Colloid and Materials Science for the Conservation of Cultural

Invited Feature Article pubs.acs.org/Langmuir

Colloid and Materials Science for the Conservation of Cultural Heritage: Cleaning, Consolidation, and Deacidification Piero Baglioni,* David Chelazzi, Rodorico Giorgi, and Giovanna Poggi Department of Chemistry and CSGI, University of Florence, via della Lastruccia 3 - Sesto Fiorentino, 50019 Florence, Italy ABSTRACT: Serendipity and experiment have been a frequent approach for the development of materials and methodologies used for a long time for either cleaning or consolidation of works of art. Recently, new perspectives have been opened by the application of materials science, colloid science, and interface science frameworks to conservation, generating a breakthrough in the development of innovative tools for the conservation and preservation of cultural heritage. This Article is an overview of the most recent contributions of colloid and materials science to the art conservation field, mainly focusing on the use of amphiphile-based fluids, gels, and alkaline earth metal hydroxide nanoparticles dispersions for the cleaning of pictorial surfaces, the consolidation of artistic substrates, and the deacidification of paper, canvas, and wood. Future possible directions for solving several conservation issues that still need to be faced are also highlighted.

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Concepts and tools7−10 belonging to the realm of colloid and materials science, which have acquired a leading role in the development of advanced and functional tools constituting a large portion of the commercial products that are consumed daily by millions of people all over the world and used in a variety of applications including cosmetics, food, and pharmaceutics,11−13 may lead to a dramatic enhancement of the effectiveness and durability of restoration interventions. Typically, works of art comprise both movable and immovable objects. The first class includes all of the documentary and historical manuscripts and books, usually made of paper or parchment, easel paintings on wood or canvas, and a large variety of objects such as statuettes, jewelry, and textiles. Immovable works of art mainly consist of mural/wall paintings, architectonic substrates, statues, and several kinds of stone-based artifacts. Regardless of their nature, artifacts are irremediably exposed to several degradation agents: physical erosion, chemical degradation, temperature, relative humidity, light, and microorganisms, all accounting for the natural aging of art materials. Moreover, anthropic activities increase the concentrations of SO2, NOx, and VOC (volatile organic compound) gases in the atmosphere that eventually lead to the corrosion of artistic substrates, contributing to the degradation of works of art. Finally, it must be outlined that in some cases conservation issues are due to past extemporized restoration interventions that were based on trialand-error practice. Depending on the type of artistic substrate, different tasks are necessary for conservation purposes. A large fraction of

INTRODUCTION Conservation science is a complex discipline that deals with the restoration and preservation of a large variety of materials constituting our cultural heritage. At first sight, one might think that science, art, and humanities are disconnected disciplines. However, our way of thinking and behaving depends strongly on the legacy of physical artifacts and intangible attributes of a society, inherited from past generations, maintained in the present, and possibly bestowed for the benefit of future generations. Because of the complexity of artistic and historical substrates, conservation science has explored different routes, developing several approaches for solving conservation issues. For instance, new resins that are more stable than natural ones, while exhibiting similar optical properties, have been developed and applied as varnishes for retouching paintings.1 Furthermore, the analysis of materials is often considered to be a preliminary step for suggesting treatments for works of art: in the conservation of matte paintings, the analysis of the physical and optical properties of the painted layers is the key to choosing the best materials for their consolidation.2 Electrochemistry and corrosion science have proven useful in conserving bronze outdoor statuary,3 and colloid science has been providing an increasingly important contribution to the development of restoration tools. Among the systems specifically tailored for conservation issues, nanoparticulate inorganic sols (nanosols),4 colloidal silica,5 and alkoxysilane6 play an important role in stone and wood conservation. In particular, because of their high surface-tovolume ratio, nanosols are metastable and usually hydrolyze to form 3D xerogel networks that improve the mechanical properties and resistance to water, fire, and microbial or insect attack of wood or stone objects. © 2013 American Chemical Society

Received: November 8, 2012 Revised: February 19, 2013 Published: February 25, 2013 5110

dx.doi.org/10.1021/la304456n | Langmuir 2013, 29, 5110−5122

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Invited Feature Article

Figure 1. Degradation of a wall painting. (A) Paint layer detachments (white spots). (B) Salt efflorescence. (C) Paint layer flaking and detachment due to the presence of organic coatings. The polymer coating acts as a barrier preventing the natural “breathing” of the surface and consistently increasing the mechanical stresses due to salt crystallization at the painting−polymer interface.

tools have been conceived and offered to conservators, constituting a new palette of treatments that exhibit high compatibility with the original artistic substrates and therefore grant long-term durability. The aim of this Feature Article is to illustrate some of the main achievements attained for the conservation of cultural heritage in the framework of surface and colloid science, providing an up-todate overview focused on the research activity of the authors of this Feature Article, including future directions and perspectives in the field. In this regard, nanostructured fluids such as microemulsions and micellar solutions, highly retentive chemical gels, and nonaqueous dispersions of alkaline nanoparticles will be described in the following sections.

