The conservation and restoration of paper is a complex and multifaceted process that seeks to preserve the integrity of historical documents, manuscripts, and books for future generations. Figure 6 represents the types of damage that can appear on an antique sheet of paper: mould, foxing, mechanical damage, and yellowing. These deteriorations are accelerated both by both the chemical composition and pH of the paper and by environmental factors such as temperature, humidity, and light, as we will discuss later.

Examples of damage that can occur on antique paper, including mould growth, foxing, mechanical deterioration, and yellowing. These forms of deterioration are influenced by the paper’s chemical composition and pH as well as environmental factors such as temperature, humidity, and light

Effective management of environmental factors, particularly climate control, is crucial in preventing the deterioration of paper, as fluctuations in temperature and humidity can accelerate its alteration. A key aspect of this process is understanding how cellulose breaks down over time, so researchers employ various analytical methods to assess the extent of cellulose deterioration. Among these, high-resolution imaging techniques, including scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS), are commonly used to analyse the structural and surface characteristics of cellulose. While a technique like SEM-EDS can provide an immediate analysis of the surface morphology of cellulose, along with EDS analysis of the elements present, it is necessary to sample a portion of the artefact for examination [33, 52,53,54]. In contrast, atomic force microscopy (AFM) in tapping mode allows for the examination of cellulose in its native state, though it may introduce tip-related artefacts that obscure structural details [29, 33, 52].

Beyond imaging techniques, non-imaging methods such as x-ray diffraction (XRD) are widely used to characterise the crystallinity and crystallite size of cellulose [34, 55]. However, XRD does not directly measure crystallinity but rather indicates the proportion of crystalline cellulose within the entire sample. Furthermore, its results may be influenced by the presence of other crystalline substances, making it difficult to isolate cellulose-specific information [29, 34, 55]. Nuclear magnetic resonance (NMR) provides valuable insights into the chemical alterations of both amorphous and crystalline cellulose. However, it requires cellulose to be dissolved in acidic or enzymatic solutions to break hydrogen bonds, rendering it a destructive technique [56, 57]. Similarly, thermo-analytical methods such as differential scanning calorimetry (DSC) and thermo-gravimetric analysis assess thermal properties by exposing samples to high temperatures, further limiting their applicability to preservation efforts [58,59,60,61,62]. To address the need for non-destructive techniques, methods such as Fourier transform infrared (FTIR) and Raman spectroscopy offer chemical sensitivity while preserving sample integrity, making them valuable tools for cellulose analysis [31, 63,64,65].

However, the most important aspect is to consider the application of a multi-analytical approach, as a single technique alone cannot fully meet conservation and preliminary analysis requirements. Each technique has its own advantages and limitations, which must be considered depending on the specific research question being addressed.

When climate control fails, diagnostic analysis takes place, and restoration becomes the only option to preserve this heritage. In the following paragraph, we will first explore the fundamental principles of environmental control for paper collections, highlighting how proper management of temperature and humidity plays a crucial role in preserving these materials. We will then discuss the latest restoration techniques that utilise modern technologies to help maintain the structural integrity of paper artefacts.

The alterations and deteriorations that paper undergoes over time are not solely due to the chemical composition of the material or the reactions with inks and adhesives applied to manuscripts. Paper artefacts interact dynamically with their surrounding environment, making climate control an essential aspect of preventive and long-term conservation. Figure 7 illustrates the key environmental factors with which paper interacts and that can lead to deterioration. Preventive conservation of cultural heritage aims to control these risks factors, and it is defined as "measures and actions aimed at avoiding or minimizing future damage, deterioration, and loss, and consequently, reducing the need for invasive interventions" (EN 15898:2019. Conservation of cultural property—Main general terms and definitions).

Environmental factors critical for paper preservation, including temperature, humidity, microorganisms, human activity (such as vandalism), pollution, and light. These factors play a significant role in the deterioration of paper materials, especially when subjected to fluctuations, and these factors are interconnected, with changes in one often leading to a cascade of effects. For example, increased humidity can promote the growth of microorganisms, while temperature fluctuations can accelerate chemical degradation

In this context, climate control falls under preventive conservation strategies, as it focuses on maintaining the optimal environmental conditions necessary to preserve paper materials effectively.

Extensive research has demonstrated the significant effects of cyclic variations in indoor conditions, outdoor climate, environmental pollution, mechanical stresses, and chemical and thermo-physical changes on cultural heritage items in libraries, museums, and historical exhibitions [66]. Studies have shown how fluctuations between indoor and outdoor climates can accelerate both deterioration and bio-deterioration processes in these collections. A schematic representation of the major risk factors for paper artefacts is shown in Fig. 6.

