Alkaline Nanoparticles for the Deacidification and pH Control of Books and Manuscripts
Manuscripts and books are susceptible to fast degradation owing to the presence of detrimental components used in the papermaking techniques, and to the action of environmental pollutants. As a result, the acidity of documents increases, promoting the acid-catalyzed depolymerization of cellulose. The latter process strongly reduces the mechanical properties of paper, reducing its long-term resistance to natural aging. The presence of inks concurs to degradation, making the conservation of manuscripts particularly demanding. In this chapter, the use of dispersions of alkaline earth metal hydroxide nanoparticles will be discussed as a method for counteracting the degradation of paper. These systems have proven efficient for the deacidification of cellulose-based artifacts, providing a mild alkaline buffer and maintaining a stable neutral environment. The palette of formulations nowadays available to conservators includes systems designed for the treatment of manuscripts featuring metal gall inks and modern industrial inks.
KeywordsCellulose Chain Magnesium Hydroxide Aqueous Treatment Hydroxide Nanoparticles Titanium Ethoxide
This chapter provides an overview on the use of nanomaterials, in particular dispersions of alkaline nanoparticles in non aqueous media, for the preservation of cellulose-based artifacts, such as those commonly found in archives, libraries, museums and private collections. First, a brief introduction will be given on cellulose chemistry and on degradation mechanisms both in acidic and in alkaline environments. We will focus on factors that influence the aging of books and manuscripts, in particular the presence of iron gall inks and their detrimental action on paper. Then the traditional conservation treatments will be discussed before describing the advantages of the methodologies developed in the framework of Colloids and Nanosciences. The syntheses, stabilization, and characterization of alkaline nanoparticles dispersions will be outlined, and applicative aspects will be illustrated in the Case Studies Sect. (5.4).
2 Chemistry and Degradation of Cellulose-Based Material
Cellulose based-materials are largely present in the cultural patrimony of all civilizations. Paper, canvas, and wood objects constitute a significant amount of the documentary, historical, and artistic heritage that needs to be preserved and transferred to future generations. Authors belonging to different countries worldwide have brought this issue to the attention of the scientific and conservation communities (Dobrodskaya et al. 2004; Strlič and Kolar 2005; Wouters 2008; Yanjuan et al. 2013; Afsharpour and Hadadi 2014). Before discussing the use of nanomaterials for the conservation of cellulose-based artifacts, a brief introduction on the properties and degradation pathways of cellulose will be given.
2.1 Cellulose Chemistry
Cellulose is a linear polymer naturally found in the walls of plants cells and plays a fundamental role in determining the mechanical properties of vegetal tissues. Cellobiose, which is the repeating unit of cellulose, consists of two molecules of D-glucose, linked through a β-1,4-glycosidic bond formed upon condensation. One of the most important parameters in evaluating the conservation status of cellulose-based artifacts is the degree of polymerization (DP), which is defined as the average number of monomer units for cellulose chain. The degree of polymerization of native cellulose depends upon the vegetal species, ranging between 9000 and 15,000 (Fengel and Wegener 1984). Typically, the manufacturing process of cellulose, i.e. its extraction from wood, significantly reduces the original DP. It is worth noting that the mechanical properties of the polymer are related to the average chain length; for instance, if DP decreases down to 300, which is a possible value for aged cellulose in historical objects, the resistance is dramatically reduced resulting in mechanical failure (crumbling, rupture).
Degradation via hydrolysis and oxidation is known to affect amorphous regions first, as the crystallites are less accessible to degradation agents. As a matter of fact, the acid-catalyzed hydrolysis of β-1,4-glycosidic bonds can proceed up to complete depolymerization of the amorphous zones, leaving the crystallites to set a lower DP limit, known as Levelling-off Degree of Polymerization (LODP) (Calvini 2005), which can be seen as a watershed between amorphous and crystalline regions of cellulose (Calvini 2014).
The crystalline structure of cellulose has been widely studied starting from the second half of the 20th century (Wilkie 1961). The structure established by von Nägeli in 1858 was confirmed by Meyer and Misch through X-ray crystallography (Meyer and Misch 1937). Four different crystalline allomorphs have been identified (Cellulose I–IV), cellulose I being the most abundant in nature. It is well known (Atalla and Vanderhart 1984) that the crystalline structure of cellulose I is composed of celluloses Iα (triclinic) and Iβ (monoclinic), whose relative amount depends on the original source. Cellulose crystallites are usually about 5 nm in width. The relative amount of crystalline portions is defined by the crystallinity index (CI), and typically ranges from 50 to 90 %, depending on the cellulose source and on the measurement method (Park et al. 2010).
