-decomposition mechanism could occur separately or simultaneously. Acid gases (NO_X) and corresponding acids could cause acid hydrolysis of leather. Compared to parchments, the deterioration degree of leather is different with different types of tanning agents. Leather tanned with condensed tannins is more susceptible to acidic pollutants than hydrolysable ones, thus intensifying leather hydrolysis. In case of severe air pollution, vegetable tannins would absorb substantial quantities of SO₂ to produce a sulfate content exceeding 5% w/w of dry leather⁸⁰. SO₂ combined with oxygen in high RH conditions could lead to the weakness or fragment of leather book covers, known as “red rot”⁸¹. The deterioration process is accompanied by chemical changes in the collagen, auxiliary such as tanning agents, and collagen-tanning agent complex. Vegetable tannins will undergo hydrolysis itself too, especially hydrolysable tannins. Severely deteriorated vegetable-tanned leather exhibited fiber structure disintegration and a powdery phenomenon that was not found in other types of leather or parchment. The pulverization of vegetable-tanned leather indicated that the tannin molecules that filled the fiber network could establish strong tannin-collagen bonds through condensation, making the fibers stiff, brittle, and prone to breakage⁸². The transition metal cations, mainly iron and copper, from dye pigments, water, or other substances in the tanning process, could catalyze leather oxidation, intensifying damage⁸³. UV light could catalyze the oxidization to lead to different degrees of collagen structure damage, destroying its main chain by bond disruption (amide, -CH₂-N = , and =CH₂) bonds breaking⁸⁴. The orientation in their molecular chains of leather fibers was lost by UV-irradiation to decrease their birefringence⁸⁵. The acid pollution inhibited the oxidation, with the minimum oxidation at the pH of 2.5, and the main hydrolysis happened in the deterioration⁸². The mechanical strength of the collagen fibers is from its triple helix molecule cross-linked via hydrogen bonds. Prolonged exposure to high temperature could disrupt the internal hydrogen bonds of collagen, opening the twisted collagen chains, which resulted in a disordered random coil configuration called thermal denaturation. Artificial aging confirmed the mechanism of heat-induced deterioration of collagen, with the irreversible transformation of triple-helical structure into a thermally destabilized intermediate conformation by hydrogen bonds and peptide bonds cleavage, followed by denaturation at 120 °C, and gradual gelatinization over 150 °C²¹. High water content in the collagen tends to break the hydrogen bonds between the collagen fibers, resulting in a more separated distribution of fibers, thus making it more pliable. RH values between 60 and 80% affect collagen stability¹⁸, so proper humidity is required to maintain the structural integrity of collagen. The breaking of hydrogen bonds between collagen fibers would ultimately lead to the destruction of the triple helix structure. Briefly, the essence of high temperature and high humidity deterioration is the transformation of the triple helix structure. When it comes to microbial deterioration, collagen-based cultural heritage is more likely than silk fabrics. Collagen-based heritages are rich in organic matter, with proteins and carbohydrates providing essential nutrients for the growth and metabolism of microorganisms, particularly bacteria and fungi. The biological deterioration of parchment is mainly caused by bacteria due to its higher pH value⁸⁶. Bacteria that produce enzymes capable of hydrolyzing collagen, such as Bacillus, Staphylococcus or Pseudomonas, played a major role. Therefore, the surface of the parchment undergoes changes, such as purple spots produced by alkaliphilic bacteria. The mechanical properties of the material are also weakened until they were completely destroyed. Leather is more complicated by microbial deterioration due to its abundant components. The presence of ammonium salts, phosphates, surfactants, and additives in chrome-tanned and vegetable-tanned leathers could promote the microbial activity on the leather surface⁸⁷. Microorganisms produced enzymes that broke down the collagen into smaller molecules, which they then use for nutrients and energy. The primary nutrients absorbed by microorganisms were the non-structural proteins in the collagen fibers. This adsorption weakened the covalent bonds between the tanning agent and collagen, leading to leather bio-deterioration⁸⁸. Hygroscopic materials could allow fungi to thrive on leather at lower RH than bacteria, making fungi particularly harmful to leather⁸⁹.