Acclimatisation of sensitive heritage objects to the environment in which they have been preserved for extended periods has been key in establishing climate control criteria. This concept has been formalised in numerous standards, which recommend optimal ranges for thermo-hygrometric parameters to prevent cultural heritage deterioration. In recent studies, the idea of "historical climate" has emerged, suggesting that paper artefacts benefit from stable environmental conditions that reflect the climate in which they were originally kept. Many studies have explored the various alteration and deterioration phenomena affecting paper, including mechanical stress, chemical and photochemical reactions, and biological mechanisms. A substantial research attributes bio-deterioration of heritage cellulosic materials to fluctuations in air temperature and humidity [66, 67]. In fact, temperature and humidity are particularly critical for maintaining the balance of paper, as they directly influence the hydrolysis of cellulose, the primary concern regarding deterioration of paper collections [67]. The rate of chemical degradation is primarily driven by temperature, making these factors crucial in preserving paper’s integrity [68]. Recent advancements have allowed researchers to calculate a dose-response function that depends on environmental factors such as temperature and humidity, as well as the inherent characteristics of the paper itself, such as pH and polymerisation degree. Verticchio and colleagues developed models to estimate the "time life" of a collection—essentially, the period during which a library collection remains fit for use before it becomes unfit because of the combined effects of handling and cellulose hydrolysis [68]. Furthermore, other factors like light exposure and particulate matter (PM) also impact paper preservation and must be closely monitored. Together, these factors can contribute to the growth of microorganisms and mould.

In recent years, the conversation surrounding environmental control has expanded to include the challenges posed by climate change. Rising global temperatures pose additional risks to paper artefacts, particularly by promoting the migration and adaptation of new species of bio-deteriorating agents. These changes, driven by global warming, intensify the risks of biodegradation and create growing challenges for the preservation of Cultural Heritage [69]. The growing concern regarding climate change further underscores the importance of proactive climate management in the preservation of paper and other sensitive materials.

The restoration of paper has a long history, evolving significantly from its early techniques to the advanced methods we use today. In ancient times, paper restoration primarily focused on basic interventions, often performed by scribes or conservators who applied simple manual techniques to mend tears and reinforce fragile documents. Common methods included the use of adhesives, paste, and stitching to hold fragments together, with the primary goal of preserving the readability and integrity of the text. As paper technology and knowledge of materials advanced, so did restoration methods, incorporating more sophisticated approaches such as washing, bleaching, and deacidification. These techniques, however, were often rudimentary and could sometimes contribute to further deterioration due to a lack of understanding of the long-term effects on paper. In the twentieth century, with the development of scientific methods and a deeper understanding of the chemical properties of paper, conservation practices became more specialised. Today, modern restoration techniques combine cutting-edge technologies to ensure the material integrity of the artefacts while maintaining their historical authenticity.

In the context of paper restoration, deacidification is an essential treatment aimed at stopping the ongoing deterioration of paper caused by acidic conditions. Over time, the acids present in paper, often originating from the paper-making process itself or from materials such as inks and adhesives, can cause the cellulose fibres to break down. This leads to brittleness, discoloration, and loss of structural integrity. In restoration practice, deacidification is one of the most performed methods to prevent further damage and improve the paper’s longevity [70,71,72].

Deacidification involves the introduction of alkaline substances to neutralise acids present in paper and ensure the retention of an alkali reserve. This reserve helps counteract any acidic compounds that may form over time, thus extending the lifespan of the paper [73, 74]. Ideally, the process should be safe for the artwork, the environment, and the conservators, with no adverse effects on original inks, pigments, or paper fibres [23, 74]. In paper restoration—and more broadly in conservation practice—two fundamental principles guide interventions: minimum intervention and reversibility. Any treatment should be as non-invasive as possible and, when feasible, should allow for future reversibility without compromising the artefact.