conservation interventions consists of cleaning surfaces, consolidating surfaces and bulk layers, and deacidification. Cleaning mainly consists of the selective removal of dirt, grime (greasy material, dust, etc.), and natural or synthetic polymers from the surface of coated movable and immovable works of art. The application of natural varnishes has been largely performed in the past and has been traditionally adopted by easel painting artists that wanted to enhance the visual properties of their works. In addition to improving the appearance through the saturation of color, traditional conservation practice foresees the use of such materials to provide a vast number of different artistic substrates with hydrophobic properties and surface protection. Starting from the second half of the 20th century, synthetic polymers have been enthusiastically adopted, mainly because they were thought to be highly resistant to aging and easily removable. Unfortunately, synthetic polymers undergo degradation similarly to that of natural resins, resulting in the decrease of their solubility in net solvents and in the significant alteration of their visual aspect (mainly yellowing).14 Because of their adhesive properties, synthetic polymers have been used to readhere detached or damaged parts in the external layers of works of art. For instance, acrylate, vinyl, silicone, and epoxy polymers are widely used for the consolidation and protection of stone and wall paintings. However, the use of synthetic adhesives results in the strong alteration of physicochemical properties of the original substrates, such as porosity, water capillarity, water vapor permeability, and surface wettability,15,16 generating in the long term enhanced degradation that can proceed even up to the loss of the artifacts. Another important class of works of art subjected to degradation are cellulose-, parchment-, and leather-based artifacts (mainly documentary and historical manuscripts and books), which are threatened mainly by hydrolysis and oxidation reactions (acidity plays a fundamental role in the catalysis of these reactions) that lead to the loss of paper/parchment mechanical resistance.17,18 Conservation practice usually involves the use of alkaline aqueous solutions for paper deacidification whereas polar solvents are discouraged on collagen-based substrates. However, water causes the swelling of cellulose fibers and the leaching of compounds associated with paper (inks, sizing, etc.). This limitation has favored the development of nonaqueous deacidification methods. In the past 15 years, tools from nanoscience and in particular from colloid and surface science were demonstrated to overcome some of the limits of traditional restoration methodologies by providing, in a new cultural framework, innovative methodologies and materials for the above-described conservation issues. As a result, several reliable, easy to use, and inexpensive

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PROTECTION AND RESTORATION OF CULTURAL HERITAGE In this section, we report three applications of colloid science that address a large fraction of conservation issues: (i) cleaning with nanostructured fluids, (ii) the use of gels to control/enhance the cleaning action, and (iii) the application of nanoparticles for consolidation and deacidification purposes. Wall Painting Cleaning: Microemulsions and Micellar Solutions. The main component of the majority of mural paintings is calcium carbonate, which is obtained by the reaction of calcium hydroxide (hydrate lime) with atmospheric CO2. In a typical wall painting belonging to the classic tradition, three layers can be individuated: the so-called arriccio constitutes the inner layer that is in contact with the wall structure. Its composition is rich in sand, which is used as a filler to improve the mechanical properties of the plaster. The second layer, called the intonaco, is obtained by mixing equal amounts of lime and sand. The paint layer, consisting of a mixture of pigments and calcium hydroxide, is located on the intonaco. Frescoes are paintings in which the pigments are applied directly to the wet intonaco layer. The application of coloring materials on dried surfaces is called the secco technique. In this case, the paint layer may also contain organic binders such as egg, milk, and animal glues. Wall paintings are commonly subjected to two important degradation pathways: (i) the formation of sulfates depleting and weakening the carbonate layer and (ii) the solubilization and recrystallization of sulfate salts during relative humidity cycles (chloride and nitrate salts can also be present, even in large amounts), leading to mechanical stress within the wall pores and eventually to the pores’ collapse. As a result, the paint layer exhibits flaking and detachment (Figure 1). As stated above, the use of synthetic polymers as coating and protective agents has been widely practiced by conservators to 5111


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Figure 2. Details of wall paintings by Masaccio and Masolino in the Brancacci Chapel, Florence. The right upper panel shows wax spots under UV light before cleaning. The right lower panel shows the same area after cleaning with a microemulsion under visible light. On the left, the entire scene after restoration is shown (courtesy of Piero Baglioni).

Table 1. Composition of Some Amphiphilic Cleaning Systems That Have Been Used for the Conservation of Cultural Heritagea Xyl21,22

Xyl-ND8,22

EAPC23,24

APG25

component

%

component

%

component

%

component

%

water SDS 1-PeOH p-xyl

85.40 4.10 7.90 2.60

water SDS 1-PeOH p-xyl ND

86.20 3.90 6.50 1.80 1.60

water SDS 1-PeOH PC EA

73.30 3.70 7.00 8.00 8.00

water AGE AGESS p-xyl

99.00 0.52 0.12 0.36

a SDS, sodium dodecyl sulfate; 1-PeOH, 1-pentanol; p-xyl, p-xylene; ND, nitro diluent, a commercial mixture of 62% toluene, 15% butyl acetate, 15% ethyl acetate, 6% n-butyl alcohol, and 2% cellosolve acetate; PC, propylene carbonate; EA, ethyl acetate; AGE, alkyl polyglycoside ester; AGESS, sodium alkyl polyglycoside sulfosuccinate.

improve both the mechanical properties of flaking frescoes and a painting’s appearance. In particular, acrylic polymers constitute the majority of synthetic products used in the restoration and conservation of inorganic substrates. However, flaking and detachment phenomena are enhanced by the presence of these hydrophobic coatings over the surface (Figure 1). Therefore, the degradation of the polymers and the severe alteration caused by these materials to artistic substrates require their removal. The first application of amphiphile-based systems for the cleaning of works of art dates back to the end of the 1980s, when a water-based microemulsion was specifically designed and used for the removal of hydrophobic wax spots from the surface of Renaissance wall paintings in the Brancacci Chapel in Florence (Figure 2). The inspiration for applying a microemulsion for cleaning came to one of the authors of this Article (P.B.) from a study by De Gennes and Taupin.19 The four-component microemulsion cleaning system used on the Brancacci Chapel paintings was a modification of the so-called French microemulsion and consisted of dodecane droplets stabilized in water by a sodium dodecyl sulfate ionic surfactant and a 1-pentanol cosurfactant. With respect to traditional cleaning with pure solvents, the main advantage in the use of microemulsions relies on an enhance-