2.2 Reactions in Acidic Medium
Degradation in acidic environments is also due to oxidation, which is interconnected to acid-catalyzed hydrolysis. Oxidized groups on cellulose chains are unstable in acidic conditions, favoring scission of bonds even at room temperature (Calvini et al. 2008). Oxidation of cellulose chains can also result in the production of acids, including glucuronic, glucaric, uronic and aldaric acids that promote hydrolysis in the so-called spiraling effect (Shanani and Harrison 2002). Conjugated carbonyl and carboxyl compounds formed during degradation are chromophores responsible for browning of aged paper (Bronzato et al. 2013).
Volatile acids are developed during the aging of cellulose-based material and, if not removed, result in an autocatalytic degradation process (Calvini et al. 2008). This can be the case in sealed environments, for instance when books are tightly packed in shelves (Carter et al. 2000). Instead, the removal of volatile acids leads to an auto retardant degradation process (Calvini et al. 2008).
2.3 Reactions in Alkaline Medium
Polysaccharides are affected by concentrated alkaline solutions at high temperature, the main steps being: (i) dissolution of polysaccharides; (ii) peeling of end-groups; (iii) alkaline hydrolysis of acetyl groups and glycosidic bonds; (iv) degradation of dissolved polysaccharides and peeled monosaccharides (Rydholm 1965; Sjostrom 1977).
3 Factors Affecting Books and Manuscripts Degradation
The degradation of archival material is due to several factors that are either internal, i.e. related to the presence of detrimental compounds from making processes, original components or even past conservation treatments, or external, i.e. environmental factors.
3.1 External Factors
There are many environmental factors that can affect paper stability. Merely day light, in combination with temperature and oxygen, plays a fundamental role in the degradation of cellulose (McKellar and Allen 1979; Feller 1994; Zervos 2010). In particular, the interaction between light and cellulose leads to the oxidation of functional groups and to the formation of oxidized compounds, as described above. At the macroscopic scale, these phenomena result in the typical browning of paper. Air pollutants such as sulfur and nitrogen oxides hydrate in moisture within cellulose fibers and form acids that promote hydrolysis (Carter 1989). The migration of degrading compounds from packing materials, cardboard as well as furniture, cabinet, and cases, in particular if treated with poly vinyl acetate based coatings, is another factor to be taken into account (Tétreault and Stamatopoulou 1997; Carter et al. 2000; Tétreault 2003). In fact, it has been shown that detrimental organic compounds can also be found inside microclimate frames (Lopez-Aparicio et al. 2010; Grøntoft et al. 2010). Environmental factors also include the attack of cellulose by fungi and bacteria, both aerobic and anaerobic, for instance through the use of hydrolytic enzymes (Coughlan 1991).
3.2 Internal Factors
The internal factors affecting paper stability include the quality of raw materials, as well as the presence of compounds introduced during the manufacturing processes. For instance, the purity and crystallinity of cellulose are important parameters in determining its resistance to degradation, as mentioned above. Up to 19th century, papers were obtained from cotton and linen rags, which are mainly composed of cellulose. On the other hand, the onset of the large-scale production of paper gradually imposed the use of wood as a raw material. This involves several conservation issues: (i) both lignin oxidation products and residual chemicals from the pulping and bleaching processes can increase the intrinsic acidity of paper; (ii) paper sheets made from chemical and mechanical pulp, containing considerable amount of lignin, tend to absorb more SO2 from polluted atmosphere than lignin-free paper (Bégin et al. 1998; Tse et al. 2002).
Regarding the manufacturing processes, it must be considered that, from the beginning of the 19th century until the mid 20th, papermaking used alum and rosin for the sizing of paper sheets. Both materials are major sources of acidity, and Chamberlain (2007) showed that the reactivity of aluminum salts in catalyzing the degradation of cellulose can be predicted with reference to the aqueous chemistry of aluminum ions. In fact, it is interesting to note that gelatin, historically used in combination with alum, has anionic groups capable of interacting with aluminum cations, and this could hinder the degrading action of alum on paper (Dupont 2002).
3.2.1 Iron Gall Ink
4 State-of-the-Art Methods for the Conservation of Cellulose-Based Artworks
Besides preventive measurements, such as the choice of appropriate display materials and storage conditions, several conservation strategies have been developed for counteracting cellulose oxidation and hydrolysis. These include deacidification treatments and antioxidants, even if there is a strong interest in finding a single method capable of hindering both degradation mechanisms in one step.
