Challenges and Prospects of Applying Nanocellulose for the Conservation of Wooden Cultural Heritage—A Review ^†

Abstract

Nanocellulose is a nanostructured form of cellulose, which retains valuable properties of cellulose such as renewability, biodegradability, biocompatibility, nontoxicity, and sustainability and, due to its nano-sizes, acquires several useful features, such as low density, high aspect ratio and stiffness, a high specific surface area, easy processing and functionalisation, and good thermal stability. All these make it a highly versatile green nanomaterial for multiple applications, including the conservation of cultural heritage. This review provides the basic characteristics of all nanocellulose forms and their properties and presents the results of recent research on nanocellulose formulations applied for conserving historical artefacts made of wood and paper, discussing their effectiveness, advantages, and disadvantages. Pure nanocellulose proves particularly useful for conserving historical paper since it can form a durable, stable coating that consolidates the surface of a degraded object. However, it is not as effective for wood consolidation treatment due to its poor penetration into the wood structure. The research shows that this disadvantage can be overcome by various chemical modifications of the nanocellulose surface; owing to its specific chemistry, nanocellulose can be easily functionalised and, thus, enriched with the properties required for an effective wood consolidant. Moreover, combining nanocellulose with other agents can also improve its properties, adding new functionalities to the developed supramolecular systems that would address multiple needs of degraded artefacts. Since the broad use of nanocellulose in conservation practice depends on its properties, price, and availability, the development of new, effective, green, and industrial-scale production methods ensuring the manufacture of nanocellulose particles with standardised properties is necessary. Nanocellulose is an interesting and very promising solution for the conservation of cultural heritage artefacts made of paper and wood; however, further thorough interdisciplinary research is still necessary to devise new green methods of its production as well as develop new effective and sustainable nanocellulose-based conservation agents, which would replace synthetic, non-sustainable consolidants and enable proper conservation of historical objects of our cultural heritage.

1. Introduction

The need to remove petrochemical-based materials from our lives is indisputable since they are non-renewable, pollute the environment, and contribute to high carbon dioxide emissions, causing the greenhouse effect and leading to climate change [1,2]. Bio-based materials, including oleo-chemicals derived from plant oils, various biopolymers of plant and animal origin, natural curing agents, or other materials based on agricultural by-products and waste biomass, are sourced from abundant and inexhaustible natural resources and guarantee biodegradability and low environmental impact [1,3,4,5,6]. They have multiple applications in various industries, from chemical production to agriculture, construction, food, pharmaceutical, biomedical, and cosmetic branches, offering a renewable, environmentally friendly, and sustainable alternative to fossil-fuel-based products [7,8,9,10,11,12].

The trend towards replacing petroleum-based products with green solutions has also entered the cultural heritage conservation area [13]. According to the United Nation’s Sustainable Development Goals for 2030, strengthening “efforts to protect and safeguard the world’s cultural and natural heritage” appears to be one of the measures taken to achieve Goal 11—“Make cities and human settlements safe, resilient, inclusive and sustainable” [14]. The Sustainable Development Goals oblige us not only to preserve our cultural heritage for future generations but also to do it sustainably, which forces the development of new, green conservation agents and methods where bio-based polymers play the first fiddle [15,16,17,18,19,20]. Among bio-based polymers of great potential for the conservation of cultural heritage is cellulose, particularly one of its forms, nanocellulose, which has already been tried for various purposes, including cleaning surfaces of stone artefacts and paintings, as protective coatings for historical paper and textiles, stabilisation of damaged painting canvases, consolidation of degraded silk, and deacidification of historical papers [21,22,23,24,25,26,27].

This review aims to present the state of the art of nanocellulose application in the field of cultural heritage conservation, particularly in the conservation of artefacts made of lignocellulosic materials, with a special focus on wooden objects. The first part describes the principal information about nanocellulose types, their properties, and applications. Then, the main wood degradation factors and patterns are presented along with individual needs and conservation problems with degraded historical wooden objects depending on their burial environment and display or storage places. The following review presents the results of recent research on nanocellulose formulations applied for the conservation of historical artefacts made of wood and paper. We highlight the up-to-date solutions based on nanocellulose, discuss their effectiveness, advantages, and disadvantages, address the challenges associated with using nanocellulose in historical wood conservation, and debate prospects of its broader use as a green alternative to petroleum-based conservation materials.

2. Cellulose and Nanocellulose

2.1. Cellulose

Cellulose is the most abundant polymer on Earth and the main structural component of plant cell walls, several algae tissues, and the membrane of epidermal cells of tunicates, where it is present in the form of microfibrils. It can also be synthesised by bacteria as a nanofiber network. Chemically, cellulose is a linear polysaccharide composed of repeated glucose units linked together via an oxygen atom covalently bonded to C1 of one glucose ring and C4 of the adjacent ring (the β-1,4-glycosidic bond) [28,29]. As a widespread, natural, non-toxic, environmentally friendly, biodegradable, and renewable organic material, cellulose has been widely used as a source of energy, clothing, and building material. The excellent properties of cellulose and its derivatives, such as good mechanical properties (mainly high tensile strength), high surface area, low density, high aspect ratio, biocompatibility, adaptable surface characteristics, easy accessibility, and low cost, make it an invaluable material for a successively increasing number of various industrial purposes (Figure 1) [30,31,32,33,34,35,36].

