Colloid and Materials Science for the Conservation of Cultural Heritage: Cleaning, Consolidation, and Deacidification

Colloid and Materials Science for the Conservation of CulturalHeritage: Cleaning, Consolidation, and DeacidificationPiero 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 afrequent approach for the development of materials andmethodologies used for a long time for either cleaning orconsolidation of works of art. Recently, new perspectives havebeen opened by the application of materials science, colloidscience, and interface science frameworks to conservation,generating a breakthrough in the development of innovativetools for the conservation and preservation of cultural heritage.This Article is an overview of the most recent contributions ofcolloid and materials science to the art conservation field,mainly focusing on the use of amphiphile-based fluids, gels,and alkaline earth metal hydroxide nanoparticles dispersionsfor 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.

■ INTRODUCTION

Conservation science is a complex discipline that deals with therestoration and preservation of a large variety of materialsconstituting our cultural heritage. At first sight, one might thinkthat science, art, and humanities are disconnected disciplines.However, our way of thinking and behaving depends strongly onthe legacy of physical artifacts and intangible attributes of asociety, inherited from past generations, maintained in thepresent, and possibly bestowed for the benefit of futuregenerations.Because of the complexity of artistic and historical substrates,

conservation science has explored different routes, developingseveral approaches for solving conservation issues. For instance,new resins that are more stable than natural ones, whileexhibiting similar optical properties, have been developed andapplied as varnishes for retouching paintings.1 Furthermore, theanalysis of materials is often considered to be a preliminary stepfor suggesting treatments for works of art: in the conservation ofmatte paintings, the analysis of the physical and optical propertiesof the painted layers is the key to choosing the best materials fortheir consolidation.2 Electrochemistry and corrosion sciencehave proven useful in conserving bronze outdoor statuary,3 andcolloid science has been providing an increasingly importantcontribution to the development of restoration tools. Among thesystems specifically tailored for conservation issues, nano-particulate inorganic sols (nanosols),4 colloidal silica,5 andalkoxysilane6 play an important role in stone and woodconservation. In particular, because of their high surface-to-volume ratio, nanosols are metastable and usually hydrolyze toform 3D xerogel networks that improve the mechanicalproperties and resistance to water, fire, and microbial or insectattack of wood or stone objects.

Concepts and tools7−10 belonging to the realm of colloid andmaterials science, which have acquired a leading role in thedevelopment of advanced and functional tools constituting alarge portion of the commercial products that are consumed dailyby millions of people all over the world and used in a variety ofapplications including cosmetics, food, and pharmaceutics,11−13

may lead to a dramatic enhancement of the effectiveness anddurability of restoration interventions.Typically, works of art comprise both movable and immovable

objects. The first class includes all of the documentary andhistorical manuscripts and books, usually made of paper orparchment, easel paintings on wood or canvas, and a large varietyof objects such as statuettes, jewelry, and textiles. Immovableworks of art mainly consist of mural/wall paintings, architectonicsubstrates, statues, and several kinds of stone-based artifacts.Regardless of their nature, artifacts are irremediably exposed toseveral degradation agents: physical erosion, chemical degrada-tion, temperature, relative humidity, light, and microorganisms,all accounting for the natural aging of art materials. Moreover,anthropic activities increase the concentrations of SO2, NOx, andVOC (volatile organic compound) gases in the atmosphere thateventually lead to the corrosion of artistic substrates,contributing to the degradation of works of art. Finally, it mustbe outlined that in some cases conservation issues are due to pastextemporized restoration interventions that were based on trial-and-error practice.Depending on the type of artistic substrate, different tasks are