ment of grime/soil removal and on the confinement of the hydrophobic material (wax) inside the oil (in the present case, dodecane) microemulsion droplets. This avoids the spreading of the dissolved wax into the wall pores, as would have occurred with pure solvents.20 Another important general feature is that aqueous amphiphilic systems allow a significant decrease in the amount of organic solvents involved in a typical cleaning procedure, therefore depressing the system’s toxicity and environmental impact. Usually, the concentration of the dispersed organic phase is below 10−15% and varies according to the different cleaning systems, as reported in Table 1. In addition to that, regardless of the type of microemulsion or micellar solution applied, the use of a confining system to trap a micellar solution or microemulsion is always encouraged in order to have fine control of the cleaning action. In the case of cleaning mural paintings, the confining system usually consists of a poultice of cellulose pulp loaded with the selected complex fluid. Whenever direct solubilization of the coatings takes place, the poultice acts as a spongelike tool, further limiting the diffusion of any solubilized material within the porous matrix of the substrate. This, together with the hydrophilic barrier provided by the continuous aqueous medium of the microemulsion, strongly 5112


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that the microemulsion consists of droplets with a hydrodynamic diameter of about 17 to 18 nm.22 The efficacy of this system (XylND, see Table 1) was positively assessed during the cleaning of a late 14th century wall painting (Cappella Guasconi) in Arezzo, Italy,22 and in several other restoration interventions where aged Paraloid was removed. Besides acrylic polymers, vinyl-based coatings have been largely used for the consolidation and protection of works of art. Xylene is not a good solvent for this class of polymers because of its low polarity; therefore, different amphiphilic systems containing polar organic phases were developed, characterized, and assessed in recent applications. Propylene carbonate (PC) was successfully included in a four-component system consisting of SDS, 1-pentanol, water, and 22%PC.22 This system has been fully characterized by self-diffusion NMR and SAXS in order to clarify the nature of its nanostructure.29 It was reported that propylene carbonate is mainly solubilized in the continuous phase (water), but a significant amount (40% w/w) is contained in the micelles, acting mainly as a cosurfactant for the SDS micelles, decreasing their size and aggregation number by increasing the mean headgroup area of SDS. This system has been successfully tested in Siena (paintings of Lorenzo di Pietro called “il Vecchietta” in the Old Sacristy of Santa Maria della Scala (15th century)) and in Conegliano (mural paintings of Pozzoserrato from the external walls of the Santa Maria dei Battuti Cathedral (16th century)), where it efficiently removed polyvinyl acetate and acrylic polymers not removable with conventional cleaning systems such as neat solvents.22,30 The four-component system (water/SDS/PC/1-pentanol) was later modified by adding ethyl acetate (EA), which is a good solvent for both vinyl and acrylic copolymers. The composition of this system, named EAPC, is indicated in Table 1. EAPC is one of the most effective tools recently developed for the removal of polymeric coatings from artistic substrates. The structure of EAPC has been recently investigated by SANS contrast variation,28 deuterating in a selective fashion the EAPC components to determine the structure and constituent location of this complex fluid. The presence of two solvents (EA and PC) that are soluble in both the dispersed and the continuous phase points to the definition of the system as composed of swollen micelles even if its structure is definitely more complicated. The interaction of EAPC and the polymeric coating is affected by the presence of a water-soluble blend of organic solvents that dynamically exchange between micelles and the continuous phase, leading to a faster and more efficient interaction with the polymer layer.23 Interesting case studies concerned the removal of acrylic−vinyl copolymers (Mowilith products) that were widely applied to mural paintings belonging to the Mesoamerican cultural heritage (Mayapan − Yucatan, Mexico; Cholula − Puebla, Mexico)24 (Figure 4). This system was also found to be efficient in the removal of acrylic−vinyl copolymers and siliconic resins that are usually impossible to remove with conventional cleaning systems. Figure 5 shows the cleaning of a wall painting in the Annunciation Church in Nazareth, Israel, where a mixture of different polymers, including siliconic resins, were successfully removed with the EAPC system.28 The formulations containing xylene and propylene carbonate solvents require the presence of significant amounts of SDS. Nonionic surfactants exhibit the advantage, over ionic surfactants, of lowering the CMC, thus allowing the use of smaller amounts of nonvolatile components that might remain as residues on the painting after the cleaning. An interesting

hinders the redeposition of solubilized hydrophobic material in the wall’s pores, as would occur with neat solvents (Figure 3).

Figure 3. Organic coatings on a porous substrate (A). On the left (B), the use of neat organic solvents causes the solubilization of the coating within the pores (B1 brown) and the redeposition of the dissolved coating within the substrate’s pores upon solvent evaporation (B2). On the right (C), the continuous aqueous phase acts as a hydrophilic (C1 blue) barrier, preventing the penetration of the removed hydrophobic material within the porous substrate. The poultice acts as a spongelike tool, further limiting the spreading of the removed polymer (C2).