2.2. Nanocellulose

Nanocellulose (NC) is a nanostructured form of cellulose, and four of its types are distinguished: cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), regenerated nanocellulose (RNC), and bacterial cellulose (BC). The main properties, particle forms and sizes, and production methods of nanocellulose types are summarised in

Table 1. Nanocellulose types and their characteristics.

Type of Nanocellulose	Cellulose Nanocrystal (CNC)	Cellulose Nanofibril (CNF)	Regenerated Nanocellulose (RNC)	Bacterial Nanocellulose (BNC)
Synonyms	whiskers, cellulose nanocrystals, rod-like cellulose crystallites [37]	nanofibrils, nanofibrillated cellulose [37]	cellulose nanospheres [38]	bacterial cellulose, microbial cellulose, biocellulose [37]
Typical sources/precursor materials	wood, cotton, hemp, flax, wheat straw, algal cellulose, bacterial cellulose, bast fibres, microcrystalline cellulose, tunicin [37,39]	wood, potato tuber, sugar beet, flax delamination, and hemp [37]	cellulose I or II molecules [38,40]	low-molecular-weight sugars and alcohols [37]
Shape and size	Rod- or needle-like particles with high crystallinity, a diameter of 3–50 nm, a length of 100 nm to several µm, and an aspect ratio of 5–50 [39,41]	intertwined, network-like structure including at least one elementary fibril consisting of both crystalline regions and amorphous regions, with crystallinity lower than CNC, a diameter of 3–500 nm, a length of up to several µm, and an aspect ratio of 60–100 [41,42]	spherical molecules with high crystallinity and a diameter ranging from 10 to 500 nm [38]	different types of nanofiber networks and ribbon-shaped fibrils with high crystallinity, a diameter of 20–100 nm, a length of up to several µm, and an aspect ratio of 600–6000 [37,41,42]
Production method	acid hydrolysis [39,41]	mechanical treatment before and/or after chemical or enzymatic treatment, electrospinning [37,43]	dissolution and precipitation/regeneration [38]	mostly synthesised by bacteria [37,41]
Properties	high hardness, cellulose chains stacked neatly, optical transparency, low thermal expansion, gas impermeability, brittleness, Young modulus of 140–160 GPa, tensile strength of 8–10 GPa, particles are rigid [39,41,42]	low density, high strength, hydrophilicity, manipulated porosity, Young modulus of 30–40 GPa, tensile strength of 0.8–1 GPa, particles are flexible [42,43]	high purity, good thermal properties [44]	relatively pure, translucent and gelatinous, Young modulus of 15–130 GPa, tensile strength of 0.2–2 GPa [42,45]

.

Nanocellulose not only retains the valuable characteristics of cellulose, such as renewability, biodegradability, biocompatibility, nontoxicity, and sustainability, but, due to its nanometre scale, it has multiple unique features, such as a high specific surface area, easy processing, low density, high aspect ratio and stiffness, and good thermal stability, which make it versatile and one of the most significant green materials for various end uses in recent years [46,47]. Many structures of NC, differing in forms and properties, permit the development of multiple types of materials with improved mechanical, rheological, and optical properties, which may be used in many inventive utilisations in diverse areas. They are applied as components for producing biomedical materials, catalytic membranes, and electronic and sensing devices, as well as in packaging, energy production, water treatment, air filtration, cosmetics, pharmaceutical, and paper and cardboard industries. Due to its low thermal expansion coefficient, high elastic modulus, and stiffness, NC is also an excellent material for polymer reinforcement, which enables the development of novel multipurpose functional materials [41,46,48,49,50,51,52]. Therefore, there is increasing interest in using nanocellulose NC in its crystalline and fibrillated forms as a natural alternative for petrochemical-based materials, and significant efforts are being made in establishing new efficient and cost-effective methods for its isolation from various plants and waste lignocellulosic biomass and synthesis [47,53,54,55].

One of the unique properties of nanocellulose is an abundance of hydroxyl groups on its surface. On the one hand, they make NC highly hydrophilic, which gives nanocellulose great potential for application in aqueous media (e.g., hydrogels, rheology modifiers). However, it can also be considered a huge disadvantage due to swelling and a decrease in the strength of nanocellulose fibres when exposed to high-humidity conditions, hindering its performance for applications where mechanical strength and dimensional stability are key features. High NC hydrophilicity also leads to its poor dispersibility in polymers and non-aqueous media, which makes it inappropriate as a reinforcing agent for hydrophobic polymers [56,57]. On the other hand, the presence of hydroxyl groups on nanocellulose surfaces can be highly beneficial. They have a high surface energy and good binding activity, which enables a wide range of surface modifications (including hydrophobisation) and functionalisation via physical adsorption, chemical modification, or polymer graft copolymerisation, leading to the development of functional materials with customisable properties [41,46,57]. Thus, nanocellulose can be used in various forms (e.g., foams, films, gels, fibrils, crystals) for multiple applications, including adhesives, filers, hierarchical materials, flexible electronics, biomaterials, food stabilizers, dietary fibres, thickeners, flavour and drug carriers, filtering membranes, sensors and detectors, and a variety of papers and packaging materials [48,58,59,60,61]. Due to its versatile and adjustable properties, it has great potential as a green alternative for adhesives, coatings, and additives in the wood and paper industry; thus, it may also be useful for the protection and conservation of wooden artefacts of cultural heritage [62,63,64,65,66,67].