Deterioration of paper and wood

The deterioration mechanisms of paper and wood are complex due to their inherent multi-component composition. Paper deterioration primarily involves oxidation and hydrolysis of cellulose. Unlike collagen hydrolysis, which inhibits oxidation, cellulose oxidation and hydrolysis are interrelated. Oxidation would convert the hydroxymethyl groups to carboxyl groups, promoting hydrolysis of cellulose, while the cleavage of glycosidic bonds generated aldehyde groups more susceptible to oxidation. From a microscopic perspective, paper weakening and discoloration result from the acid hydrolysis of the β-d-(l,4)-glycosidic bond and oxidation of the β-d-glucopyranose unit in the cellulose polymer. Numerous intra- and inter-molecular hydrogen bonds created a completely cross-linked molecular structure of cellulose, making its hydrolysis less likely than that of fibroin and collagen⁹⁰. Paper degradation predominantly occurs within the amorphous region of cellulose, attributed to the high stability and durability of areas within high crystallinity (in Fig. 5c)⁹¹. As aging proceeds, the amorphous and crystalline regions of cellulose are degraded sequentially. Elevated temperatures facilitated the production of free radicals that catalyzed the cellulose oxidation. The primary dry heat aging of paper involves the transformation of ketones into conjugated diketones and aldehydes into a carboxylic acid complex through oxidation reaction. Additionally, intramolecular esterification between the acid and the alcohol could generate lactone³⁸. Oxidation and hydrolysis occur simultaneously during dry heat and wet heat aging. However, the presence of water molecules accelerated the generation of carboxyl groups, which promoted the hydrolysis. The carbonyl content increased rapidly, indicating that hydrolysis played a primary role in the wet heat deteriorated process. Elevated RH in air could accelerate the breakage of hydrogen bonds, ultimately decreasing the paper’s strength⁹². The deterioration of paper is exacerbated by the combined effects of temperature and humidity. Cellulose primarily absorbed in the near-UV spectra⁹³. The spectrum of sunlight filtered into the room by glass is the near-ultraviolet, visible, and near-infrared at wavelengths higher than 340 nm. In this case, cellulose will not undergo direct photolysis. Light exposure activated the photosensitizer, initiating cellulose degradation. Consequently, under light with wavelengths longer than 340 nm, cellulose underwent extensive oxidative deterioration, producing hydroxyl radicals, carbonyl groups, or even depolymerization⁹⁴. Lignin in wood would deteriorate due to UV light absorption, leading to wood yellowing⁹⁵. Acidic and oxidizing pollutants (SO₂, NO_X, O₃) in the air further exacerbated the deterioration of paper and wood⁹⁶. These compounds could form acids with residual moisture in the paper, inducing acid hydrolysis of cellulose. Free radicals produced by oxides could contribute to cellulose chain breaks. Additionally, substances contained in heritages could promote deterioration through acid hydrolysis and oxidation. For example, the iron gall inks on paper are inherently acidic⁹⁷, inducing acid hydrolysis and a severe Fenton oxidation reaction, damaging the cellulose and causing perforation in the inked areas. Paper biodeterioration was primarily associated with fungi, typically slow-growing ascomycetes and xerophilic species which could metabolize even in low water content environments⁹⁸. Microbial activity also caused a discolored spot on paper, compromising esthetic appeal. For example, the formation of brown spots on paper was produced by the metabolic activity of xerophilic species including Aspergillus, Cladosporium, Penicillium and Eurotium⁹⁸. In addition, enzymes could degrade cellulose, causing structural damage. Unlike paper, wood deterioration also involved hemicellulose and lignin. Hemicellulose, particularly fragile, is preferentially lost during deterioration. Hemicellulose hydrolysis involved the hydrolysis of β-1,4-D mannose in the main chain is hydrolyzed, leading to the formation of monomers⁹⁹. Lignin has a stable phenolic cross-linking network, minimizing its susceptibility to depolymerization¹⁰⁰. Archeological wood is typically characterized by a high lignin content, with hemicellulose and cellulose often deteriorating completely in extreme cases. However, wood composed solely of lignin residues exhibited significantly diminished mechanical strength. Common archeological wood usually was submerged in waterlogged environments, leading to rotting by anaerobic bacteria. Three groups of wood-deteriorating bacteria, erosion, cavitation, and tunneling bacteria, had been identified¹⁰¹. Erosive bacteria are the primary agents responsible for the degradation of cellulose and hemicellulose, as they are capable of surviving in environments with extremely low oxygen concentrations. In terrestrial environments, fungi could penetrate wood through mycelium by the utilization of cellulose and lignin as nutrients⁹⁸. Wood is subject to different types of microbial deterioration in aquatic and non-aquatic environments compared to paper, which is primarily affected by fungi. Submerged archeological wood deteriorated solely due to anaerobic bacteria, resulting in a slower deterioration process.

Modern protective approaches on organic cultural heritages

Prolonged soil burial leads to poor physical and chemical conditions of most historic cultural heritage items. To prevent the destruction of organic cultural heritages and extend their lifespan, various protective materials and methods have been developed, including cleaning, cementing, adhesives, and protective coatings. Contact synthetic protective approaches achieve conservation through chemically synthesized materials. These materials penetrate or encapsulate the organic cultural heritages, thus inevitably interfering with them. Contactless protective approaches use physical methods or materials deposited on the protective glass, to achieve the aim. However, Contactless protective approaches are limited, as they do not reach full protection for all purposes.