Traditional deacidification methods are categorised into gaseous and liquid treatments, with the latter performed in aqueous or organic solutions [2, 72]. However, each method presents inherent risks. Aqueous treatments can cause fibre swelling and colour bleeding, particularly in delicate or water-sensitive manuscripts, while organic solvent-based methods pose concerns regarding flammability and toxicity. To address these challenges, ongoing research focuses on refining deacidification techniques. One approach explores the modification of aerosol-based treatments, adjusting the size and composition of aerosol particles to improve penetration and efficiency while minimising damage [75, 76]. Another innovative strategy, presented by Zhang and colleagues [74], involves controllable enzymatic mineralisation. Their method facilitates the deposition of calcium carbonate (CaCO₃) and hydroxyapatite, two alkaline compounds that effectively neutralise acidic content in paper. The resulting mineralised membrane is reported to be thermally stable and reversible, aligning with key restoration principles. Recent developments were made regarding the use of alkaline nanoparticles (vide infra), with specific deacidifying properties, applied to the cellulosic materials with the double aim of reducing the pH of the surface while improving the paper strength [77,78,79]. The work of Baglioni and Giorgi [80] reported that MgO nanoparticles are highly effective as a paper de-acidification agent, offering excellent physico-chemical compatibility with the substrate. Their application not only neutralises acidity but also preserves the integrity of the treated material, ensuring that no undesirable side effects, such as alterations in texture, strength, or colour, occur. This makes MgO nanoparticles a promising solution for long-term conservation efforts, maintaining the material's original properties while extending its lifespan. However, these are cutting-edge solutions that are still in the experimental phase, while even more advanced solutions are being investigated. For instance, Li and colleagues are exploring the use of bacterial cellulose impregnated with zinc oxide nanoparticles [81]. This approach not only offers deacidification but also serves as a fungal inhibitor and reinforcement for fragile paper structures.

While promising, such techniques require further testing to ensure compatibility with historical paper and adherence to conservation standards before they can be widely implemented in real-world restoration.

Over time, paper can accumulate dirt, stains, and pollutants that not only obscure its readability but also contribute to its deterioration and the growth of mould and micro-organisms. Various cleaning methods have been developed to address these issues while preserving the integrity of the material. These range from dry cleaning techniques, which remove surface contaminants without introducing moisture, to wet cleaning methods that can mitigate acidic deterioration and discoloration. However, these traditional methods present certain limitations, particularly in relation to the reduction of the paper's mechanical strength and the use of toxic chemical agents [82, 83]. In this paragraph, we will explore the recent advancements in cleaning methods, focusing on cutting-edge techniques that hold promise for the field of scientific conservation and restoration.

In recent years, ionic liquids have gained attention as a promising alternative due to their non-toxic and eco-friendly nature. Despite these advantages, challenges remain, especially concerning their impact on the structural integrity of the paper [83]. Ionic liquids are effective in removing lignin and hemicellulose—key components responsible for paper yellowing—but their extraction can weaken the material, as these substances also contribute to the paper's mechanical stability. However, the study by Schmitz and co-workers highlights the potential of ionic liquids as antifungal agents, demonstrating their ability to inhibit the growth of five fungal species on the paper surface [84]. His finding underscores the importance of selecting a cleaning method based on the specific conservation challenge being addressed. While ionic liquids effectively prevent fungal growth, their use may also lead to mechanical deterioration, emphasising the need for a careful balance between efficacy and material preservation. Other advanced approaches regarding laser-cleaning, enzyme-based treatments and nanomaterials have been explored to enhance the efficacy and safety of the cleaning process. Here, laser cleaning and enzyme-based treatments will be discussed, while the use of nanoparticles is addressed separately in another section because of their significant technological impact on heritage materials.

Laser cleaning of paper relies on physical effects, specifically photo-ablation, a process in which contaminant particles are ejected as photons are absorbed by the dirt material targeted for removal. This technique enables the selective elimination of unwanted substances from the surface based on differences in optical absorption. The first step in developing laser technology for paper restoration is selecting laser parameters that ensure efficient cleaning of books and documents while avoiding damage. It is crucial to fine-tune the laser output to prevent adverse effects such as carbonisation, discoloration, changes in acidity, porosity, or alterations to the paper’s physical and chemical properties. Given the fragility of paper, careful consideration of the laser type and its operational characteristics is essential to minimise the risk of damage [85,86,87,88].