necessary for conservation purposes. A large fraction of

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

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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 polymersfrom the surface of coated movable and immovable works of art.The application of natural varnishes has been largely performedin the past and has been traditionally adopted by easel paintingartists that wanted to enhance the visual properties of theirworks. In addition to improving the appearance through thesaturation of color, traditional conservation practice foresees theuse of such materials to provide a vast number of different artisticsubstrates with hydrophobic properties and surface protection.Starting from the second half of the 20th century, syntheticpolymers have been enthusiastically adopted, mainly becausethey were thought to be highly resistant to aging and easilyremovable. Unfortunately, synthetic polymers undergo degrada-tion similarly to that of natural resins, resulting in the decrease oftheir solubility in net solvents and in the significant alteration oftheir visual aspect (mainly yellowing).14 Because of theiradhesive properties, synthetic polymers have been used toreadhere detached or damaged parts in the external layers ofworks of art. For instance, acrylate, vinyl, silicone, and epoxypolymers are widely used for the consolidation and protection ofstone and wall paintings. However, the use of synthetic adhesivesresults in the strong alteration of physicochemical properties ofthe original substrates, such as porosity, water capillarity, watervapor permeability, and surface wettability,15,16 generating in thelong term enhanced degradation that can proceed even up to theloss of the artifacts.Another important class of works of art subjected to

degradation are cellulose-, parchment-, and leather-basedartifacts (mainly documentary and historical manuscripts andbooks), which are threatened mainly by hydrolysis and oxidationreactions (acidity plays a fundamental role in the catalysis ofthese reactions) that lead to the loss of paper/parchmentmechanical resistance.17,18 Conservation practice usually in-volves the use of alkaline aqueous solutions for paperdeacidification whereas polar solvents are discouraged oncollagen-based substrates. However, water causes the swellingof cellulose fibers and the leaching of compounds associated withpaper (inks, sizing, etc.). This limitation has favored thedevelopment of nonaqueous deacidification methods.In the past 15 years, tools from nanoscience and in particular

from colloid and surface science were demonstrated to overcomesome of the limits of traditional restoration methodologies byproviding, in a new cultural framework, innovative method-ologies and materials for the above-described conservationissues. As a result, several reliable, easy to use, and inexpensive

tools have been conceived and offered to conservators,constituting a new palette of treatments that exhibit highcompatibility with the original artistic substrates and thereforegrant long-term durability.The aim of this Feature Article is to illustrate some of the main

achievements attained for the conservation of cultural heritage inthe framework of surface and colloid science, providing an up-to-date overview focused on the research activity of the authors ofthis Feature Article, including future directions and perspectivesin the field. In this regard, nanostructured fluids such asmicroemulsions and micellar solutions, highly retentive chemicalgels, and nonaqueous dispersions of alkaline nanoparticles will bedescribed in the following sections.

■ PROTECTION AND RESTORATION OF CULTURALHERITAGE

In this section, we report three applications of colloid science thataddress a large fraction of conservation issues: (i) cleaning withnanostructured fluids, (ii) the use of gels to control/enhance thecleaning action, and (iii) the application of nanoparticles forconsolidation and deacidification purposes.

Wall Painting Cleaning: Microemulsions and MicellarSolutions. The main component of the majority of muralpaintings is calcium carbonate, which is obtained by the reactionof calcium hydroxide (hydrate lime) with atmospheric CO2. In atypical wall painting belonging to the classic tradition, threelayers can be individuated: the so-called arriccio constitutes theinner layer that is in contact with the wall structure. Itscomposition is rich in sand, which is used as a filler to improve themechanical properties of the plaster. The second layer, called theintonaco, is obtained by mixing equal amounts of lime and sand.The paint layer, consisting of a mixture of pigments and calciumhydroxide, is located on the intonaco. Frescoes are paintings inwhich the pigments are applied directly to the wet intonaco layer.The application of coloring materials on dried surfaces is calledthe secco technique. In this case, the paint layer may also containorganic binders such as egg, milk, and animal glues.Wall paintings are commonly subjected to two important

degradation pathways: (i) the formation of sulfates depleting andweakening the carbonate layer and (ii) the solubilization andrecrystallization of sulfate salts during relative humidity cycles(chloride and nitrate salts can also be present, even in largeamounts), leading to mechanical stress within the wall pores andeventually to the pores’ collapse. As a result, the paint layerexhibits 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