4.1 Deacidification Treatments
There is a long tradition on deacidification in the conservation practice, and conservation scientists have developed and proposed many methodologies throughout the last decades (Baty et al. 2010). Neutralization of acidity needs to be achieved because in neutral conditions both the random cleavage of cellulose chains catalyzed by acids and the oxidation reactions favored by an acidic environment proceed at a much slower pace. Hydroxides, applied directly or formed in situ, are used to neutralize acidity on the substrates. Sometimes, carbonates or bicarbonates are directly used as milder neutralizing agents. Deacidification methods should use solvents that are neither toxic to the operator nor dangerous for the environment. Moreover, the deacidification treatment must be physically and chemically compatible with the substrate and its components, i.e. sizing, fillers, inks etc., which means that the treatment must not produce any undesired alteration of the substrate’s original characteristics. To this end, it is important to carry out preliminary tests to evaluate the compatibility between the neutralizing system and the object to be treated. The cost of a deacidification treatment is a crucial factor in most conservation scenarios. For instance, when entire library or archival collections need to be neutralized, interventions on single paper sheets are simply not feasible. Therefore mass deacidification methodologies were developed in order to keep the costs low and increase the number of books deacidified with a single intervention. Mass deacidification methods are typically based on non-aqueous solvents that produce fewer undesired effects than water. In the next sections we will briefly introduce the most important and common deacidification methods in conservation practice.
4.1.1 Aqueous Treatments
During aqueous deacidification treatments, paper sheets (separated from the binding) are either immersed into the neutralizing solution or sprayed with it (Barrow and Sproull 1959); aqueous solutions of calcium (or magnesium) bicarbonate and calcium hydroxide are commonly used. Neutralization of cellulose-based material by aqueous alkaline treatments is very effective because of the high mobility of the hydroxide ions.
Moreover, aqueous treatments can improve the visual aspect of samples owing to direct washing of hydro-soluble dirt and of sizing compounds usually involved in the browning of paper (Sequeira et al. 2006); in fact, a resizing bath is usually needed after aqueous treatments (Giorgi et al. 2005).
On the other hand, aqueous treatments also show severe drawbacks.
Any excess of free hydroxide ions will readily interact with cellulose, which might be depolymerized and degraded in highly alkaline aqueous environments as described in the previous sections. Besides, both the aqueous medium and the alkaline environment can lead to the removal of original material (inks, sizing), and in some cases to ink discoloration due to the decomposition of the ink complex (Kolar 1997; Neevel 2000; Malesič et al. 2002; Sequeira et al. 2006; Stefanis and Panayiotou 2007).
4.1.2 Non Aqueous Treatments
Non-aqueous wet treatments are based on neutralizing agents dispersed or dissolved in solvents that are less polar than water, in order to limit or completely avoid ink bleeding or solubilization of sizing and other hydrophilic paper components (Sequeira et al. 2006). The most inert solvents are usually employed in mass deacidification methods.
The Bookkeeper (Preservation Technologies, L.P.) is a commercial dispersion of micron-sized particles of MgO (concentration of about 4 g/L) in a blend of perfluoroalkanes. Fluorinated solvents are completely inert to paper components, including modern inks. However, the stabilization of magnesium oxide microparticles in the dispersing media requires several additives, including surfactants, probably either a perfluorinated Mg-soap, a perfluoropolyether derivative or a non-ionic fluorinated acyl ester (Zumbühl and Wuelfert 2001).
After the application, magnesium oxide is converted into hydroxide in contact with moisture; the hydroxide neutralizes acidity and, if in excess, forms a carbonate buffer against recurring acidity.
The Bookkeeper is available for single sheet deacidification, i.e. spray products, or for mass deacidification. The latter has been used in the National Congress Library of Washington, D.C., since its development in 1992.
The influence of surfactants on the neutralization process has been studied by Zumbühl and Wuelfert (2001) that showed how the presence of a hydrophobic coating hampers the reactivity of magnesium oxide. Slow conversion of magnesium hydroxide into carbonate might create a too alkaline environment that could be risky for the cellulose fibers. Despite these drawbacks, Bookkeeper is widely used and it is one of the best non-aqueous mass deacidification treatments available.
The Battelle or Papersave Process
The National German Library, back in 1987, asked the Battelle Ingenieurtechnik GmbH to investigate different deacidification methods and to develop an innovative system. In 1994, the Battelle process was tested and approved by the Library; the treatment was based on a combination of magnesium ethoxide and titanium alkoxides in hexadimethyl disiloxane (HMDO), a colorless organic silicon compound. The complex formulation is referred to as METE (Magnesium Ethoxide Titanium Ethoxide). Some authors also reported that a surfactant is added to the system to improve paper impregnation (Dufour and Havermans 2001). Magnesium ethoxide reacts with moisture to form magnesium hydroxide that neutralizes cellulose acidity and, if present in excess, is transformed into carbonate by reaction with carbon dioxide. Titanium ethoxide, on the other hand, hydrates to form titanium oxide and water, thus not contributing to the deacidification process (Blüher and Grossenbacher 2006). In fact, the role of titanium alcoholate is to increase the solubility of magnesium ethoxide in HMDO. This method is also known as Papersave and is currently being developed by the German company ZFB. Finally, the Ink save process targets ink corrosion through the use of neutralizing agents (calcium and magnesium alkoxides) in conjunction with quaternary ammonium halides as antioxidants (Baty et al. 2010).