3. Wooden Cultural Heritage—Degradation Factors and Main Conservation Problems

Cultural heritage is an intrinsic part of our lives, of who we are and what we identify with, providing us with a sense of unity and belongingness within individual groups or societies. It is a record of human existence, history, and development that helps us understand our ancestors. It gives us a sense of personal identity and a thread of continuity that connects past, present, and future generations and manifests in our language, traditions, and performance. Through tangible artefacts, we experience intangible values, feelings, and impressions. We are obliged to care about our heritage and preserve it for posterity. Therefore, we are taking all available measures and saving historical artefacts in collections, museums, archives, libraries, monuments, and archaeological sites to preserve them and make them accessible to people. Wooden historical artefacts in the form of architecture, furniture, everyday objects, panel paintings, sculptures, musical instruments, shipwrecks, coffins, and several others are important parts of cultural heritage that require careful, dedicated conservation because of the natural material they are made of.

3.1. Degradation of Wooden Artefacts

Wood is a natural material composed of organic matter. As such, when exposed outdoors, it is susceptible to natural weathering (a complex combination of chemical, mechanical, and light energy abiotic factors) and biodegradation (biotic factors such as fungi, bacteria, and insects) [68,69].

The activity of different abiotic and biotic factors can cause two categories of damage to wooden artefacts:

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chemical, when quantitative and qualitative changes in wood chemical composition occur,

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physical damage to the wood tissue, including swelling, shrinking, cracking, peeling, surface roughening, checking, colour changes, or holes and corridors caused by living organisms boring or gnawing on wood organic matter [70]. Figure 2 shows examples of physical damage caused by various insects to wood.

Chemical decomposition is mainly caused by UV radiation (so-called photo-chemical degradation or photo-oxidation) or microbial and fungal decay. Photo-oxidation occurs on the wood surface since light cannot penetrate deeper than 200 µm and involves a sequence of complex free-radical reactions that lead to wood discolouration, changes in surface texture, and loss of brightness [71]. Chemical changes caused by organisms depend on their type. Among wood-decaying bacteria, divided into three groups based on their degradation pattern, erosion and cavitation bacteria decompose polysaccharides (hemicellulose and cellulose), while tunnelling bacteria degrade both polysaccharides and the lignin-rich primary cell wall and middle lamella [20,72,73,74]. The chemical composition of wood can also be altered by different types of fungi, including wood-decaying fungi (white-rot, brown-rot, soft-rot), mould, and blue-stain fungi [75,76]. White-rot fungi, the most effective lignin degraders on Earth, are equipped with unique extracellular nonspecific ligninolytic systems and intracellular oxidative enzymes. Among them, both selective lignin degraders and all wood polymer decomposers can be found. As a result of their activity, wood becomes soft, stringy or spongy, and white due to the presence of light cellulose which remains, with strength properties decreasing as decay progresses [77,78,79]. Brown-rot decay involves the degradation of carbohydrates and partial modification of lignin by demethylation, which results in wood shrinking and cracking into brown cubical pieces made mainly of lignin remains with decreased mechanical properties. Soft-rot fungi decompose polysaccharides and only partially lignin, causing cracking patterns, discolouration, and deterioration of mechanical strength like brown-rot [80]. Moulds do not alter wood structural polymers but decompose easily available sugars, causing wood discolouration and minor degradation of the wood surface, while blue stain fungi degrade both easily available sugars and proteins present in parenchyma cells, causing discolouration and increased permeation of wood due to the degradation of pit membranes [81]. Biodegradation strongly depends on factors such as water (e.g., fog, rain, snow, or water vapour in humid air), temperature, and oxygen availability [82]. The fungal attack occurs mostly in terrestrial environments where oxygen and enough moisture are present; however, soft-rot fungi can decompose wood also under partially oxygenated waterlogged conditions. The bacterial decomposition of wood in terrestrial environments is negligible, and their contribution is more pronounced by the synergistic or antagonistic effect they have on other microorganisms that inhabit wood. However, bacteria are the main cause of wood biodeterioration in anoxic or nearly anoxic waterlogged conditions [20,83,84].

Chemical changes ultimately lead to changes in the physical properties of wooden tissue, such as density, mechanical performance (compression, stretching, bending, shear strength), and moisture behaviour. They often cause aesthetic alterations of wooden artefacts, including various distortions and changes in surface texture and colour. However, most importantly, degradation processes often cause the disintegration of wooden tissue, which requires thorough consolidation procedures to save the object and keep it in one piece.

Biotic and abiotic damage to wood exposed outdoors affects its suitability for ongoing use, and the damage can alter wood responses to fluctuating relative humidity, even within a single artefact. For example, brown rot fungi cause prismatic cracks in wood, altering mechanical properties compared to non-decayed wood. Further complicating the restoration of wood affected by fungal decay, each prism responds differently to changes in air relative humidity. Wood damage caused by different types of organisms and abiotic factors affects it in unique ways. Moreover, the behaviour of “old” wood in varying environmental conditions, especially air relative humidity, differs from that of “new” wood, often used to fill gaps in the conservation process. This is due to differences in the chemical and physical properties between undegraded and degraded wood [85]. All these make the conservation of wooden artefacts a challenging and complex issue, which requires broad, interdisciplinary knowledge at the border of material science, chemistry, physics, history, conservation, and art.