Contact synthetic protective approaches

Synthetic polymers are broadly utilized as consolidating agents or adhesives in the conservation of organic cultural heritages due to their superior mechanical properties and adhesion to the substrate. For example, polymers derived from acrylic and methacrylic esters are widely used in adhesives for their optical clarity and overall stability¹⁰². Aging silk fabrics had been treated by spraying with an aqueous solution of epoxide-ethylene glycol diglycidyl ether, which cross-links with tyrosine and lysine in the amorphous structural domains of the fibroin for mechanical strengthening¹⁰³. However, this procedure usually leads to a yellowing of the silk fabric over time. The mechanical and physical properties of polymers were decreased over time, eventually diminishing their ability to protect historical silk heritages, and even causing side effects due to the difficulty in removing deteriorated synthetic polymers. Considering excellent mechanical properties and removability, biopolymers presented an appealing and sustainable alternative as consolidating agents. The water-saturated silk unearthed from the tomb of Shisong of the Southern Song Dynasty was reinforced by using fibroin extracted from silk and the additive ethylene glycol diglycidyl ether¹⁰⁴. The fibroin reinforcement technology successfully enabled the smooth unfolding of the heavily laminated fabrics and effectively improved the mechanical and thermal stability of the silk fabrics. The homologous reinforcing and protecting materials of silk will not damage to the silk heritages as other synthetic polymers do. Biopolymer-based composites thus represent a promising alternative to traditional conservation methods. Bacterial cellulose (BC), a biodegradable alternative to conventional synthetic materials, exhibits the potential to reinforce fragile silk heritage items¹⁰⁵. The abundant hydroxyl side groups, high crystallinity, and elastic modulus of BC could foster effective interfacial interactions with the silk matrix, enhancing the crystallinity, thermal stability, and tensile strength of the repaired samples. It is of particular significance that BC could be entirely removed after 24 h of UV light and ozone treatment without any impairment to the intrinsic properties of silk fibers¹⁰⁵. Given BC’s propensity for modification due to its abundance of side groups, it can be widely applied as a multifunctional composite material. Similarly, self-regenerated silk fibroin (SRSF), an environmentally friendly, sustainable biopolymer, is a viable material for reinforcement applications¹⁰⁶. Instead of embedding fibers in a thick coating, SRSF was applied to silk fabric surfaces, preserving the esthetic appeal of treated heritages. To prevent performance damage and deterioration from UV irradiation, a polyaniline/chitosan coating was developed, in which bio-based chitosan improved the binding capacity and dispersion uniformity of polyaniline on the silk surface. (in Fig. 6)¹⁰⁷. Surface modification had been employed to improve hydrophobicity and thermal stability of silk fabrics. Polymer coatings were designed and fabricated on the surface of silk using surfactant-assisted polymerization of 1H, 1H, 2H, and 2H-perfluoroalkyl acrylate¹⁰⁸. Super-hydrophobicity and super-oleophobicity were induced on silk via a silica nanoparticle-enriched siloxane aqueous coating¹⁰⁹. This coating minimally impacted the silk’s visual color and could be removed through methanol-carbon dioxide compression, making it an economical and green chemical approach that fulfilled the basic principles of conservation science, including reversibility. Nanomaterials offer advantages such as low interference and durable antibacterial properties for preserving silk heritages. Silver core-shell nanoparticles with favorable dispersion and stability were fabricated to prevent microbial deterioration of silk heritages¹¹⁰. This material exhibited superior antimicrobial effects without altering the color or mechanical properties of the simulated silk fabrics. Among the methods available to meet specific restoration requirements, some chemicals penetrate deep into the silk, potentially damaging its esthetic qualities and structure, despite providing excellent restoration effects. Therefore, surface modification techniques are preferred as they involved only the surface of the silk without compromising its appearance.