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

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improve both the mechanical properties of flaking frescoes and apainting’s appearance. In particular, acrylic polymers constitutethe majority of synthetic products used in the restoration andconservation of inorganic substrates. However, flaking anddetachment phenomena are enhanced by the presence of thesehydrophobic coatings over the surface (Figure 1). Therefore, thedegradation of the polymers and the severe alteration caused bythese 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, whena water-based microemulsion was specifically designed and usedfor the removal of hydrophobic wax spots from the surface ofRenaissance 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 byDe Gennes and Taupin.19 The four-component microemulsioncleaning system used on the Brancacci Chapel paintings was amodification of the so-called French microemulsion andconsisted of dodecane droplets stabilized in water by a sodiumdodecyl sulfate ionic surfactant and a 1-pentanol cosurfactant.With respect to traditional cleaning with pure solvents, the mainadvantage in the use of microemulsions relies on an enhance-

ment of grime/soil removal and on the confinement of thehydrophobic material (wax) inside the oil (in the present case,dodecane) microemulsion droplets. This avoids the spreading ofthe dissolved wax into the wall pores, as would have occurredwith pure solvents.20

Another important general feature is that aqueous amphiphilicsystems allow a significant decrease in the amount of organicsolvents involved in a typical cleaning procedure, thereforedepressing the system’s toxicity and environmental impact.Usually, the concentration of the dispersed organic phase isbelow 10−15% and varies according to the different cleaningsystems, 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 amicellar solution or microemulsion is always encouraged in orderto have fine control of the cleaning action. In the case of cleaningmural paintings, the confining system usually consists of apoultice of cellulose pulp loaded with the selected complex fluid.Whenever direct solubilization of the coatings takes place, thepoultice acts as a spongelike tool, further limiting the diffusion ofany solubilizedmaterial within the porous matrix of the substrate.This, together with the hydrophilic barrier provided by thecontinuous aqueous medium of the microemulsion, strongly

Figure 2.Details of wall paintings by Masaccio and Masolino in the Brancacci Chapel, Florence. The right upper panel shows wax spots under UV lightbefore cleaning. The right lower panel shows the same area after cleaning with a microemulsion under visible light. On the left, the entire scene afterrestoration 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 85.40 water 86.20 water 73.30 water 99.00SDS 4.10 SDS 3.90 SDS 3.70 AGE 0.521-PeOH 7.90 1-PeOH 6.50 1-PeOH 7.00 AGESS 0.12p-xyl 2.60 p-xyl 1.80 PC 8.00 p-xyl 0.36

ND 1.60 EA 8.00aSDS, 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.



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

When the cleaning fluids produce only the swelling of thecoatings, as occurs with silicon polymers, gentle mechanicalaction is required to complete the removal process. Interestingly,the latter case indeed represents an ideal condition for selectivecleaning because the coating is peeled off without beingsolubilized 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 (polyoxyethylenesorbitan monoleate, TWEEN 80)21 surfactant. The oil phaseconsisted of xylene26,27 because of its high affinity for aged acrylicpolymers. In particular, the system containing SDS and 1-pentanol as cosurfactants (Xyl, see Table 1) has proven to beeffective in many practical case studies. The interactionmechanism between the cleaning system and acrylic polymericcoatings has been investigated in order to achieve a thoroughcomprehension of the different steps involved in the cleaningprocess.24 Upon interaction with the polymer, a fraction of thesolvents (1-pentanol and xylene) migrates from the micro-emulsion droplets to the coating; because of the loss of theorganic fraction, the micelle size decreases and the swollenpolymer detaches from the surface, resulting in surface cleaning.The structure of the microemulsion has been characterized downto the nanoscale by small angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and photocorrelation spec-troscopy (QELS).28