3.2. Conservation of Degraded Wood—Main Problems and Solutions

Conservation of historical artefacts is guided by several principles, the most difficult of which is that the treatment should be reversible and not affect the original structure of an object. The chemicals used in restoration must ensure maximum durability and chemical inertness while stopping processes that would further degrade an artefact, all without changing the item’s chemical composition and physicochemical and mechanical properties. The substances must be compatible with the artefact’s materials to achieve this effect [86]. All these are not easy, especially since conservation treatment should address the specific needs of an artefact, and those differ depending on the pattern and extent of degradation, the type of an artefact, and the place and conditions of its exposition or storage. Multifarious problems and dilemmas continually confront conservationists in preserving artefacts of cultural significance, creating a need to search for new, more reliable solutions.

One of the most crucial conservation procedures is the structural consolidation of highly degraded wood tissue (where “highly degraded” means that it is completely perforated by insects, its anatomical elements are almost entirely destroyed, or it reaches an almost powdery state). It aims to restore or strengthen the physical and mechanical wood properties to keep an object’s integrity and prolong its life whilst preserving its aesthetic and historical values following conservation ethics. There are plenty of consolidating agents and methods that are useful for different wood types and degrees of degradation. Nowadays, the most common consolidants for dry wood are synthetic resins (e.g., Paraloid B72, Acryloid B72, Butvar B98, AYAT, Regalrez 1126, used with various solvents), which help improve the wood integrity and mechanical properties by fully or partially filling voids in wooden tissue. Although the method is irreversible in a strict sense, some resins can be removed using specific solvents [87,88,89,90,91,92]. In the case of waterlogged artefacts, the consolidation of delicate wood tissue requires simultaneous dimensional stabilisation to prevent wood form shrinkage, cracking, and disintegration during drying after its excavation from a wet environment and exposition to air. To achieve that, water molecules inside the wood structure must be replaced with a proper consolidation agent [20]. Historically, alum was applied for this purpose, and more recently, polyethylene glycol (PEG) and various sugars have mainly been used in conservation practice, but they all have their drawbacks. PEG’s shortcomings include high hygroscopicity and easy leachability from wood under changeable temperature and moisture conditions, which causes dimensional changes leading to permanent wood deformations or even disintegration, dark wood colouration and its heavy weight, waxy finish on the wood surface, plasticising effect on wood decreasing its mechanical properties, and vulnerability to oxidative and iron-catalysed degradation into low-molecular acidic compounds causing further chemical degradation of treated wood [93,94,95]. Therefore, since there is a need for more reliable alternatives, several other consolidants for waterlogged wood have been tried, including cellulose and lignin derivatives [20,96,97,98], chitosan [99], guar [100], oligo- and polyamides [101], and organosilicon compounds [102], as well as combinations of various agents, including Halloysite nanotubes with wax, calcium hydroxide, or esterified colophony [103,104,105], as well as multifunctional complexes that address multiple needs of a treated object [106], and more research is in progress.

Other problems with historical wooden artefacts are gaps and holes in wood tissue formed due to various degradation factors (e.g., insects, fungal decay). When they cause the loss of an object’s aesthetic and historical values that may alter its proper interpretation, then aesthetic concerns are of the utmost importance, and soft, nonstructural fills are used to restore the original artefact’s appearance. However, if multiple or extensive gaps and cracks threaten the wholeness of a wooden item, conservation concerns predominate, and structural fills are needed to reinforce the wooden structure and provide it with integrity [107,108]. In the early days of art restoration, natural materials were used to fill voids and cracks in artefacts, including wooden fills, waxes, bitumen, plaster, chalk, concrete, gesso, or oil-based putties. In time, due to their better performance, they were replaced by synthetic materials such as polyurethane, silicon rubbers, and a variety of acrylic, methacrylic, epoxy, and vinyl resins mixed with natural sawdust or synthetic microballoons. Nowadays, as a general trend towards sustainability and eco-friendliness in all areas of our life occurs, also in conservation, a return to nature can be seen, where natural materials such as minerals and lignocellulosic materials become an increasingly frequent choice alongside natural binders and biocides. Moreover, various mixtures are commonly used, including both commercial ready-mixed pastes consisting of a filler, adhesive, and additional compounds (e.g., emulsifiers, pigments, biocides, thickeners) and home-made compositions of confidential content prepared to address the specific needs of individual artefacts [108,109,110].

Sometimes, only the surface of a wooden object requires protection against water, air pollutants, and biocorrosion. Protective coatings are not permanent and usually require periodic replacement. There are two types of coatings—solutions and suspensions or curable monomers and oligomers, and they can be applied by spraying or paintbrushing. Among commercially available resins, thermoplastics are preferred over thermosetting polymers [111,112].

Conserving wooden objects constantly exposed to weather conditions, e.g., in open-air museums, is particularly challenging. It requires selecting an appropriate biocide and preservatives, identifying a method of impregnation suitable to the condition and state of preservation of the artefact, and choosing a proper filler to seal gaps. Determining appropriate methods that combine modern materials and approaches along with existing methods to preserve wooden objects of cultural value is necessary here, together with interdisciplinary knowledge [108,113].

4. Nanocellulose for the Protection of Cultural Heritage

The durability, sustainability, and excellent mechanical properties of cellulose nanomaterials make them increasingly popular for multiple applications. Nanocellulose is a perfect example of a novel eco-friendly alternative to conventional materials used in many industries (Figure 1). The variety of structures that can be obtained using nanocellulose is extensive and far-reaching [114]. Although using cellulosic materials in nano-size form is a relatively new and rapidly developing area, its effectiveness for conserving wall paintings and paper deacidification provides evidence of the great potential of this emerging technology for cultural heritage conservation [115].