In the field of conservation science, only a limited number of reducing agents, namely mild agents, can be applied on paper in order to preserve the status of the fibers. In this regard, borane amine complexes, where the high boron reactivity is reduced by complexation with nitrogen, are thought to reduce oxidized groups. In particular, the borane tert-butylamine complex in a non-aqueous solvent, has been widely used in the ICRCPAL Laboratories of Rome (previously known as ICPL) as a reducing agent for carbonyl functionalities (Bicchieri and Brusa 1997; Bicchieri et al. 1999, 2000). More recently, its capability of neutralizing carboxyl groups has been evaluated (Sanna et al. 2009) with the aim of developing a single treatment for inhibiting acid-catalyzed hydrolysis and oxidation of paper. Further data are needed to confirm these interesting preliminary results (Sanna et al. 2009).
Alternatively, J. Neevel proposed the usage of phytates as metal deactivators capable of inhibiting metal gall ink corrosion, back in 1995 (Neevel 1995). Phytate is the commonly used term to indicate myo-inositol hexaphosphate that is the salt of myo-inositol hexaphosphoric acid, a substance present in various food of plant origin. Being a metal deactivator, phytate reduces the oxidizing potential of iron(II) ions, inhibiting its oxidation to iron(III), hence hampering the Fenton reaction.
The method has become popular among conservators and many scientific papers have been published on the subject (Neevel 2000; Botti et al. 2005; Kolar et al. 2005; Zappalà and Stefani 2005; Henniges et al. 2008; Potthast et al. 2008). More recently, J. Kolar proposed the usage of magnesium phytate (Kolar et al. 2007), which can be completely dissolved at standard working conditions, avoiding the risk of forming surface deposits.
According to Kolar et al. (2003), the chelating action of phytate is specific to iron ions, which could hinder the effectiveness of the method in the presence of metal gall inks containing substantial amount of copper ions. However, some authors reported that phytate copper ions complexes are very stable between pH 5 and 7 (Persson et al. 1998). In fact, the phytate treatment increased the resistance to aging of paper featuring metal gall inks with both copper and iron ions (Henniges et al. 2008). Treatment with phytate does not hamper acid-catalyzed hydrolysis of cellulose, therefore, a subsequent deacidification treatment is carried out commonly using calcium hydrogen carbonate aqueous solutions.
In recent years, Kolar et al. proposed the application of quaternary ammonium halides that are specifically designed for the inhibition of cellulose oxidation induced by metal gall inks. Halides are known to act as antioxidants, as they are catalytic peroxide decomposers; therefore they can limit the degradation of paper that contains metal gall inks, regardless of the transition metal ions contained in the inks. The first new antioxidant proposed was tetrabutylammonium bromide (TBABr) (Kolar et al. 2003). In 2005, experimental data acquired on several quaternary ammonium halides showed that the efficiency of the treatment is related to the size of the associated cation. In particular, halides with large counter ions were demonstrated to be more efficient in hampering cellulose degradation; among the tested quaternary ammonium halides, TBABr and TBACl (tetrabutylammonium chloride), in water and dichloromethane, resulted the best systems for the preservation of inked paper (Malešič et al. 2005a, b). More recently, the usage of alkylimidazolium halides and alkaline compounds (i.e. magnesium and calcium ethoxide in ethanol) has been proposed (Kolar et al. 2008). This method is interesting as it combines deacidification with antioxidants in a low-polar system.
5 Alkaline Nanoparticles for pH Control
As highlighted in the previous sections, the ideal deacidification treatment must fulfill several requirements in terms of effectiveness, physico-chemical compatibility with the artifacts, feasibility, and low impact both on the operator and the environment. In this sense, a significant contribution to the development of advanced solutions has come from colloids and nanoscience. The first works date back to the early 2000s, and introduced the use of nanoparticles of alkaline earth metal hydroxides as neutralizing agents for paper and canvas (Giorgi et al. 2002a, b, 2005).
For what concerns the solvents, short chain alcohols were initially selected (and are still used) as dispersing media because they exhibit several advantages: (i) they exhibit good wetting properties; (ii) they favor the dispersion of nanoparticles, probably due to the presence of a hydrophobic layer on the surface of the particles; this layer is formed upon physisorption of the alcohol hydroxyl group on the surface of particles (Ambrosi et al. 2001), resulting in inhibition of the particles face-to-face sticking (Fratini et al. 2007).
In the next sections, we will illustrate the main synthetic routes of nanoparticles dispersions, as well as the characterization of these systems and their application to deacidification of works of art throughout the last 15 years.