When the cleaning fluids produce only the swelling of the coatings, as occurs with silicon polymers, gentle mechanical action is required to complete the removal process. Interestingly, the latter case indeed represents an ideal condition for selective cleaning because the coating is peeled off without being solubilized and no residues are left on the substrate. At the end of the 1990s, two oil-in-water microemulsion systems were developed, differing from each other in the ionic (sodium dodecyl sulfate, SDS) or nonionic (polyoxyethylene sorbitan monoleate, TWEEN 80)21 surfactant. The oil phase consisted of xylene26,27 because of its high affinity for aged acrylic polymers. In particular, the system containing SDS and 1pentanol as cosurfactants (Xyl, see Table 1) has proven to be effective in many practical case studies. The interaction mechanism between the cleaning system and acrylic polymeric coatings has been investigated in order to achieve a thorough comprehension of the different steps involved in the cleaning process.24 Upon interaction with the polymer, a fraction of the solvents (1-pentanol and xylene) migrates from the microemulsion droplets to the coating; because of the loss of the organic fraction, the micelle size decreases and the swollen polymer detaches from the surface, resulting in surface cleaning. The structure of the microemulsion has been characterized down to the nanoscale by small angle X-ray scattering (SAXS), smallangle neutron scattering (SANS), and photocorrelation spectroscopy (QELS).28 To enhance the removal effectiveness, the Xyl microemulsion can be modified by adding to the oil phase variable amounts of nitro diluent (ND), a commercial blend of solvents (mainly toluene, butyl acetate, and ethyl acetate). QELS analyses showed 5113


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Figure 4. Removal of organic coatings from a Mesoamerican mural painting in Cholula (upper panel). On the bottom left (A), the presence of the organic coating significantly alters the readability of the painting. On the right (B), the same area partially cleaned after the application of a microemulsion (courtesy of Piero Baglioni).

Figure 5. Wall paintings in the Conon Apse (Annunciation Church in Nazareth, Israel). On the left, the effect of the polymer coating on the pictorial surface. On the right, the same area after complete removal achieved by using the EAPC system.28

surfactants with the aim of producing more environmentally friendly, but still effective, systems. Easel Painting Cleaning: Gels. A stratigraphy of a typical easel painting is depicted in Figure 6. Starting from the bottom, the supporting material consists of linen (or another natural cellulosic fiber); a preparation layer (ground layer) made of gypsum (or lead carbonate) mixed with animal glue is laid on the support in order to create a homogeneous substrate upon which the artist paints by using pigments, in most cases, dispersed in oil (paint layer). Finally, a varnish layer is laid over the paint layer for both aesthetic and protective purposes.

application of nonionic surfactants concerned the use of alkyl polyglycoside (APGs) for the formulation of cleaning systems (Table 1); in particular, an oil-in-water microemulsion, formed from less than 1% of a mixture of polyglucoside surfactants and about 0.5−1% oil, was successfully applied for the removal of Paraloid B72 (70:30 ethyl methacrylate/methyl acrylate copolymer) from the wall paintings in Santa Maria della Scala, Siena, Italy.25 Research efforts are currently devoted to the development of novel formulations involving the use of biodegradable nonionic 5114


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Figure 6. (Left) Stratigraphy of a typical easel painting. (Right) Stratigraphy of an easel painting restored using a traditional approach. Usually, during the restoration, a second canvas is glued to the back side of the painting (relining adhesive and canvas), and after the thinning of the aged varnished using neat solvents or solvent gels, fresh varnish is often applied to the front of the canvas.

Figure 7. (Left) SEM image of the relining glue covering the linen sample. (Right) Linen sample after the removal of the acrylic vinyl copolymer glue by the application of an acrylamide/bisacrylamide hydrogel loaded with the EAPC system.44

Because of photooxidation and thermal oxidation,31 the varnish layer naturally ages, undergoing both discoloration and cracking, altering the readability and visual appearance of the work of art. In addition to that, oil and varnish may release acid products that catalyze the hydrolysis of the cellulose constituting the canvas, resulting in the loss of the mechanical properties of the canvas support.32 The traditional conservation approach foresees the thinning of the aged varnish using neat solvents or solvent gels, even though the varnish solubility strongly decreases upon aging; after this operation, fresh varnish, natural or synthetic, is often applied. The reinforcement of the degraded canvas is carried out by gluing a new canvas on the back of the easel painting. (This operation is called relining.) Figure 6 shows the stratigraphy of the easel painting after restoration with traditional materials. Nowadays synthetic polymers, including glues and varnishes, are frequently used for canvas relining. Synthetic polymers, in particular, poly(vinyl acetate) (PVAc) adhesives, favor further degradation of the support because of the products formed during their own degradation, making their removal compulsory.33,34 Varnishes and glues can be directly removed using neat solvents. As for wall paintings, the use of neat solvent has two main contraindications: (i) the solubilized materials diffuse within the work of art layers; (ii) several solvents are harmful to both the operator and the environment. To avoid these issues, amphiphilic aqueous systems could be used. However, aqueous systems cannot be directly used on water-sensitive works of art (such as easel paintings) because of the possible swelling and mechanical stress of the canvas, the ground, and the paint layers. The confinement of these cleaning fluids in highly retentive gels allows the thinning of natural and synthetic polymeric layers and their controlled removal. In recent years, several authors have largely investigated the synthesis of new classes of gels, exploring their potential as cleaning tools for cultural heritage objects.9,35−39 The application