Table 2. Selected examples of recent research on nanocellulose applications in cultural heritage conservation.

Researcher, Year	Aim of the Study	Research Material Used in the Studies	Nanocellulose form Applied	Tests Performed	Results
Christensen et al., 2012 [116]	application of CNC to consolidate degraded wood	degraded Viking Age wood	CNC	scanning electron microscopy (SEM)	CNC does not penetrate thoroughly into the wood but adheres well to its surface CNC can act as a gap-filler and form the desired open structure PEG1000 used as a stabiliser improves CNC penetration
Santos et al., 2016 [117]	verification of the suitability of BNC to rebuild degraded old papers	three books made with chemical pulp from cereal straw, chemical and semi-chemical pulp from softwood, softwood mechanical pulp	BNC	burst and tear strength brightness, opacity and yellowness air permanence specular gloss static and dynamic water contact angles (WCA) SEM	mechanical properties of paper lined with BNC are as good as those obtained with traditional Japanese paper letters in books lined with BNC are more legible BNC improves the quality of deteriorated paper without altering the information contained therein, and this improvement is maintained over time
Camargos et al., 2017 [118]	application of CNC-based paper pulp for filling of lacunae of documents and artworks on paper	wood cellulose paper sheets with lacunae	CNC CNC-based paper pulp	surface pH measurements Fourier transform infrared spectroscopy (FT-IR) SEM tensile testing visual examinations with fluorescent light	CNC grafting shows very similar stress–strain behaviour to those presented by wood cellulose pulp paper, confirming the compatibility factor between restored paper and CNC grafting CNC grafting allows for achieving very regular and uniform filling surfaces
Gregory et al., 2017 [119]	determination of the potential of growing BCN directly onto waterlogged wood	model paper: pure cellulosic Munktell filter paper, glossy poster paper and Japanese Kozo paper wood: ash spear shafts from the waterlogged site of Nydam Mos	BNC		BNC does not bond to glossy poster paper but attaches well to Kozo paper and Munktell filter paper (after pre-treatment with acetone) BNC can grow on the surface and inside the pores of heavily degraded waterlogged wood
Völkel et al., 2017 [120]	application of nanocellulose suspensions for historical papers stabilisation	rag paper Whatman filter paper book paper without lignin book paper with lignin newsprint paper	BNC CNF	surface pH measurements FT-IR SEM tensile testing visual and haptic characterisation ISO brightness	BNC and CNF provide stabilisation for mechanically damaged papers BNC and CNF coat small losses and cracks in degraded paper BNC and CNF films formed on the paper surface induce small optical and haptic changes the use of pure nanocellulose suspension eliminates side effects caused by additives
Basile et al., 2018 [121]	application of CNC as a consolidant for degraded wood	old beams of Norway spruce (Picea abies L.) taken from a dismantled roof of a 17th-century villa in North Italy	CNC CNC mixed with lignin and/or siloxane derivatives	X-ray diffraction (XRD) FT-IR and Raman spectroscopy dynamic light scattering DLS Dynamic Mechanical Analysis (DMA) AFM	consolidation efficiency of CNC has been confirmed on old rotted wood CNC treatment was most effective on the highest degraded wood due to its best penetration into degraded wood tissue the effectiveness of CNC treatment depends on the degree of wood degradation and the number of impregnation cycles CNC treatment increases the stiffness of treated wood
Hamed and Hassan 2019 [122]	application of hydroxypropyl cellulose (Klucel E) and nanocellulose for consolidation of based artefacts	beech wood—hardwood	mixture of hydroxypropyl cellulose Klucel E and nanocellulose	retention of consolidant in treated wood colourimetry SEM FT-IR compression test	NC addition provides increasing penetration within the wood structure and compression strength of the treated samples NC addition does not cause colour changes the use of NC has no side effects, even after ageing NC addition effectively enhances wood consolidation
Jia et al., 2019 [123]	application of ZnO nanoparticles in CNC to improve antibacterial and mechanical properties of historical papers	historical paper: the school newspapers of Renmin University of China 1960	composite of ZnO nanoparticles and CNC	FT-IR and UV–vis absorption spectroscopy SEM, XRD EDS elemental mapping image folding endurance, tensile strength antibacterial and antifungal effect	coating paper with ZnO/CNC provides good colour stability CNC provides an effective dispersion matrix for ZnO nanoparticles ZnO/CNC nanocomposites have stronger antibacterial properties than unmodified ZnO ZnO/CNC nanocomposites can be modified to meet the desired application
Antonelli et al., 2020 [97]	application of lignin nanoparticles (LNP), BNC and CNC for consolidation of waterlogged archaeological wood	softwood: stone pine (Pinus pinea L.), silver fir (Abies alba Mill.), cypress (Cupressus sempervirens L.) hardwood: elm (Ulmus sp.) and probably holm oak (Quercus ilex L.)	LNP BNC CNC	anti-shrink efficiency equilibrium moisture content	LNP and BC addition cause colour changing problems with LNP and CNC penetration can be solved by modifying the impregnation conditions BNC addition provided a satisfying penetration, but the consolidating effect was not substantial