Conventional treatments for leather conservation include the application of leather dressing and saddle soap washes. Dressings rich in oils, fats, and waxes are necessary when leather loses its elasticity or needs protection against future humidity changes. However, the fatty substances in these dressings could result in oxidation, hardening, and discoloration of the leather¹¹¹. Combinations of wax and acrylic resin (SC 6000), and hydroxypropyl cellulose were used to address leather consolidation problems¹¹². However, these products have limited application for consolidation due to their inability to penetrate the leather’s interior, thereby altering its appearance and water balance. Nanoscale particles, being well-dispersed and aligned in the polymer network, could provide a long-lasting curing effect. For example, a novel collagen nanostructured nanotube had been synthesized for enduring treatment outcomes¹¹³. The nano-collagen solution formed a bond and filled the cavities between the collagen fibers, enhancing the mechanical properties of repaired leather. Additionally, the shrinkage temperature of leather increased, partially restoring lost elasticity and flexibility. Collagen as the main component of leather heritages exhibits minimal side effects. It also has filling, covering, and film-forming properties, making it ideal for repairing tanned leather. Consequently, collagen hydrolysate had been adopted as a reinforcing agent to improve the structure and thermal stability of collagen fibers¹¹⁴. Simultaneously, leather impregnation with a mixture of glutaraldehyde and tannins strengthened collagen molecular cross-linking as well as inter- and intramolecular hydrogen bonding. However, this method may be limited because glutaraldehyde tends to condense into water-insoluble substances, causing the leather to yellow. In general, collagen hydrolysates offer less post-repair strength and flexibility compared to nano collagen. The nanosized natural halloysite is equally biocompatible. Therefore, it has also been practically applied for the conservation of cultural heritages. Halloysite nanotubes were dispersed into different media and tested for their restorative properties on ancient leather bindings⁶. The dispersions acted as restorative filler materials for ancient leather, improving the thermal stability of the leather and partially restoring collagen fiber cohesion. Ideal leather reinforcement materials are able to enhance the cross-linking of the collagen fibers within them without altering their own state, thus further improving their hydrothermal stability and physical and mechanical properties. Biodeterioration leads to the hydrolysis of collagen fibers, culminating in surface cracking. To prevent microbial persistence on leather heritages, biodeterioration agents are used for sterilization. In recent years, many green sterilization methods have been employed to protect cultural heritage. A three-dimensional hydrogel network of gellan gum had been used to capture and remove contamination from parchment¹¹⁵, including both mycelium and spores. The gum mechanically removes the contaminated organic material with a similar effect to water washing. Compared to chemical methods, this approach is simple, economical, and preserves the character of the heritages to the greatest extent possible. Based on the cleaning effect of gellan gum, other materials with antimicrobial properties can be added to achieve synergistic effects. Spherical silver nanoparticles (Ag-NPs), sized between 20 and 50 nm, were prepared using tea tree leaf extract for the treatment of historical parchments (in Fig. 7)¹¹⁶. The findings indicated efficient bacterial and fungal eradication by Ag-NPs, with negligible alterations in the chemical and mechanical properties of the parchment manuscripts. Green lemongrass oil was applied for parchment sterilization, and its nano-emulsion form improved its antimicrobial traits without adversely impacting the optical and mechanical properties of parchment¹¹⁷. Nanomaterials have emerged as novel antimicrobial agents due to their unique chemical and physical properties, inspiring more consideration of nanotechnology as a green and sustainable approach to antimicrobials. To prevent the absorption of airborne pollutants by leather heritages in museum and library environments, beeswax-paraffin blends and methyl acrylate-ethyl methacrylate copolymer coatings were developed for the treatment of vegetable-tanned leather¹¹⁸. These coatings provided remarkable protection for leather from SO₂ damage and improved the mechanical properties of treated leather. Additionally, monitoring air pollutants in heritage institutions for preventive conservation of leather and parchment is also a new research perspective.

Chemical interventions for paper preservation primarily include deacidification, reinforcement, and the use of antioxidants. Deacidification treatments mitigate acid hydrolysis during the natural deterioration of paper by introducing an alkaline substance to neutralize the acid and retain some alkalinity to prevent future acidification. Liquid deacidification typically involves an aqueous or organic solution containing an alkaline substance for paper treatment. However, excessive alkalinity could lead to alkaline hydrolysis of cellulose or pigments used in writing. An alkali-exchanged Y zeolite material with adjustable alkali ion content and concentration was coated on the surface of deteriorated paper, effectively improved its pH¹¹⁹. The material exhibited long-term stability of deacidification without causing pigment discoloration. For the consolidation of deteriorated paper, cellulose derivatives such as cellulose nanofibrils, cellulose nanocrystals, and bacterial cellulose have been used due to their remarkable mechanical properties and optical transparency¹²⁰. Additionally, these compounds exhibit a deacidifying effect when combined with alkaline substances. Bacterial cellulose, used as a consolidating agent due to its compatibility with paper and excellent mechanical properties, formed composite coatings with zinc oxide nanoparticles, offering favorable deacidification and consolidation effects¹²¹. This coating provided deacidification and curing properties while also demonstrating strong anti-light, anti-heat, and anti-fungal properties, providing innovative insights for the preparation of multifunctional composites. Hydroxyapatite and calcium carbonate mineralized BC films with adjustable alkalinity and alkali reserve were prepared, respectively¹²². The mineralized BC films provided efficient and sustained deacidification performance. This is in line with the principle of reversibility in heritage conservation, which means that BC films can be removed without damaging paper. In addition, due to their unique organic-inorganic composite structure, mineralized BC films had excellent flame-retardant properties on paper. Nanocellulose and CaCO₃ were used to prepare transparent coatings with deacidifying and superhydrophobic properties towards historical papers from an old wheat straw pulp book that published in 1954¹²³. Coating paper must ensure that fonts are visible on the paper. As the primary concern of transparency, coating paper must ensure that fonts are visible on the paper. Antioxidants are generally employed to alleviate the oxidation problem of paper. Antioxidants include blocking and chain-breaking types²⁷. Blocking antioxidants such as ethylenediaminetetraacetic acid and phytic acid, prevent the formation of free radicals through chelation. Chain-breaking antioxidants resist oxidation by blocking the chain reaction of free radicals, with tetrabutylammonium bromide demonstrating excellent stabilizing effects¹²⁴. However, direct contact between these chemicals and the paper can inevitably cause secondary damage. From a preventive perspective, a removable protective coating of graphene was deposited on the paper. which can be removed with a soft rubber. This coating blocks UV light, oxygen, water vapor, and other corrosive gases without damaging the cultural heritage¹²⁵. Wood and paper exist in vastly different environments and thus require distinctive conservation methods. Most wooden heritages preserved in waterlogged and moist environments are rapidly destabilized by light and oxygen post-excavation. To maintain the mechanical stability of wood, synthetic polymers, resins (such as polyethylene glycol), and inorganic substances such as potassium aluminum sulfate have been used as consolidants¹²⁶. However, these traditional treatments compromise the esthetics and repeatability of historic wood. Currently, nanomaterials are being utilized as consolidants and protective coatings due to their superior attributes and deep penetration into wood pores. Halloysite nanotubes, prepared as a composite material with beeswax, have been shown to effectively strengthen water-soaked archeological wood, demonstrating the enormous potential of nanomaterial applications and promoting the trend of selecting nanomaterials for composite protection¹²⁷. Additionally, inorganic particles such as zinc oxide, titanium dioxide, gold, and Ag-NPs could enhance the consolidation of wood¹²⁸. Due to the complexity of the preservation environment, protective materials must offer specific hydrophobic, anti-UV, and antimicrobial effects. For instance, silicon dioxide nanoparticles combined with cellulose nanofibrils could form a composite coating that improved the wood hydrophobicity (in Fig. 8a)¹²⁹. The incorporation of cellulose nanofibrils improved the dispersion stability of the inorganic particles and the coating’s durability. The superior mechanical properties of cellulose derivatives have led to their widespread use in the consolidation of silk, paper, and wood, with promising future applications in leather and parchment. Ethyl methacrylate and methyl acrylate copolymers blended with hydrophobic silica nanoparticles which were sprayed onto fabric, paper, and wood substrates to create transparent, superhydrophobic coatings¹³⁰. These coatings are convenient to prepare and exhibit UV and weathering resistance compared to those with cellulose nanofibrils. Zinc oxide is particularly suitable for preserving wood from UV deterioration due to its chemical stability and optical properties. The uniform distribution of nanoscale inorganic particles in the dispersion medium significantly enhanced the UV-blocking performance. Consequently, composite coatings of nanoscale zinc oxide with polymers have been effectively applied to reduce wood discoloration induced by photo-degradation¹³¹. Regarding wood decay, chitosan extracted from biomass had effectively protected wood heritages from fungal decay¹³². Mixed emulsions of chitosan and cinnamaldehyde derived from cinnamon significantly increased the antifungal activity (in Fig. 8b)¹³³. The film-forming properties of chitosan prolong the antifungal duration of cinnamaldehyde. Lignin nanoparticles encapsulating the essential oil of thymus vulgaris were sprayed onto wood samples¹³⁴, effectively capturing and retaining the essential oil by filling surface pores. Bio-based materials and bioactive substances are emerging as sustainable and green options for sterilization.