To enhance the removal effectiveness, the Xyl microemulsioncan be modified by adding to the oil phase variable amounts ofnitro diluent (ND), a commercial blend of solvents (mainlytoluene, butyl acetate, and ethyl acetate). QELS analyses showed

that the microemulsion consists of droplets with a hydrodynamicdiameter of about 17 to 18 nm.22 The efficacy of this system (Xyl-ND, see Table 1) was positively assessed during the cleaning of alate 14th century wall painting (Cappella Guasconi) in Arezzo,Italy,22 and in several other restoration interventions where agedParaloid 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 ofits low polarity; therefore, different amphiphilic systemscontaining polar organic phases were developed, characterized,and assessed in recent applications. Propylene carbonate (PC)was successfully included in a four-component system consistingof SDS, 1-pentanol, water, and 22%PC.22 This system has beenfully characterized by self-diffusion NMR and SAXS in order toclarify the nature of its nanostructure.29 It was reported thatpropylene carbonate is mainly solubilized in the continuousphase (water), but a significant amount (40% w/w) is containedin the micelles, acting mainly as a cosurfactant for the SDSmicelles, decreasing their size and aggregation number byincreasing the mean headgroup area of SDS. This system hasbeen successfully tested in Siena (paintings of Lorenzo di Pietrocalled “il Vecchietta” in the Old Sacristy of Santa Maria dellaScala (15th century)) and in Conegliano (mural paintings ofPozzoserrato from the external walls of the Santa Maria deiBattuti Cathedral (16th century)), where it efficiently removedpolyvinyl acetate and acrylic polymers not removable withconventional 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 goodsolvent for both vinyl and acrylic copolymers. The compositionof this system, named EAPC, is indicated in Table 1. EAPC is oneof the most effective tools recently developed for the removal ofpolymeric coatings from artistic substrates. The structure ofEAPC has been recently investigated by SANS contrastvariation,28 deuterating in a selective fashion the EAPCcomponents to determine the structure and constituent locationof this complex fluid. The presence of two solvents (EA and PC)that are soluble in both the dispersed and the continuous phasepoints to the definition of the system as composed of swollenmicelles even if its structure is definitely more complicated. Theinteraction of EAPC and the polymeric coating is affected by thepresence of a water-soluble blend of organic solvents thatdynamically exchange between micelles and the continuousphase, leading to a faster and more efficient interaction with thepolymer layer.23 Interesting case studies concerned the removalof acrylic−vinyl copolymers (Mowilith products) that werewidely applied to mural paintings belonging to the Mesoamer-ican 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 usuallyimpossible to remove with conventional cleaning systems. Figure5 shows the cleaning of a wall painting in the AnnunciationChurch in Nazareth, Israel, where a mixture of differentpolymers, including siliconic resins, were successfully removedwith the EAPC system.28

The formulations containing xylene and propylene carbonatesolvents require the presence of significant amounts of SDS.Nonionic surfactants exhibit the advantage, over ionicsurfactants, of lowering the CMC, thus allowing the use ofsmaller amounts of nonvolatile components that might remain asresidues on the painting after the cleaning. An interesting

Figure 3. Organic coatings on a porous substrate (A). On the left (B),the use of neat organic solvents causes the solubilization of the coatingwithin the pores (B1 brown) and the redeposition of the dissolvedcoating within the substrate’s pores upon solvent evaporation (B2). Onthe right (C), the continuous aqueous phase acts as a hydrophilic (C1blue) barrier, preventing the penetration of the removed hydrophobicmaterial within the porous substrate. The poultice acts as a spongeliketool, further limiting the spreading of the removed polymer (C2).



application of nonionic surfactants concerned the use of alkylpolyglycoside (APGs) for the formulation of cleaning systems(Table 1); in particular, an oil-in-water microemulsion, formedfrom less than 1% of a mixture of polyglucoside surfactants andabout 0.5−1% oil, was successfully applied for the removal ofParaloid B72 (70:30 ethyl methacrylate/methyl acrylatecopolymer) from the wall paintings in Santa Maria della Scala,Siena, Italy.25

Research efforts are currently devoted to the development ofnovel formulations involving the use of biodegradable nonionic

surfactants with the aim of producing more environmentallyfriendly, but still effective, systems.