Bergamonti et al., 2020 [124]	application of CNC for paper preservation and consolidation	Whatman paper	CNC with Ag nanoparticles	DLS and electrophoretic light scattering (ELS) transmission electron microscope (TEM) and XRD FT-IR and Raman spectroscopy colourimetry antibacterial and mechanical tests	CNC/Ag coating prevents the growth of Aspergillus niger on Whatman paper CNC coating improves the mechanical properties of Whatman paper (stretch and toughness) presence of Ag does not affect the aesthetic appearance of the paper
Völkel et al., 2020 [125]	integration of the nanocellulose application into a multi-stage calcium phytate/calcium hydrogen carbonate treatment to combine deacidification and stabilisation of historic papers	rag papers from a collection of handwritten sermons from the years 1839 and 1840	CNF with calcium phytate/calcium hydrogen carbonate	colourimetry water contact angle SEM-EDX laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)	CNF is effective in the mechanical stabilisation of iron gall ink paper and can be combined with calcium phytate/calcium hydrogen carbonate treatment for the chemical stabilisation of iron gall inks CNF forms a stabilising network, has little to no optical interference and does not change the haptic properties of the manuscripts CNF stabilises and protects paper during accelerated degradation
Xu et al., 2020 [27]	application of ethanol-based nanocellulose (CNC) and alkaline nanoparticle “hybrids” on strongly degraded cellulosic artworks for its deacidification and consolidation	filter paper	ethanol-based “hybrids” containing grafted nanocellulose (CNC) and alkaline nanoparticles (Ca(OH)2 or CaCO3)	rheology small-angle X-ray scattering	ethanol and alkaline nanoparticles increase the interactions between grafted CNC, leading to the formation of dispersions suitable for conservation purposes treatment of filter paper with “hybrids” improves its mechanical properties and increases the pH
Ma et al., 2021 [126]	application of CNC and polyhexamethylene guanidine (PHMG) for reinforcing and improving mould-resistant properties of ancient books	historical paper published in 1954	suspension of CNC and PHMG	mechanical tests brightness test SEM XRD anti-mould test	CNC-PHMG enhances mechanical properties and improves the folding strength of treated paper CNC-PHMG treated paper slightly changes the whiteness CNC-PHMG treated paper retained good mechanical properties after ageing CNC-PHMG treated paper has excellent biocidal activity
Operamolla et al., 2021 [127]	application of sulphated nanocellulose (S_CNC) for consolidation of degraded paper	Whatman filter paper A book from 1738	S_CNC CNC	AFM, FE-SEM glancing incidence X-ray diffraction colourimetry pH experiments FT-IR ζ-potential measurements elemental analysis tensile tests	S_CNC can be removed from the surface using Gellan hydrogel S_CNC improves the optical quality and mechanical properties of paper pH of S_CNC treated paper decreases under accelerated ageing, posing a negative effect on the conserved paper artefact in the long-term non-sulphated CNC do not compromise paper pH and mechanical properties with ageing
Abdel-Hamied et al., 2022 [128]	evaluation of nanocomposites of mesoporous silica nanoparticle (MPSNP)/polyvinyl alcohol (PVA) and CNF/PVA for the consolidation of dyed paper manuscripts with madder extract	deteriorated dyed manuscript “DiwanShaar”	MPSNP/PVA CNF/PVA	SEM, TEM, AFM and XRD DLS FT-IR colourimetry mechanical tests (tensile strength and elongation)	MPSNP/PVA was more effective than CNF/PVA in consolidating degraded manuscript MPSNP/PVA treatment gave lower colour change, improved tensile strength and elongation, and stabilised and protected cellulosic fibres in the document against accelerated thermal ageing
Camargos et al., 2022 [129]	application of CNC, CNF and nanolignin for protecting diverse cellulose-based substrates	balsa wood (Ochroma pyramidale) white cotton paper	coating formulations of CNC in water nanolignin in water carnauba wax nanoparticles in water	moist-heat accelerated ageing thickness measurements water vapour permeability diffuse reflectance spectroscopy colourimetry surface morphology analyses water contact angle 2D Micro-FT-IR	nanocomposite coatings effectively protected the coated substrates against deterioration and moist-heat ageing coatings ensure effective presented UV-blocking properties nanocomposite-coated surface retained its morphology, roughness and vapour permeability nanocomposite-coated hydrophilic substrates demonstrated a more controlled wettability nanocellulose/nanolignin coatings are reversible treatments
Elmetwaly et al., 2022 [130]	application of CNC and inorganic nanotubes for historical paper consolidation	historical paper	colloidal solutions of CNC Halloysite nanotubes CNC-HNT	SEM TEM FT-IR thermogravimetric analysis tensile strength colourimetry	significant reinforcement of historical paper by using CNC-HNT coating CNC-HNT coating provides exceptional clarity and good optical properties of coated paper CNC-HNT coating provides superior protection against harmful UV rays CNC-HNT coating forms hydrogen bonds with the historic paper fibre matrix
Völkel et al., 2022 [131]	application of CNF coatings to preserve historical paper severely damaged by fire	historical paper samples damaged in an undefined and complex way during the fire of the Duchess Anna Amalia Library Weimar 2004	CNF	thermogravimetric analysis X-ray photoelectron spectroscopy (XPS) WCA roughness measurements SEM, AFM gel permeation chromatography	CNF coating enables the reliable preservation of the paper and retrieval of the contained historical information CNF fibres form a flexible, transparent film on the paper surface and adhere strongly to it, reducing its fragility, providing stability, and enabling digitalisation and handling