**Fig. 8: The contact protection approaches of wood.**

Contactless protective approaches

Synthetic chemical materials directly coat or penetrate organic cultural heritages, inevitably producing secondary damage to them. Therefore, some physical methods are applied to minimize the intervention on the heritages. Principal methods include gamma irradiation, high-frequency currents, low-oxygen environments, and ultraviolet radiation. Gamma rays, with their exceptional penetration, can traverse materials without leaving residue or causing thermal effects, making them an effective and sustainable method for treating microorganisms on silk fabrics, leather, parchment, paper, and wood^135,136,137. Doses up to 10 kGy were applied for sterilization without affecting the inherent mechanical properties and color of the material. For example, fungi failed to proliferate on paper books after appropriate radiation even in conducive conditions for two months¹³⁵. Nevertheless, irradiation could impair dye adsorption in silk fibers, particularly with natural dyes, causing silk fabrics to become darker than unirradiated samples¹³⁸. Therefore, it is essential to assess the source of dyes in treated fabrics when using irradiation techniques for fabric preservation. X-ray irradiation of leather fragments showed that doses below 1 kGy suppressed bacterial growth while leaving the stability of collagen molecules unaffected¹³⁹. UV radiation mitigates fungal-induced leather deterioration, resulting in minimal post-irradiation damage³⁵. Irradiation with UV-C for 15–60 min effectively combated leather infection without compromising its performance. Unaged paper and two books from the late 19th and early 20th centuries (containing lignin phases) provided by the National and University Library of Zagreb were gamma irradiated within the commonly used absorbed dose range (up to 8 kGy)¹⁴⁰. The effectiveness of irradiation in removing biological detergents depends on the initial oxidation level of the samples and their structural and chemical composition. Therefore, it is required to establish and optimize the optimal handling of organic cultural heritages in different states. Atmospheric plasma technology, used for non-destructive paper deacidification¹⁴¹, preserved the original paper appearance of the paper by avoiding cellulose degradation due to the rapid plasma treatment time. Meanwhile, plasma also inactivated microorganisms (in Fig. 9)¹⁴². The above methods extend the life of organic cultural heritages while avoiding changes to their originality to the maximum extent possible. These physical methods primarily focus on sterilization and deacidification of organic cultural heritages but lack broader protective applications. To prevent the oxidation of organic heritages under UV exposure, nano-TiO₂, with UV absorption capabilities, could be spray-coated onto the protective glass surface¹⁴³. Graphene could also be deposited on protective glass to achieve a non-contact method of protecting heritages from UV and oxidant damage¹²⁵. These effective and portable measures could avoid direct contact with fragile organic heritages, ensuring no secondary damage to the precious non-renewable cultural heritage. In addition, vapor phase sterilization methods have been advanced. Ethylene oxide fumigation is the common method of sterilization; however, it is extremely hazardous to humans. To avoid the destruction of silk fabrics by conventional disinfection methods and the use of hazardous chemical substances, vapor phase disinfection was carried out with plant-derived cinnamon essential oil¹⁴⁴. This method demonstrated effective antimicrobial activity without causing significant changes in the optical, mechanical, and structural parameters of the silk fabrics. As a novel, effective, and green approach, it offers new perspectives on antimicrobials for cultural heritage preservation. Origanum vulgare and Thymus vulgaris essential oils as green fungicides were used to protect two wooden heritages from the 18th century¹⁴⁵. They are excellent at removing Aspergillus flavus colonies isolated from the bases of wood carvings, and Anobium punctatum insects from the heads of Sicilian puppets. Wooden heritages are especially in need of insect repellent treatments due to their susceptibility to insect decay. Plant essential oils, as a green protection method, provide a non-polluting and insect-free environment for wooden heritages with high sterilization efficiency.