5.1 Synthesis of Alkaline Nanoparticles Dispersions
Several different strategies can be used to prepare inorganic particles. Synthetic methods are usually divided into top-down and bottom-up, based respectively on breaking particles down to the nanoscale, or on building up nanoparticles atom by atom (or molecule by molecule). A detailed description of the methods for the preparation of inorganic nanoparticles is beyond the scope of this chapter. Therefore, in this section, we will focus on the synthetic routes of alkaline nanoparticles for the pH control of cellulose-based artworks.
A completely different synthetic strategy for the preparation of calcium hydroxide nanoparticles was recently proposed (Poggi et al. 2014), based on a solvothermal reaction. The main advantage of this process relies in the possible future up-scale of the method, with great benefits in terms of costs. The use of dispersions obtained by the solvothermal process to hinder the degradation of cellulose-based materials will be discussed in Sect. 5.4.
As previously discussed, several state-of-the-art deacidification methods (e.g., Bookkeeper and PaperSave) are based on the usage of magnesium compounds as precursors for magnesium hydroxide, which then acts as the neutralizing agent. Alternatively, a coprecipitation reaction in aqueous solutions was achieved in 2005 (Giorgi et al. 2005) with the aim of synthesizing Mg(OH)2 nanoparticles to be dispersed in alcohols and directly applied on cellulose-based materials. Aqueous solutions of sodium hydroxide were added to solutions of different magnesium salts, such as sulfate, chloride, nitrate and perchlorate. The size of the obtained Mg(OH)2 particles range from ca. 50 to 200 nm, and was shown to be dependent on the counterions, following the Hofmeister anion series. Magnesium hydroxide nanoparticles were then dispersed in 2-propanol and applied by brushing on paper samples. The resistance of paper to photo-oxidative and hydrothermal aging was evaluated in terms of percentage of broken bonds in cellulose and tensile strength, showing that the nanoparticles dispersion is more efficient than the other state-of-the-art methods in preventing degradation.
A modified procedure to synthesize magnesium hydroxide nanoparticles has been recently proposed, where the removal of residual water (by drying) from particles before dispersion in propanol was demonstrated a key factor in further reducing the particles’ size. Particles obtained with this method were used to control the pH of paper containing metal-gall inks (see Sect. 5.4; Poggi et al. 2010).
5.2 Stabilization of Alkaline Nanoparticles Dispersions
Depending on the synthetic routes, the stability of the dispersions commonly used for deacidification purposes may range from several hours to months. It is worth noting that the stability of dispersions should be at least on the same timescale as the application onto artifacts, which normally requires from some minutes up to few hours. Another important factor is the reversibility of particles sedimentation. In many cases, after settling calcium and magnesium hydroxide nanoparticles can be easily re-dispersed again by gently shaking the container. After long storage time, sonication of dispersions might be used to favor the re-dispersion of settled nanoparticles in the organic solvent.
5.3 Characterization of Alkaline Nanoparticles Dispersions
Size, shape, polydispersity, specific area, superficial charge, composition and crystallinity, all are fundamental properties of the nanoparticles, which have to be characterized before application for conservation purposes. For what concerns solid particles, the crystallinity grade is usually assessed by XRD (X-Ray Diffractometry), while chemical composition and purity grade are commonly checked by Fourier-Transform InfraRed Spectroscopy (FTIR). Gas-porosimetry and Z-potential are used to measure the specific area of particles and their superficial charge distribution, respectively.
5.4 Application of Alkaline Nanoparticles Dispersions
When manuscripts and documents are treated, both sides of the paper sheets (recto and verso) should be treated if possible. Artworks can also be immersed in the dispersion. The choice of the most appropriate application procedure is usually made by conservators. As previously indicated, before any deacidification treatment, compatibility tests should be carried out, to check solvent interactions with the original components of artworks.
For the deacidification of cellulose-based artifacts, a concentration of 2.5 g/L usually grants the neutralization of present acidity and, at the same time, avoids the formation of white veils on the surface. The use of less concentrated nanoparticles dispersions is advisable when low porosity substrates, e.g. calendered paper, need to be treated.
Before applying the nanoparticles, pH measurements should be carried out on the artifacts. Several methods are commonly used to measure pH, including standardized procedures such as cold extraction (ASTM D788-97 2002; TAPPI T 509 Om-02 2002) and surface determination (TAPPI T 529 Om-04 2004). If pH is below 5.5, a deacidification intervention is advisable.
As previously indicated, hydroxide nanoparticles react with artworks acidity and, if in excess, turn into carbonate, which acts as a mild alkaline buffer against reoccurring acidity. The amount of particles applied can be tuned so to obtain a final pH of 7.0–8.0 after carbonation, as checked with pH measurements. This value is optimal for counteracting acidity and avoiding excessive alkalinity on cellulose fibers. More detailed information has been recently provided by Baglioni et al. (2015) regarding application procedures and examples of quantities typically applied in paper deacidification.