A cutting-edge advancement in conservation techniques is bio-cleaning, which utilises bacterial enzymatic resources to clean artefacts. While the first applications of bio-cleaning were primarily on frescoes, recent studies have extended this approach to paper artefacts as well [89, 90]. One of the pioneering works in this field is that of Barbabietola and colleagues, who successfully applied a microbe-based procedure to remove aged glues from historical papers. They used a living bacterial cell to remove the non-desired aged glue on the ancient paper. Their research highlighted several advantages of this method, including its low environmental impact, high selectivity in targeting unwanted materials, absence of toxic agents, safety for both conservators and artefacts, and overall cost-effectiveness. Notably, the removal of aged glue has traditionally been performed through mechanical methods, which are often aggressive and lack selectivity, posing a risk to delicate artworks. While the introduction of hydrogels represents a significant improvement over chemical solvents, bio-cleaning offers an even more refined, non-invasive alternative for conservation treatments [91]. This approach represents a true revolution in the field and has the potential to be widely adopted in restoration practices. However, the use of microorganisms, enzymes, and living bacterial cells requires the expertise of highly skilled operators to ensure safe and effective application [92]. Table 6 summarises and compares the key features and limitations of the cleaning methods discussed.

Nanoscience has brought significant advancements to conservation science, introducing nanoparticles, composite nanomaterials, and soft-condensed matter as valuable tools in the field. Research has led to the development of tailored materials designed to address specific conservation challenges. As noted by Chelazzi and Baglioni [93], material deterioration originates at the nanoscale before spreading to the entire artwork. By targeting alteration at its earliest stages, nanomaterials offer an innovative and effective approach to preserving cultural heritage.

One of the earliest applications of nanoparticles in conservation focused on the deacidification of ancient paper, using calcium hydroxide (Ca(OH)2) nanoparticle dispersion in non-aqueous solvents [94]. This was the very first attempt in using nanoparticles for the deacidification of paper; the success of this application led to the development of other formulations, such as the magnesium hydroxide (Mg(OH)₂) nanoparticles to treat both artificially aged and historical documents [95]. Both studies highlighted that these nanoparticles significantly enhanced the deacidification process because of their high reactivity. More importantly, the treatment did not require special precautions, and the nanoparticles effectively preserved the mechanical and aesthetic properties of the artefacts. Additionally, the authors emphasised the economic advantages of this approach, as the nanoparticles can be synthesised through low-cost methods, making them a more affordable alternative to traditional deacidification techniques, which are often both time-consuming and expensive.

Beyond their essential role in deacidification, nanoparticles have also been employed for their antimicrobial and/or antifungal properties to prevent the bio-deterioration of ancient manuscripts [96, 97]. By inhibiting the growth of fungi and bacteria, these nanoparticles serve as a protective barrier, safeguarding delicate paper artefacts from biological damage. As previously mentioned, microbial bio-deterioration primarily results from the synergistic action of cellulolytic enzymes, which break down the cellulose structure, compromising the integrity of the paper. This process can also lead to the formation of rust-coloured stains and discoloration on paper surfaces, a phenomenon commonly known as “foxing” [98]. The growing interest in nanoparticles and their application as bio-protective agents stems from their unique physical and chemical antimicrobial properties. These nanomaterials have demonstrated remarkable efficacy in preventing microbial colonisation and deterioration of historical paper artefacts. Their antimicrobial activity is attributed to several mechanisms, including direct cell membrane damage, the release of metal ions that disrupt cellular functions, leakage of intracellular components such as reducing sugars and proteins, and the induction of oxidative stress. Additionally, non-oxidative mechanisms, influenced by factors such as particle size, shape, and morphology, also contribute to their effectiveness in inhibiting microbial growth [99, 100]. Among the various nanoparticles explored for antimicrobial purposes, silver nanoparticles (Ag-NPs) and zinc oxide nanoparticles (ZnO-NPs) stand out as two of the most extensively studied. Silver nanoparticles are particularly valued for their broad-spectrum antimicrobial properties, effectively targeting bacteria, fungi, and even viruses. Zinc oxide nanoparticles, on the other hand, exhibit strong antibacterial and antifungal activity while also offering additional benefits, such as UV protection and photocatalytic properties that contribute to the alteration of organic pollutants [81, 94, 101]. These characteristics make nanoparticles a promising avenue for the long-term preservation of historical documents, minimising microbial-induced degradation without relying on more invasive or chemically aggressive conservation treatments. As research progresses, the optimisation of nanoparticle-based treatments could offer safer, more sustainable, and more efficient methods for protecting paper artefacts from bio-deterioration. [96].

A significant frontier in the field of paper conservation involves the use of nano-fibrils and nanocrystals as protective coatings for lignin-cellulosic materials [102]. These innovative materials offer a promising solution for enhancing the durability and stability of historical and archival documents while adhering to the fundamental principles of restoration—namely, reversibility and minimal intervention.

A pioneering study by Camargos and co-workers [