At present, the main contactless protective approaches are: disinfection and deacidification through physical methods, attaching protective substances to intermediate media, and diffusing disinfecting chemicals onto the surface of organic heritages through gas-phase evaporation. The above contactless methods provide protection while avoiding damage to fragile and precious organic heritages. However, there are also some limitations. As an indirect contact method, its protective performance is limited, and not all protection measures can be used in contactless manner, such as reinforcement. In addition, in some respects, contactless protective measures are not as effective as contact synthetic protective measures, for example, in terms of vapor phase sterilization, they are not as strong as applying an antimicrobial coating on heritage surface. Therefore, in the future, contactless protective measures should be explored to achieve more aspects of protection, such as UV resistance and hydrophobicity, while ensuring the effectiveness of the method.

Conclusion and perspective

The characteristics, identification, deterioration mechanisms, and protective approaches of diverse organic cultural heritages are summarized. Organic cultural heritages play a pivotal role in the transmission of history and culture, with main components of fibroin, collagen, and cellulose. Both fibroin and collagen are proteins composed of amino acids, but the different types and arrangements of amino acids lead to distinctions in mechanical properties. Cellulose has a composition and structure distinct from both of these. Consequently, different species of organic heritages exhibit unique deterioration mechanisms. Further scientific research is crucial for preserving organic cultural heritages, understanding deterioration mechanisms, and formulating strategies for novel conservation methods and materials. A range of identification and analysis methods have been adopted to assess the existing state and deterioration mechanism of cultural heritages. For example, morphological analysis of fungal-deteriorated leather indicated roughened surfaces and fiber damage. Combined spectral and thermal analyses qualitatively assess the molecular structure, composition, crystal structure, and thermal stability of organic cultural heritages. Elemental and amino acid analyses quantify the severity and process of deterioration. Employing omics tools to comprehend microbial types and metabolic functions, enabling the inhibition of their growth. These methods range from non-destructive techniques for in situ analysis to destructive methods such as thermal analysis. Deterioration is an inevitable long-term effect triggered by environmental conditions, causing significant changes in the appearance, mechanical properties, composition, and internal structure of organic cultural heritages. Regardless of their specific deterioration mechanisms, oxidation and hydrolysis are involved. Understanding the deterioration process is key to refining long-term preservation strategies using non-destructive techniques. Synthetic polymers are utilized as reinforcements or adhesives, and materials with specific effects such as antimicrobial and UV resistance are applied as protective coatings. Additionally, several nanomaterials are extensively employed due to their excellent properties. Gamma rays, plasma, and protective coatings on glass covers preserve fragile organic heritages in a non-contact manner.

Although the modern analytic techniques for understanding the deterioration mechanism of organic cultural heritages under diverse factors and for developing preservation approaches have made remarkable progress, certain challenges remain:

(1) Reduce Irreversible Damage by Characterization Techniques: Many current techniques cause irreversible damage, limiting their application in the characterization and preservation of organic cultural heritage. For example, while thermal analysis can be used to assess the molecular stability, it can be fatal to heritages because the sample is completely destroyed. Therefore, it is important to identify the target to be analyzed before using this analytical technique in order to minimize or even eliminate its use. If necessary, choose other non-destructive methods that can be substituted to achieve the analytical goal. Given the complexity of these materials, a comprehensive multi-technique analysis is required, such as using SEM-EDS to analyze the artifacts’ morphology along with the compositional analysis. Additionally, advanced characterization methods from other fields should be adapted and combined for use in organic heritages.