Finally, it must be noticed that the carbonation of hydroxide nanoparticles depends upon several factors including temperature, relative humidity and carbon dioxide concentration. In standard conditions (room temperature, RH = 60 %), full carbonation requires approximately two weeks.
5.5 Case Studies
In this section we will discuss some recent case studies on the use of nanomaterials for the pH control of cellulose-based artifacts.
Macroscopically, at the end of the aging the deacidified samples retained their original mechanical resistance, while untreated samples showed dramatic discoloration and severe mechanical failure. Overall this showed that an enduring stabilization of pH around neutrality is effective in preventing cellulose fibers degradation in manuscripts that feature metal and iron gall inks.
One of the recent upgrades of this methodology is the investigation of hybrid systems where the nanoparticles dispersions are mixed to gelatin and applied onto paper that already exhibits reduced mechanical properties. In fact, hydroalcoholic solutions of gelatin are commonly used in paper conservation for the lamination of manuscripts fragments, carried out by gluing Japanese paper tissue to the degraded paper sheets. The mechanical resistance of degraded manuscript is thus increased. In order to prevent the degradation of the laminated manuscript, nanoparticles can be easily added to the hydroalcoholic solutions of gelatin, creating a combined treatment that allows mechanical strengthening and hampering of cellulose degradation. Preliminary test conducted on mockups samples and on 16th and 18th century manuscripts showed that the combined method may improve on the traditional conservation method, prolonging the useful life of historical manuscripts and granting, at the same time, their manipulation (Poggi et al. 2015).
Acidification of Whatman paper lowered cellulose Tp of about 35 °C as compared to pristine samples. As can be seen in Fig. 12, treatment of acidic paper with nanoparticles (either dispersed in ethanol or propanol) kept cellulose Tp 20 °C higher than that of non-protected acidic samples throughout 48 h of aging. As a matter of fact, neutralization of acidity inhibits the dehydration of cellulose, which is the first step of the thermal degradation process (Fengel and Wegener 1984). In addition to that, it has been hypothesized that bivalent calcium ions are capable of interacting with cellulose carboxylate groups (formed upon neutralization of carboxylic groups). These interactions could create a network between the deacidified cellulose chains, increasing both paper resistance to thermal degradation (Bukovský 2000; Baglioni et al. 2012) and to folding, which has been observed during tests on treated paper samples.
As previously discussed, manuscripts featuring iron and metal gall inks exhibit good compatibility with alcohols, and can be successfully treated using formulations of calcium and magnesium nanoparticles both in ethanol and in 2-propanol. On the contrary, only few available treatments can be safely used on contemporary drawings or contemporary artworks on paper. As a matter of fact, during the 20th century, the notion of drawing underwent great changes. The use of paper changed from a simple support for studies or sketches to form the basis for autonomous work, at times torn, burnt, folded, perforated, twisted or scraped. At the same time a large number of new media, such as acrylic and vinyl resins, pressure sensitive adhesives, ballpoint and felt-tip pens and markers, entered the world of art. All of these media are rarely compatible with traditional restoration procedures. This makes the conservation and restoration of of contemporary drawings a widely unexplored field.
Therefore, even in the case of creased or folded paper, such as Simon Schubert, Kiki Smith or Stefano Arienti artworks, the treatment will not cause any changes in the original visual aspect of the object. Promising results obtained on preliminary laboratory tests led to the application of this innovative formulation on contemporary drawings from a private collection, potentially paving the way for the treatment of a significant portion of contemporary artworks.
To conclude, dispersions of nanoparticles in non aqueous solvents have proven in the last fifteen years as feasible and effective tools for the preservation of cellulose-based artifacts. As a matter of fact, these systems are able to counteract the main degradation mechanisms affecting cellulose (acid-catalyzed hydrolysis and oxidation), and represent valid alternatives to traditional conservation treatments. Moreover, the potential applications of nanomaterials in this field have been recently expanded, with new perspectives opened by the preparation of nanoparticles-polymer hybrid systems and by the dispersion of nanoparticles in inert solvents.
This work was partly funded by NANOFORART – Nano-materials for the conservation and preservation of movable and immovable artworks, FP7-NMP European project, (http://www.nanoforart.eu); and by NANORESTART, NANO materials for the REStoration of works of ART, H2020-NMP-2014 project (http://www.nanorestart.eu).