of gels for the removal of dirt and coatings from artistic substrates underwent a major advancement in the late 1980s when Richard Wolbers proposed the use of polymers (e.g., poly(acrylic acid)) as gellants.40 These systems (called solvent gels) enabled the control of components such as cosolvents, enzymes, and detergents, limiting the evaporation of solvents and their penetration into the artifact to be cleaned. However, the network of these physical gels is based on weak interactions (dipole− dipole interactions and/or hydrogen bonding), leading to the presence of residues left on the artifact after gel application.41 The removal of these residues would require the use of solvents, reproducing the problems connected to the application of neat solvents.41 An elegant solution to the residue issue is the use of chemical gels whose network is formed by covalent bonds. Such materials can be loaded with micelles or microemulsions or with polar neat solvents while exhibiting high retention capability. For instance, it has been shown that acrylamide-based chemical gels can be obtained through the radical polymerization of acrylamide monomer and N,N′-methylene bisacrylamide (cross-linker).42,43 The resulting tridimensional network of covalent bonds produces systems that can be loaded with water or micellar solutions/microemulsions, exhibiting good retention properties. In recent work, acrylamide gels have been synthesized and loaded with the EAPC amphiphile-based fluid for the selective removal of adhesives used for canvas relining.44 Model samples simulating the back side of canvases treated with adhesives have been relined using an acrylic vinyl copolymer such as Mowilith DMC5 (a commercial copolymer consisting of 65% vinyl acetate and 35% n-butyl acrylate).44 The removal of the adhesive from the surface was achieved by using two different acrylamide/bisacrylamide hydrogels loaded with EAPC. In particular, the two gels have a constant cross-linker/monomer ratio but differ for the polymer network concentration and were characterized by means of differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and small-angle 5115


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Figure 8. pHEMA/PVP thin gel film application over a model sample simulating the Tibetan painting technique of Thang-Ka (tempera magra), which is very sensitive to water. The panel on the left is the artificially soiled Thang-Ka. Soil is removed without damaging the pictorial layer, using the gel loaded with water.46

produced by the hydrolysis of poly(vinyl acetate), and depending on its degree of hydrolysis, several HVPDs can be produced and loaded with polar solvents, enzymes, and amphiphilic systems.37−39 The main feature of these systems relies on the possibility of their removal by peeling without the use of neat solvents, as would occur with solvent gels. In conclusion, by coupling highly retentive supporting gels with nanostructured amphiphilic systems, a new palette of cleaning tools has been offered to the conservator community. These new cleaning systems have tremendous advantages when easy, selective, and controlled removal of undesired layers (dirt, grime, polymers, etc.) is required. Consolidation and Deacidification: Alkaline Earth Metal Hydroxide Nanoparticles. Alkaline earth metal hydroxides can be used for a twofold task in the field of cultural heritage conservation. As reported in the previous sections, wall paintings and carbonatic stones can degrade for several reasons. A valid alternative to the use of polymers for consolidation is constituted by the use of inorganic consolidants. Among these, nanoparticles of alkaline earth metal hydroxides can be considered to be the most reliable and durable systems. In wall painting consolidation, nanoparticles replace the original pigment’s binder lost during the degradation process, reconsolidating the painting in a fully compatible way. In addition to that, calcium and magnesium hydroxides have proven to be excellent compounds for the deacidification of cellulosic works of art. A wide range of different synthesis pathways have been developed for the preparation of calcium, magnesium, strontium, and barium hydroxide nanoparticles dispersed in short-chain alcohols.47−55 The main rationale behind the research efforts concerning the synthesis of nanostructured materials for both consolidation and deacidification purposes is the control of the size and shape of the particles that eases the penetration into the substrate (i.e., wall painting or paper) and leads to enhanced reactivity associated with the high surface area of the nanoparticles. The use of hydroxide particle dispersions in short-chain alcohols presents some advantages over the application of saturated aqueous solutions such as limewater (a saturated Ca(OH)2 solution), a widely used inorganic method for consolidation. Calcium hydroxide has a low solubility in water, and a large amount of limewater is usually necessary for the treatment; however, a large amount of water in the wall painting for porous matrices can favor the collapse of the pores related to freeze−thaw cycles, the transport of salts, and the growth of microorganisms. On the contrary, nanoparticles dispersed in short-chain alcohols exhibit good penetration within the

X-ray scattering (SAXS). A higher concentration of polymer resulted in a more compact structure that allowed the confinement of aqueous systems in smaller pores and thicker pore walls, resulting in different mechanical properties and water retention capability. The hydrogel with the more compact structure swelled the coating layer that was then removed by gentle mechanical action. The absence of any detectable gel residues, determined via FT-IR ATR measurements, is also clearly shown by SEM images of the samples (Figure 7), where the excellent cleaning and removal of the adhesive are evident.44 The same acrylamide-based polymeric network has been considered to formulate a magnetically responsive gel tailored to clean marble, mural, and easel paintings. This gel consists of a network functionalized with CoFe2O4 nanoparticles, which can be loaded with an oil-in-water microemulsion for the selective removal of Paraloid B72 coatings. Besides the control of the cleaning action, these responsive systems allow the minimization of the mechanical action needed to remove them from the treated substrate, as required in the cleaning of very precious artifacts. In fact, these gels can be easily removed in the presence of a magnetic field such as that of a simple permanent magnet.43,45 Gels having a higher solvent retention capability are required when water-based cleaning fluids are used for the cleaning of hydrophilic and highly water-sensitive works of art (manuscripts and watercolor paintings). For these artifacts, properly designed gels were recently synthesized to prevent solvents from spreading over the artifact surface and to release fluids at a slow rate so as to perform gradual, nonaggressive cleaning at the interface. A system with high control of water confinement can be obtained with a semi-interpenetrating poly(2-hydroxyethyl-methacrylate) (pHEMA) and poly(N-vinyl-1-pyrrolidone) (PVP) structure. The rationale behind the choice of these components relies on the balance between the mechanical strength, provided by pHEMA, and the hydrophilic properties, granted by PVP, to obtain a system that gradually releases the cleaning fluids while avoiding gel residues on the treated surface. Moreover, these gels are transparent, allowing the visual control of their cleaning action, and can be realized as thin films (1 to 2 mm) and shaped as wished (Figure 8).46 Another important class of materials for the selective cleaning of synthetic coatings consists of high-viscosity polymeric dispersions (HVPDs). These systems are not truly physical gels according to their rheological behavior. It is well known that poly(vinyl alcohol) (PVA) forms HVPDs in the presence of borate ions, which act as cross-linkers between polymer chains. The resulting 3D network is thermally reversible. PVA is 5116


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Figure 9. (Left) TEM image of calcium hydroxide nanoparticles as obtained via an alcohol thermal synthesis. (Right) SEM image of nanoparticles adhering to cellulose fibers; the panel in the right upper corner is an enlargement (2-fold).