Walsh-Korb et al., 2022 [15]	analysis of the drying behaviour of CNC used as a wood consolidant	fresh and archaeological waterlogged oak wood (Quercus robur L.)	CNC	light microscopy, SEM, freeze-drying microscope (FDM) thermal analysis	CNC coats wood with an open, porous network, but it does not completely fill the wood cells despite its low viscosity CNC has a relatively low affinity for wood surface chemistry CNC interacts solely with the cellulose component of the treated wood, which makes it potentially unsuitable for waterlogged wood conservation, where cellulose is usually highly degraded
Bellia et al., 2023 [132]	application of suspension made of CNC with silver nanoparticles to preserve paper against fungal degradation; analysis of the suspension effect on the optical characteristics of the paper	Whatman and Amalfi paper	CNC and Ag-functionalized CNC	ATR-FR-IR colourimetry	chromatic variations in CNC/Ag-treated samples are higher than those treated with CNC the two types of paper reacted differently to the treatments alterations are not stable over time—for Whatman paper, they become more evident after one month
Chen et al., 2023 [133]	application of BNC for the conservation of creased Chinese painting scrolls	Xuan paper of Jinpi type	BNC	folding resistance tensile and tear strength pH SEM and XRD thickness	BCN treatment improved the flexibility, strength and folding resistance of paper BCN treatment was more effective than the traditional paper strip reinforcement method BNC treatment had an excellent effect on conserving the creased artwork on Xuan paper aesthetically and improved its physical properties
Harandi and Moradienayat 2023 [134]	application of polyvinyl butyral (PVB) nanocomposite with nanocrystalline cellulose/ZnO nanofibers for wood coating	silver fir (Abies alba Mill.) wood	PVB/CNC/ZnO nanocomposite	SEM colourimetry ATR-FT-IR, SEM	PVB/CNs/ZnO improve wood protection against UV radiation and humidity high amounts of ZnO and CNC protect better against photochemical degradation
Lisuzzo et al., 2023 [135]	application of supramolecular systems based on chitosan and CNF with a different surface modification (TEMPO-oxidation and carboxymethylation) as paper consolidants	paper	chitosan and CNF with a different surface modification (TEMPO-oxidation and carboxymethylation)	isothermal titration calorimetry (ITC) ζ-potential and DLS conductivity and rheological measurements mechanical tests, flame resistance WCA	the electrostatic interactions between chitosan and functionalised nanocellulose drive the formation of hybrid fillers suitable for paper consolidation CNF coated with chitosan have improved capacity to penetrate the paper structure, enhancing its mechanical resistance and hydrophobising its surface chitosan/CNF create a protective barrier for heat transfer that prevents the combustion of paper
Younis et al., 2023 [136]	synthesis of Klucel E/CNF nanocomposites with enhanced mechanical properties for degraded wood consolidation	Ficus sycomorus L. wood	CNF/Klucel E nanocomposites	TEM, SEM and XRD mechanical testing	addition of CNF to Klucel E improves its mechanical properties increase in CNF within the composite dramatically increases the Young’s modulus and hardness wood treatment with Klucel E/30% CNF increases the compression strength value by 14.5% compared to untreated wood
Younis et al., 2023 [137]	synthesis of Klucel E/CNC films with the best mechanical properties to enhance the properties of degraded wood	Ficus sycomorus L. wood	CNC/Klucel E nanocomposite films	SEM and XRD mechanical testing (tensile strength, elongation at break, elastic modulus) viscosity colour change, FT-IR compression strength	Klucel E/CNC composite with a 30% CNC content has the highest mechanical strength, acceptable crystallinity and dispersion and is suitable for wood conservation CNC added as a filler to Klucel E significantly improves its strength, reduces its viscosity and improves the penetration into the wood tissue enhancing the compression strength of treated wood
Gmelch et al., 2024 [138]	testing the performance of CNF and CNC in stabilising fragile papers	Whatman filter paper naturally aged newsprint paper	CNF CFC a mixture of CNF and CNC	fluorescence microscopy pH, conductivity and rheology of suspensions pH of paper tensile strength	treatment with NC suspensions increases the tensile strength of the paper in general surface inhomogeneity and hydrophobicity make a historical newspaper more difficult to treat than Whatman CNF/CNC mixture increased the strength of historical newspapers but had a minimal effect on Whatman
Hu et al., 2024 [139]	application of BNC with xylitol to reinforce simulated waterlogged wooden artefacts	artificially degraded birch (Betula) veneers	BNC and xylitol	anti-shrinking efficacy (ASE) SEM mechanical strength (a three-point bending test)	BNC alone had limited effectiveness but showed enhanced reinforcing properties when mixed with xylitol BNC/xylitol improved bending strength and reduced deformation of treated wood the use of BC for wood reinforcement may darken the wood surface BNC alone was ineffective due to the inability to fill the gaps in the wood and the strong acidity of the solution