- Altaner CM, Thomas LH, Fernandes AN, Jarvis MC (2014) How cellulose stretches: synergism between covalent and hydrogen bonding. Biomacromolecules 15:791–798. http://pubs.acs.org/doi/pdf/10.1021/bm401616n)
- ASTM D788-97 (2002) Standard test methods for hydrogen ion concentration (pH) of paper extracts (hot-extraction and cold-extraction procedures)Google Scholar
- Baglioni P, Chelazzi D, Giorgi R, Poggi G (2012) Nanoparticles for the conservation of cultural heritage: paper and wood. In: Somasundaran P (ed) Encyclopedia of surface and colloid science, 2nd edn. Taylor & Francis, New York, pp 1–16Google Scholar
- Baglioni P, Chelazzi D, Giorgi R, Poggi G (2013) Colloid and materials science for the conservation of cultural heritage: cleaning, consolidation, and deacidification. Langmuir 29:5110–5122Google Scholar
- Baglioni P, Chelazzi D, Giorgi R (2015) Nanotechnologies in the conservation of cultural heritage—a compendium of materials and techniquesGoogle Scholar
- Barrow WJ, Sproull RC (1959) Permanence in book papers: investigation of deterioration in modern papers suggests a practical basis for remedy. Science 80–129:1075–1084. doi: 10.1126/science.129.3356.1075
- Baty JW, Maitland CL, Minter W, Hubbe MA, Jordan-Mowery SK (2010) Deacidification for the conservation and preservation of paper-based works: a review. BioResources 5:1955–2023Google Scholar
- Blüher A, Grossenbacher G (eds) (2006) Save paper! mass deacidification, today’s experiences, tomorrow’s perspectives: paper given at the international conference, 15–17 Feb 2006. Swiss National Library, BernGoogle Scholar
- Boone T, Kidder L, Russick S (1998) Bookkeeper® for spray use in single item treatments, 17Google Scholar
- Botti L, Mantovani O, Ruggiero D (2005) Calcium Phytate in the Treatment of Corrosion Caused by Iron Gall Inks: Effects on Paper. Restaurator 26:44–62. doi: 10.1515/REST.2005.44
- Calvini P, Gorassini G (2012) Surface and bulk reactions of cellulose oxidation by periodate. A simple kinetic model. Cellulose 19:1107–1114Google Scholar
- Calvini P, Gorassini A, Merlani AL (2007) Autocatalytic degradation of cellulose paper in sealed vessels. Restaurator 28:47–54Google Scholar
- Dobrodskaya TV, Egoyants PA, Ikonnikov VK, Romashenkova ND, Sirotin SA, Dobrusina SA, Podgornaya NI (2004) Treatment of paper with basic agents in alcohols and supercritical carbon dioxide to neutralize acid and prolong storage time. Russ J Appl Chem 77:2017–2021. doi: 10.1007/s11167-005-0211-5 CrossRefGoogle Scholar
- Dupont (2002) The role of gelatine/alum sizing in the degradation of paper: a study by size exclusion chromatography in lithium chloride/N,N-dimethylacetamide using multiangle light scattering detection. In: Daniels V, Donnithorne A, Smith P (eds) Preprint of IIC Baltimore congress 2002, works of art on paper, books, documents and photographs: techniques and conservation. International Institute for Conservation, Baltimore, pp 59–64Google Scholar
- Feller (1994) Accelerated ageing in conservation science. Getty Conservation Institute, Los AngelesGoogle Scholar
- Fengel D, Wegener G (1984) Wood: chemistry, ultrastructure, reactions. Walter De Gruyter, BerlinGoogle Scholar
- Gabrowsky E, Morrison I (1983) Particle size distributions from analysis of quasi-elastic light scattering data. In: Dahneke BE (ed) Wiley-Interscience, New YorkGoogle Scholar
- Giorgi R, Dei L, Schettino C, Baglioni P (2002b) A new method for paper deacidification based on calcium hydroxide dispersed in nonaqueous media. In: Daniels V, Donnithorne A, Smith P (eds) Preprint of IIC Baltimore congress 2002, works of art on paper, books, documents and photographs: techniques and conservation. International Institute for Conservation, Baltimore, pp 69–73Google Scholar
- Grøntoft T, Odlyha M, Mottner P, Dahlin E, Lopez-Aparicio S, Jakiela S, Scharff M, Andrade G, Obarzanowski M, Ryhl-Svendsen M, Thickett D, Hackney S, Wadum J (2010) Pollution monitoring by dosimetry and passive diffusion sampling for evaluation of environmental conditions for paintings in microclimate frames. J Cult Herit 11:411–419. doi: 10.1016/j.culher.2010.02.004 CrossRefGoogle Scholar
- Harris JF (1975) Acid hydrolysis and dehydration reactions for utilizing plant carbohydrates. Appl Polym Symp 28:131Google Scholar
- Hunter RJ (1981) Zeta Potential in Colloid Science. Elsevier, LondonGoogle Scholar