Calcium Hydroxide Nanoparticles. The preparation of calcium hydroxide nanoparticles has been achieved either through a breakdown process or by a bottom-up strategy. Fairly stable dispersions of calcium hydroxide in alcohol were obtained by grinding slaked lime (a putty lime produced by mixing calcium oxide with excess water) with a mill. The breakdown process leads to broad submicrometer/micrometer particle size distributions that were applied, for the first time, to the consolidation of degraded frescoes in the cathedral of Santa Maria Novella, Florence, Italy.47 Recently, an improved preparation method based on the thermomechanical treatment of slaked lime has been proposed, where the completion of lime’s hydration is favored by high temperature and pressure.55 As a matter of fact, the unreacted calcium oxide core in lime particles undergoes a strongly exothermic reaction causing the fragmentation of agglomerates that produces nanoparticles with an average size of up to 300 nm, which are stably dispersed in propanol. A different approach involves the building up of particles (bottom-up procedure) in aqueous solutions48 or in organic solvents.49 Control of the nucleation of particles over their growth is achieved by a high degree of supersaturation that is strongly dependent on temperature and pressure.59 Despite the fact that these methodologies produce nanoparticles with a smaller and narrower size distribution (ranging from 20 to 60 nm for the synthesis in organic solvents and from 50 to 400 nm for the synthesis in water), the required purification steps necessary to eliminate the NaCl byproduct make these procedures timeconsuming. Nonetheless, stable dispersions of calcium hydroxide nanoparticles in alcohol obtained from an aqueous homogeneous phase reaction have been applied in a variety of case studies involving both consolidation48 and deacidification purposes.50,53,58 A different approach to the synthesis of small calcium hydroxide nanoparticles involves the use of alkoxides as reaction intermediates; highly concentrated stable dispersions in alcohol, not requiring any purification step after the preparation, can be directly obtained by a high-pressure alcohol thermal reaction starting from calcium and short-chain alcohols. Because of their physicochemical characteristics, these particles are particularly suitable for application on porous artistic substrates (Figure 9). Moreover, the synthesis pathway can be potentially scaled up to the industrial level. In contrast, traditional deacidification methods (i.e., calcium hydroxide solutions) have been shown

painting’s porous matrix at concentrations sufficient for a complete consolidation process, making hydroxide nanoparticles an optimal system for wall painting consolidation. The second use of nanoparticles in conservation is related to deacidification. Cellulose-based artifacts are usually threatened by the concomitant action of hydrolysis and oxidation that leads to the loss of the mechanical resistance of the fibers and to discoloration phenomena of the substrate. As in the case of consolidation of classical wall paintings, traditional deacidification methodologies involve the use of highly alkaline aqueous solutions. The ideal deacidifying agents are alkaline earth metal hydroxides and carbonates, which are very compatible with the substrate and its components and at the same time readily neutralize acidity. However, the application of alkaline aqueous solutions implies two significant drawbacks: (i) several paper components, such as inks and sizing, are water-sensitive and (ii) the high alkalinity of aqueous solutions may favor the alkaline depolymerization of cellulose, which takes place at room temperature on oxidized substrates.56 Moreover, the traditional deacidification treatments are usually performed through immersion, and this implies poor control of the amount of applied deacidifying agent. The use of nonaqueous methods overcomes these drawbacks; the commercial nonaqueous treatments that are nowadays available are all based on solutions or dispersions of oxides and carbonates precursors.57 For instance, in the case of paper preservation, magnesium and calcium hydroxide nanoparticles in propanol produce safe and stable deacidification, leaving a mild alkaline carbonate buffer reserve against reoccurring acidity on the treated document.10,50,52,54 Recently, alkaline earth metal hydroxide nanoparticles dispersed in alcohols have been shown to be an efficient deacidification system for paper and have proven to be promising tools for the deacidification of archeological wood. The application of these systems to waterlogged wood deacidification has been investigated in the context of the preservation of the Swedish 17th century warship Vasa, whose timbers contain high quantities of sulfuric acid, developed after salvaging from the oxidation of sulfur-reduced compounds formed from bacterial activity.53,58 In the next sections, the main nanoparticles’ synthesis pathways developed for application in the field of cultural heritage conservation are briefly reviewed. 5117


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Figure 10. (A) Size distribution of Ca(OH)2 nanoparticles in 2-propanol obtained through high-temperature milling. (B) Size distribution of Mg(OH)2 nanoparticles in 2-propanol obtained through a homogeneous phase reaction with excess magnesium ions. (C) Size distribution of Ba(OH)2 nanoparticles in 1-propanol obtained through milling.