summarises recent examples of research on using nanocellulose-based solutions for conserving artefacts of cultural heritage made of lignocellulosic material or natural fibres.

5. Discussion

Nanocellulose has proved useful for various applications in many industrial branches. Being a natural, renewable, and sustainable material, it continuously attracts attention and popularity as a green alternative to petrochemicals for producing modern, often hi-tech, products.

As shown in the previous chapter, a growing interest has recently been observed among conservators in using either pure nanocellulose or supramolecular nanocellulose-based systems to stabilise and preserve historical artefacts made of lignocellulosic materials. It seems a natural and reasonable choice since its chemical structure is identical to cellulose—one of the main structural components of wood and other lignocellulosic biomass—hence, it is compatible with wood or paper structures. Due to the nano-size of NC molecules, they are expected to penetrate well into wood cells and paper fibres, which is necessary for the proper conservation of these materials. The good mechanical properties of nanocellulose (especially in the case of CNC and BNC) should reinforce and strengthen treated objects. All these make NC a potentially useful stabilisation agent for wood and paper. However, since it is susceptible to the same degradation agents as wood and paper, its broad application in conservation raises some concerns [15,42,116].

Regarding the conservation of degraded historical papers of different types, using nanocellulose seems to be a better solution than any other traditional method. It is compatible with the paper structure, improves its mechanical properties, provides protection against moisture, neither thickens the treated paper considerably nor alters the legibility of the characters written on it, and is entirely eco-friendly. By adding specific metal nanoparticles or other chemicals to it, supplementary protection against UV radiation, thermal degradation, or antimicrobial decay with simultaneous deacidification can be obtained that limits further degradation of the treated paper [23,117,125,131].

In general, in the conservation practice, nanocellulose works much better when applied as a coating than a consolidant used for wood impregnation. The experiences so far from the attempts of wood conservation using NC, although they show some benefits, also point out its disadvantages and challenges. First of all, CNC tends to flocculate and, despite the small particle size and low viscosity of a conservation solution, exhibits poor penetration into the wood structure, which probably results from the filtering effect of wood on CNCs. Although the treatment provides some filling of wooden cells by forming an open network structure, it is scarce and rather random, insufficient for effective stabilisation, and highly dependent on the degree of wood degradation and the number of impregnation cycles. CNC has a low affinity for wood surface chemistry and interacts only with the cellulosic component, which in historical wood is usually a highly degraded and minor ingredient [97,106,116,121]. From this point of view, nanocellulose, in its pure form, does not seem sufficiently suitable for conserving wooden artefacts of cultural heritage. However, there are ways to overcome the mentioned deficiencies of nanocellulose. One of them is applying it in a mixture with other compounds that improve nanocellulose performance, e.g., it was shown that the addition of PEG improves NC penetration [116]; a mixture of CNC or CNF with Klucel E has good penetration properties and improves the mechanical strength of treated wood [122,136,137], and the addition of xylitol improves BNC penetration and reinforcing properties when applied to degraded wood [139]. Moreover, as reported, wood coating with a PVB/CNCs/ZnO nanocomposite provides effective surface protection against UV radiation and humidity [134], similar to the coating made of a CNF/CNC/lignin nanoparticles composite [129]. Another method to improve the nanocellulose performance and tailor its properties to the requirements of an effective wood consolidant is its functionalisation. Several studies have shown that the NC surface can be easily hydrophobised via acetylation, amidation, etherification, silylation, and urethanisation or can be enriched with ionic charges by carboxymethylation, oxidation, phosphorylation, and sulphonation. Additionally, various polymers can be grafted on NC, enriching it with new properties, including transparency, mechanical, and water vapour barrier characteristics [140,141].

The general challenge concerning using nanocellulose is directly related to its production. Due to multiple manufacturing methods and various natural sources from which nanocellulose is extracted, NC properties vary greatly, even within its forms (CNC, CNF, BNC). This complicates the applications of nanocellulose and forces us to adjust the application protocols accurately to the type of NC currently in use. Therefore, the unification of the large-scale production of nanocellulose is necessary, along with developing green production methods to make it cost-effective and widely available. This will allow broader sustainable application of NC for various uses, including the conservation of cultural heritage artefacts and replacing petrochemicals with this versatile nanomaterial for the benefit of people and the environment [13,23,41].

6. Conclusions

Nanocellulose, as a natural, renewable, and sustainable material, has drawn the increasing attention of researchers for various applications and seems to be a good alternative to the petrochemicals that have so far been used in the industry. This review describes the basic characteristics of nanocellulose types and their properties and presents the recent research on using nanocellulose for the conservation of historical artefacts made of lignocellulosic materials, showing advantages and discussing challenges and prospects related to the broader use of this nanomaterial in the conservation practice.

The nanocellulose properties, such as high strength and stiffness, surface reactivity derived from a large number of reactive hydroxyl groups, compatibility, and small dimensions, make it a potentially good conservation agent for degraded lignocellulosic materials. In its pure form, nanocellulose has proved particularly useful for conserving historical paper since it can form a durable, stable coating that consolidates the surface of a damaged artefact. However, it is not as effective as a wood consolidant, particularly because of its poor penetration into the wood structure. Owing to its specific chemistry, though, nanocellulose can be easily functionalised and, this way, enriched with various properties required for an effective wood consolidant. Moreover, combining nanocellulose with other agents can improve its properties and add new functionalities to create supramolecular systems that would address multiple needs of degraded paper or a wooden object, including stabilisation, deacidification, and protection against moisture, UV radiation, and other degrading factors.

To broaden the use of nanocellulose in conservation practice and other areas, the development of new, effective, and green large-scale production methods is required to ensure the standardised properties of individual NC forms, increase their availability on the market, and reduce their price.

In conclusion, although nanocellulose is an interesting and very promising solution for the conservation of cultural heritage artefacts made of paper and wood, further thorough interdisciplinary research is necessary to produce cost-effective, industrial-scale NC particles with specific properties and to develop sustainable and effective nanocellulose-based conservation agents, which would replace synthetic, non-sustainable consolidants and enable proper conservation of historical objects. Research on green nanocellulose functionalisation seems particularly needed to transform its particles into effective consolidants fully compatible with lignocellulosic materials.