- Kolar J, Strlič M, Budnar M, Malesič J, Šelih VS, Simčič J (2003) Stabilisation of corrosive iron gall inks. Acta Chim Slov 50:763–770Google Scholar
- Kolar J, Šala M, Strlič M, Šelih VS (2005) Stabilisation of Paper Containing Iron-Gall Ink with Current Aqueous Processes. Restaurator 26:181–189. doi: 10.1515/rest.2005.26.3.181
- Kolar J, Možir A, Strlič M, de Bruin G, Pihlar B, Steemers T (2007) Stabilisation of iron gall ink: aqueous treatment with magnesium phytate. e-Preservation Sci 4:19–24Google Scholar
- Lopez-Aparicio S, Grøntoft T, Odlyha M, Dahlin E, Mottner P, Thickett D, Ryhl-Svendsen M, Schmidbauer N, Scharff M (2010) Measurement of organic and inorganic pollutants in microclimate frames for paintings. e-Preservation Sci 7:59–70Google Scholar
- Malešič J, Kolar J, Strlič M, Polanc S (2005b) The use of halides for stabilisation of iron gall ink containing paper—the pronounced effect of cation. e-Preservation Sci 2:13–18Google Scholar
- McKellar JF, Allen NS (1979) Photochemistry of man-made polymers. Elsevier, LondonGoogle Scholar
- Neevel JG (2000) (Im)possibilities of the phytate treatment. In: Brown JE (ed) Newcastle upon Tyne. The University of Northumbria, pp 127–134Google Scholar
- Neevel JG, Mensch CTJ, Cornelis TJ (1999) The behaviour of iron and sulphuric acid during iron gall ink corrosion. In: Bridgland J (ed) ICOM committee for conservation triennial meeting. James and James, London, pp 528–533Google Scholar
- Nisizawa K (1973) Mode of action of cellulases. J Ferment Technol 51:267–304Google Scholar
- Pauk S (1996) Bookkeeper mass deacidification process—some effects on 20th-century library material. Abbey Newsl 20:50–53Google Scholar
- Persson H, Türk M, Nyman M, Sandberg A-S (1998) Binding of Cu2+, Zn2+, and Cd2+ to Inositol Tri-, Tetra-, Penta-, and Hexaphosphates. J Agric Food Chem 46:3194–3200. doi: 10.1021/jf971055w
- Poggi G, Toccafondi N, Melita LN, Knowles JC, Bozec L, Giorgi R, Baglioni P (2014) Calcium hydroxide nanoparticles for the conservation of cultural heritage: new formulations for the deacidification of cellulose-based artifacts. Appl Phys A 114:685–693. doi: 10.1007/s00339-013-8172-7 CrossRefGoogle Scholar
- Poggi G, Sistach MC, Marin E, Garcia JF, Giorgi R, Baglioni P (2015) The GEOLNAN, a combined deacidification and reinforcement treatment for metal gall ink manuscriptsGoogle Scholar
- Potthast A, Henniges U, Banik G (2008) Iron gall ink-induced corrosion of cellulose: aging, degradation and stabilization. Part 1: model paper studies. Cellulose 15:849–859. doi: 10.1007/s10570-008-9237-1
- Rydholm S (1965) Pulping processes. Interscience Publisher, New YorkGoogle Scholar
- Shanani CJ, Harrison G (2002) Spontaneous formation of acids in the natural aging of paper. In: Daniels V, Donnithorne A, Smith P (eds) Works of art on paper: books, documents and photographs. International Institute for Conservation of Historic and Artistic Works, London, pp 189–192Google Scholar
- Sjostrom E (1977) TAPPI J 60:151Google Scholar
- Strlič M, Kolar J (eds) (2005) Ageing and stabilization of paper. National and University Library, LjubljanaGoogle Scholar
- Strlič M, Kolar J, Šelih VS, Kocar D, Pihlar B (2003) A comparative study of several transition metals in Fenton-like reaction system at circumneutral. Acta Chim Slov 50:619–632Google Scholar
- TAPPI T 509 Om-02 (2002) Hydrogen ion concentration (pH) of paper extracts (cold extraction method)Google Scholar
- TAPPI T 529 Om-04 (2004) Surface pH measurement of paperGoogle Scholar
- Tétreault J (2003) Airborne pollutants in museums, galleries and archives: risk assessment, control strategies and preservation management. Canadian Conservation Institute, OttawaGoogle Scholar
- Tse S, Bégin P, Kaminska E (2002) Highlights of paper research at the Canadian Conservation Institute. International Institution for Conservation of Historic and Artistic Works, London, pp 193–198Google Scholar
- Wouters J (2008) Coming soon to a library near you? Science 80–322:1196–1198. doi: 10.1126/science.1164991
- Zappalà A, Stefani C De (2005) Evaluation of the Effectiveness of Stabilization Methods. Treatments by Deacidification, Trehalose, Phytates on Iron Gall Inks. Restaurator 26:36–43. doi: 10.1515/REST.2005.36
- Zervos S (2010) Natural and accelerated ageing of cellulose and paper: a literature review. In: Lejeune A, Deprez T (eds) Cellulose: structure and properties, derivatives and industrial uses. Nova Science Publishers Inc, New YorkGoogle Scholar