(2) Promote the Interdisciplinarity Collaboration: The interdisciplinary research area, encompassing materials science, biology, and history, aims to elucidate specific chemical reactions and deterioration mechanisms. For instance, the mechanism behind irreversible chemical reactions and the evolution of the multilayered structure during the deterioration of leather and parchment remain unclear. In addition, computer technology helps to understand the physical behavior of changes in composition or structure of cultural heritages and can provide assistance in the decision-making process for their conservation. A more detailed understanding of these processes is needed, requiring collaborative efforts among researchers from various disciplines.

(3) Develop Preventive and Reversible Conservation Materials: Conservation materials should be able to prevent or delay changes in the physical and chemical properties of cultural heritages by monitoring the preservation environment before deterioration occurs. Protective materials should be able to be removed without affecting the cultural heritages if the protective materials are necessarily used. Furthermore, the preparation of protective materials should consider both the substrate and additives. The base material should have good biocompatibility with the protected material. For materials used to enhance the consolidation of cultural heritage, the substrate should be stable over time or not be destructive to the cultural heritage, even if it degrades. Additives should be as green, non-polluting, and cause no secondary damage. Thus, appropriate base materials and additives should be thoroughly explored.

(4) Formulate Targeted Conservation Task: Differential environments cause various modes of deterioration, such as damage from dry heat, moist heat, light, and microorganisms, which require conservation measures. Targeted protection is conducive to reducing damage to cultural heritages. Therefore, cultural heritages with different causes of deterioration should be treated with specific conservation methods. While valuable cultural heritages deserve costly conservation measures, efficient and low-cost solutions are significant for research and development.

This review is expected to provide novel ideas and inspiration in a modern perspective to facilitate the identification, understanding of deterioration mechanism, and conservation of organic cultural heritages, ultimately aiding in cultural transmission.

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Spatial evolution of traditional waterside settlements south of the Yangtze River and the distribution of settlement heritage: evidence from the Nanxi River Basin

The study of ancient settlements in the traditional waterside towns of Jiangnan is an important part of scientific research on architectural heritage. This study examines ancient settlements in the Nanxi River Basin during various historical periods, such as the Neolithic Age, Eastern Han Dynasty, Tang-Five Dynasty, Song-Yuan Dynasty, Ming Dynasty, and Qing Dynasty. It investigates their temporal and spatial evolution and the factors that influence their distribution, with a particular focus on the role of intangible cultural heritage. This study focuses on the relationship between the spatial evolution of traditional waterside settlements in the Nanxi River Basin and the distribution of intangible heritage and analyzes the driving factors of their development. How settlements changed over time and space was examined with geographic information systems (GIS) software and by using kernel density, elliptical variance, and spatial autocorrelation methods on 204 ancient settlement points. This study also employs buffer and data overlay methods to analyze the factors that affect settlement distribution by elevation, slope, water system distance, and distance to intangible cultural heritage points. The study reveals the following. (1) During the Ming and Qing Dynasties, clans, culture, and the economy drove the expansion of early settlements, which relied on water systems and flat terrain, to form a multicenter distribution. (2) The settlement distribution in the Nanxi River Basin has undergone a transformation from single-point distribution to multipoint aggregation and divergence during the evolution from the Neolithic Age to the Qing Dynasty. The overall center of gravity of the settlements shifts from south to north and east, and the overall distribution of the settlements is in a state of aggregation. (3) The spatial and temporal evolution of settlements is jointly influenced by the natural environment and cultural factors. The natural environment determines the spatial distribution of early settlements, while cultural factors promote the evolution and development of the settlement space. This study further clarifies the key role of intangible cultural heritage in the formation and development of settlements and provides a reference framework for future heritage protection policies.

AdMaPlace March 16, 2025

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The spatial coupling and its influencing mechanism between rural human-habitat heritage and key rural tourism villages in China

Exploring the influencing factors and its influencing mechanism of the spatial coupling between rural human-habitat heritage (RHH) and key rural tourism villages (RTV) at county scale from the perspective of space can expand the theoretical research on the spatial coupling mechanism between RHH and RTV, and further provide theoretical reference and data support for the coordinated development and high-quality development of RHH and RTV in China. At the same time, previous studies have failed to systematically analyze the influencing factors and influencing mechanisms of the spatial coupling between RHH and RTV at the county scale, which restricted decision makers from formulating coordinated development measures between RHH and RTV at the macro level. In this study, bivariate spatial autocorrelation model and spatial coupling coordination model were used to quantitatively analyze the spatial coupling level between RHH and RTV at the county scale in China. Then, the linear regression (OLS) model, geographically weighted regression (GWR) model, and optimal parameter GeoDetector (OPGD) model were integrated to systematically analyze the linear influencing, spatial heterogeneity effect and interactive effect of natural environment and socioeconomic factors on the spatial coupling level between RHH and RTV in China, and explore the interactive influencing mechanism. The results show that the spatial coupling level of RHH and RTV in China show a significant east-west differentiation. There were 2024, 473, 293, 55 and 6 areas of severe, moderate, mild, basic and moderate coordination between RHH and RTV, respectively. Among them, severe and moderate discoordination areas are mainly distributed in Northeast China, arid and semi-arid areas in Western China, plateau areas in Southwest China, densely populated urban agglomerations and plains agricultural areas in the Middle East China. Mild discoordination areas and basic and moderate coordination areas are mainly located in transition zones in mountainous and plain areas, economically developed mountainous and hilly counties along the southeastern coast, and coastal tourist cities. Economic and population factors are the fundamental factors that affect the spatial coupling between RHH and RTV. Rural tourism facilities and rural public service facilities are important external driving forces for the coupling development of RHH and RTV, and Sociocultural environment factors are the important internal driving forces. Different surface forms, different climate conditions and different ecological environment conditions can form different natural textures and spatial organizations. Suitable climate conditions, sufficient water sources and ecological environment conditions can form more suitable rural settlement construction conditions and production and living conditions, and ultimately affect the protection and activation of rural human settlement heritage and the development and layout of key tourist villages. The spatial coupling relationship between RHH and RTV is the result of the complex interaction between the natural directivity law caused by natural environmental factors and the humanistic directivity law caused by human social and economic activities.