Figure 11. (A) Wall painting belonging to a Mesoamerican archeological site. (B) Details of a flaking surface exhibiting sulfate efflorescence. (C) The same surface after the desulfation treatment with ammonium carbonate and the application of a mixed calcium and barium hydroxide nanoparticle dispersion.

to produce uneven depositions on cellulose fibers with particles having diameters of several micrometers.60 Barium Hydroxide Nanoparticles. Barium hydroxide has been used as a consolidant for carbonaceous materials since the end of the 19th century.61 Ferroni proposed the use of barium hydroxide aqueous solutions for the consolidation of frescoes that were heavily damaged in the Florence flood of 1966.62,63 Its use is recommended when large amounts of sulfates are present in the wall painting matrix. Typically, a two-step procedure is considered: (1) desulfation with an ammonium carbonate solution loaded in a cellulose pulp poultice and (2) the application of a Ba(OH)2 solution that fixes the residual soluble sulfates into insoluble barium sulfate. Furthermore, the presence of excess barium hydroxide converts the powdery calcium carbonate (formed in step 1) into calcium hydroxide that reacts with CO2 to reform a crystalline network of calcium carbonate that acts as a binder, resulting in the consolidating action. Besides, barium hydroxide can be used together with calcium hydroxide nanoparticle dispersions for the consolidation of heavily degraded and sulfated wall paintings. In fact, in this case, Ca(OH)2 nanoparticles alone are not very efficient because the consolidating action of calcium hydroxide is hindered by its partial transformation to the more stable calcium sulfate. In this case, the most simple and elegant solution is the application of mixed calcium and barium hydroxide nanoparticle dispersions in alcohol. The synthesis of Ba(OH)2 nanoparticles from the aqueous homogeneous phase reaction is hindered by the low degree of supersaturation that can be achieved in water as a result of barium hydroxide fair solubility (Ksp = 2.55 × 10−4). Instead, a

heterogeneous approach based on the breakdown of Ba(OH)2 macrocrystals has been recently developed.55 Typically, the milling of commercial barium hydroxide in 1-propanol leads to fairly stable dispersions in alcohol for nanoparticles whose size ranges from 200 to 400 nm (Figure 10). Recently, mixed calcium and barium hydroxide nanoparticle dispersions have proven to be highly effective in the consolidation of mural paintings heavily contaminated by salts (mainly sulfates and chlorides) in the Mesoamerican area (Figure 11).55,64 An additional advantage of the use of nanoparticle dispersions of Ba(OH)2 in nonaqueous solvents is represented by the very low toxicity as compared to that of aqueous solutions of barium salts. Magnesium Hydroxide Nanoparticles. Magnesium hydroxide microparticles are employed in many industrial applications as flame retardants and oxide precursors.65 In the past 10 years, several “classical” synthesis pathways were modified and tailored to produce magnesium hydroxide nanoparticles for the conservation of cultural heritage. In particular, Mg(OH)2 nanoparticles, obtained via aqueous homogeneous phase reactions and dispersed in alcohol, have been used for the deacidification of paper, canvas, and wood.52,54,58,66,67 It was reported that the particle size can be tuned from 50 to 300 nm by changing the counterions’ nature52 and their concentration in the chemical reaction.54 Despite the fact that these particles are usually applied for deacidification purposes, they have been recently considered for the consolidation of dolomite stones (calcium−magnesium carbonate stones).64 By analogy to calcium hydroxide, magnesium hydroxide nanoparticles can also be obtained from an alcohol thermal 5118


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Figure 12. SEM images of the conversion of magnesium methoxide micrometer-sized particles into magnesium hydroxide nanoparticles. On the left, octahedral-shaped magnesium methoxide micrometer-sized crystals. In the middle, the formation of hexagonal nanoplatelets of magnesium hydroxide on {111} crystal planes during the hydration process of magnesium methoxide. On the right, magnesium hydroxide nanoparticles of about 100 nm after the hydration reaction.

reaction of nanoparticles with CO2 produces a carbonate buffer to protect cellulose from aging. The protective action of alkaline earth metal hydroxide nanoparticles against ink corrosion was evaluated on paper model samples featuring iron gall writing fluids. The degradation of these systems was monitored before and after accelerated hydrothermal aging (T = 90 °C, RH = 75%) by measuring the cellulose degree of polymerization (via viscosimetric determinations) and pH.54,67 In Figure 14, the comparison between the

reaction starting from bulk metal. The conversion of a micrometer-sized alkoxide intermediate (Figure 12) into nanometric magnesium hydroxide particles is favored by high pressure and temperature, leading to stable concentrated dispersions in short-chained alcohols. Currently, magnesium hydroxide nanoparticles are mainly used for the deacidification of paper and iron gall-inked manuscripts,54,67 one of the more important open problems in the field of paper conservation.68,69 As can be seen in Figure 13,

Figure 14. Inked paper samples after artificial aging: (A) an unprotected sample and (B) a sample treated with nanoparticles to protect it from aging. Replica samples were made by the application of iron gall ink on 99.9% cellulose unsized Whatman paper (adapted from ref 54). Figure 13. Corrosion of paper due to the presence of iron gall inks on a historical manuscript (adapted from ref 54).

unprotected paper sample (i.e., not deacidified previous to aging and having a pH of 3) and the sample neutralized with magnesium hydroxide nanoparticles clearly shows how nanoparticle application protected the paper from ink corrosion. After 48 h of artificial aging, the unprotected sample exhibited severe damage (e.g., brittleness) and could not be manipulated, whereas sample B, treated with magnesium hydroxide nanoparticles, retained its original mechanical properties. Therefore, a single-step deacidification treatment, aimed at stabilizing the pH around neutrality, can inhibit both metalcatalyzed oxidation and the acid hydrolysis of paper, ensuring the long-term preservation of iron gall-inked paper.