AdMaPlace March 16, 2025

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How digital technologies have been applied for architectural heritage risk management: a systemic literature review from 2014 to 2024

This systematic literature review critically examines the application of digital technologies in architectural heritage risk management from 2014 to 2024, focusing exclusively on English-language publications. As the significance of architectural heritage continues to be recognized globally, there is an increasing shift towards integrating digital solutions to ensure its preservation and management. This paper explores the evolution and application of digital technologies such as Building Information Modeling (BIM), Geographic Information Systems (GIS), and advanced imaging techniques within the field. It highlights how these technologies have facilitated the non-destructive evaluation of heritage sites and enhanced accessibility and interaction through virtual and augmented reality applications. By synthesizing data from various case studies and scholarly articles, the review identifies current trends and the expanding scope of digital interventions in heritage conservation. It discusses the interplay between traditional conservation approaches and modern technological solutions, providing insights into their complementary roles. The analysis also addresses the challenges and limitations encountered in the digital preservation of architectural heritage, such as data integration, the compatibility of different technologies, and the need for more comprehensive frameworks to guide the implementation of digital tools in heritage conservation practices. Ultimately, this review underscores the transformative impact of digital technology in managing architectural heritage risks, suggesting directions for future research and the potential for innovative applications in the field.

AdMaPlace March 16, 2025

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From lignin self assembly to nanoparticles nucleation and growth: A critical perspective

Recently, self-assembly processes operating in lignin have been the subject of intense research with the aim to prepare high-value, environmentally sustainable, nanomaterials for diverse material applications such as antimicrobial, antibacterial, UV-shielding, photonics etc. Lignin offers structural features that make these potential applications possible once well-tailored aggregates are synthesised. However, the intricate nature of lignin and the ensuing complexity of its aggregation behaviour, coupled with the plethora and diversity of techniques used, still requires systematic attempts to unify the scattered and at times contradictory data. This effort attempts to enumerate the various parameters affecting self-assembly phenomena of lignins by critically considering the science behind its solvation behaviour in organic solvents, in aqueous media, and its eventual self-assembly in forming nano-systems. Finally, knowledge gaps that cause present limitations and potential future efforts, are identified so as to arrive at a thorough comprehension of these phenomena.

AdMaPlace March 17, 2025

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The texture of Chinese garden rockery stones: based on 3D point cloud and 3D printing technology

Research on the surface texture of traditional Chinese garden rockeries focuses mainly on painting and photography, which significantly limits archeological research. This study proposes the use of a three-dimensional point cloud combined with a 3D printing method to obtain and analyze the surface texture of Chinese garden rockeries of different scales and sizes. Compared with traditional painting and photo recording methods, 3D point clouds can indiscriminately record the changes in surface roughness and texture of objects and are not affected by light and shadow interference. Simultaneously, modeling was conducted based on the scanning results of the 3D point cloud, and attempts were made to use 3D printing to print the models at different scales. To accurately restore the surface texture, this study used model segmentation combined with local structural reinforcement for printing and stitching large-volume models. Small-volume models were printed using integrated printing, and different materials were used for printing attempts. This study explored a new approach to heritage research regarding the recording and reproduction of the surface texture of rockeries through a combination of 3D point clouds and 3D printing, providing possibilities for heritage protection and sustainable research.

AdMaPlace March 16, 2025

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Introduction

Organic cultural heritages

Silk fabrics

Leather and parchment

Paper and wood

Other organic cultural heritages

Identification of organic cultural heritages from diverse perspectives

Qualitative analysis

Quantitative analysis

Microbiological analysis

Deterioration of organic cultural heritages by environmental factors

Various factors affecting the organic cultural heritages

Distinct deterioration mechanism

Deterioration of silk fabrics

Deterioration of leather and parchment

Deterioration of paper and wood

Modern protective approaches on organic cultural heritages

Contact synthetic protective approaches

Contactless protective approaches

Conclusion and perspective

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