the synergistic effect of acidity and cellulose oxidation mainly as a result of the free metal ions introduced into the substrate with the application of the ink leads to the corrosion of cellulose fibers, resulting in the loss of readability of the manuscript. It has been demonstrated that the metal ion-catalyzed oxidation (due to a radical formation mechanism) is enhanced by acidity: this process has a high rate at pH values below 4.5 whereas the minimum catalytic activity of metal ions (iron and copper) is in the 6.5−7.5 pH range.70 According to Strlič et al.70 and Baty et al.,57 the ideal requirement for a conservation treatment of manuscripts is to stabilize the paper pH around neutrality to hinder both acid-catalyzed hydrolysis and metalcatalyzed oxidation. The application of alkaline earth metal hydroxide nanoparticles dispersed in nonaqueous solvents provides this feature, inhibiting the two different degradation mechanisms through a single, simple, safe treatment that gradually takes to a neutral pH, which results in a significant increase in the inked-paper resistance to weathering. The nonaqueous treatment54,67 prevented the leaching of watersensitive writing fluids and allowed an efficient distribution of the deacidifying nanoparticles in the substrate. Moreover, the

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CONCLUSIONS AND OUTLOOK In the past 15 years, colloids and material science generated a number of innovative, functional, and fairly inexpensive tools for the conservation of movable and immovable cultural heritage. The use of materials compatible with those constituting the works or art leads to the long-term stability of the treated artifact whose useful life can be extended with strong societal and economical benefits. Research in science of cultural heritage is still far from being mature. The main possible future scenarios include (a) new “green” surfactant-based self-assembled systems, (b) water-in-oil, oil-in-water, and waterless cleaning micro5119


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author of about 30 publications in the field of nanotechnology and colloids science applications to the conservation of cultural heritage.

emulsions/emulsions for the treatment of highly sensitive hydrophilic substrates (e.g., acrylic paintings), (c) organogels, as a complement to the already popular hydrogels, to be used as support systems for the above-mentioned fluids, (d) gels that are responsive to an external stimulus, (e) nanoparticle dispersions in apolar solvents for the deacidification of water-sensitive substrates (parchment, leather), and (f) hybrid organic− inorganic nanocomposite systems. The new polyfunctional tools for the conservation of cultural heritage might impact several fields, where the detergency and chemical reactivity of the nanomaterials play a leading role. However, the preservation and valorization of the cultural heritage legacy lead to the outcome of consistent economical resources (i.e., tourism) and, from a different point of view, improve the image and perception of science and, in particular, chemistry.

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Rodorico Giorgi, Ph.D. in science for cultural heritage conservation at the University of Florence and B.S. in chemistry, is currently a research fellow in the department of chemistry at CSGI, University of Florence. Giorgi’s background is in colloids science. His main research interests are the development of methodologies for the conservation of cultural heritage materials such as wall and easel paintings, stone, paper, and archaeological wood. Giorgi is the author of about 80 publications in the field of science for conservation. Giovanna Poggi holds a Ph.D. in science for cultural heritage conservation from the University of Florence and obtained a Master’s in technology for the conservation of cultural heritage in 2007. She is currently working as a postdoctoral fellow at CSGI in the development and characterization of nanotechnology for conservation. Her research mainly deals with the synthesis and characterization of nanoparticles and their application to paper and wood deacidification.

AUTHOR INFORMATION

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Corresponding Author

ACKNOWLEDGMENTS We thank all of the conservators involved in the application of the conservation methodologies presented here to real case studies for their help and, in particular, Tiziana Dell’Omo and Lucia Di Paolo for the intervention in Nazareth (Israel) and Florence Gorel for the experiments on Thang-Ka paintings. Moreover, Ramon Carrasco Vargas (Proyecto Arqueologico Calakmul, Mexico), Lilia Rivero Weber, Diana Medellin, Yareli Jaidar Benavides, and Maria del Carmen Castro Barrera (Coordinación Nacional de Conservación del Patrimonio Cultural, CNCPC, Mexico), and Diana Magaloni (UNAM and Museo National de Antropologia, Mexico) are acknowledged for the Mesoamerican archaeological areas. CSGI, MIUR, and European Union (project NANOFORART, FP7-ENV-NMP-2011/282816) are acknowledged for financial support.

*Tel: +39 0554573033. Fax: +39 0554573033. E-mail: [email protected]fi.it. Notes

The authors declare no competing financial interest. Biographies

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REFERENCES

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Michele Baglioni, photographer

Left to right: Giovanna Poggi, Rodorico Giorgi, David Chelazzi, and Piero Baglioni Piero Baglioni has been the Chair of Physical Chemistry in the Department of Chemistry at the University of Florence since 1994 and is an MIT affiliate. He was appointed as visiting scientist/professor by the Department of Chemistry of the University of Houston, the Weizmann Institute, the Collège de France, and MIT. He is the Director of the National Center for Colloids and Nanosciences (CSGI), and he is on the advisory boards of several international journals and a member of the scientific board of several national and international institutions and societies. He is the author of more than 350 publications in books and largely diffused international journals. He is also the author of 21 patents. In the field of conservation, he is a pioneer in the application of colloids and soft matter to the conservation of cultural heritage. He has produced several innovative methods for the consolidation and cleaning of paintings and the deacidification of historical documents. David Chelazzi, Ph.D. in science for cultural heritage conservation at the University of Florence in 2007 and Master’s in chemistry in 2003, is currently working as a postdoctoral fellow in the department of chemistry at the University of Florence and CSGI. His main research interests are the development of methodologies for the consolidation, cleaning, and pH control of works of art such as wall and canvas paintings, stone, paper, and archaeological wood. He is the author or co5120


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