Easel Painting Cleaning: Gels. A stratigraphy of a typicaleasel painting is depicted in Figure 6. Starting from the bottom,the supporting material consists of linen (or another naturalcellulosic fiber); a preparation layer (ground layer) made ofgypsum (or lead carbonate) mixed with animal glue is laid on thesupport in order to create a homogeneous substrate upon whichthe artist paints by using pigments, in most cases, dispersed in oil(paint layer). Finally, a varnish layer is laid over the paint layer forboth aesthetic and protective purposes.

Figure 4. Removal of organic coatings from a Mesoamerican mural painting in Cholula (upper panel). On the bottom left (A), the presence of theorganic coating significantly alters the readability of the painting. On the right (B), the same area partially cleaned after the application of amicroemulsion (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 pictorialsurface. On the right, the same area after complete removal achieved by using the EAPC system.28



Because of photooxidation and thermal oxidation,31 thevarnish layer naturally ages, undergoing both discoloration andcracking, altering the readability and visual appearance of thework of art. In addition to that, oil and varnish may release acidproducts that catalyze the hydrolysis of the cellulose constitutingthe canvas, resulting in the loss of the mechanical properties ofthe canvas support.32 The traditional conservation approachforesees the thinning of the aged varnish using neat solvents orsolvent gels, even though the varnish solubility strongly decreasesupon aging; after this operation, fresh varnish, natural orsynthetic, is often applied. The reinforcement of the degradedcanvas is carried out by gluing a new canvas on the back of theeasel painting. (This operation is called relining.) Figure 6 showsthe stratigraphy of the easel painting after restoration withtraditional materials.Nowadays synthetic polymers, including glues and varnishes,

are frequently used for canvas relining. Synthetic polymers, inparticular, poly(vinyl acetate) (PVAc) adhesives, favor furtherdegradation of the support because of the products formedduring their own degradation, making their removal compul-sory.33,34

Varnishes and glues can be directly removed using neatsolvents. As for wall paintings, the use of neat solvent has twomain contraindications: (i) the solubilized materials diffusewithin the work of art layers; (ii) several solvents are harmful toboth the operator and the environment. To avoid these issues,amphiphilic aqueous systems could be used. However, aqueoussystems cannot be directly used on water-sensitive works of art(such as easel paintings) because of the possible swelling andmechanical stress of the canvas, the ground, and the paint layers.The confinement of these cleaning fluids in highly retentive gelsallows the thinning of natural and synthetic polymeric layers andtheir controlled removal.In recent years, several authors have largely investigated the

synthesis of new classes of gels, exploring their potential ascleaning tools for cultural heritage objects.9,35−39 The application

of gels for the removal of dirt and coatings from artistic substratesunderwent a major advancement in the late 1980s when RichardWolbers proposed the use of polymers (e.g., poly(acrylic acid))as gellants.40 These systems (called solvent gels) enabled thecontrol of components such as cosolvents, enzymes, anddetergents, limiting the evaporation of solvents and theirpenetration into the artifact to be cleaned. However, the networkof these physical gels is based on weak interactions (dipole−dipole interactions and/or hydrogen bonding), leading to thepresence 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 neatsolvents.41

An elegant solution to the residue issue is the use of chemicalgels whose network is formed by covalent bonds. Such materialscan be loaded with micelles or microemulsions or with polar neatsolvents while exhibiting high retention capability. For instance,it has been shown that acrylamide-based chemical gels can beobtained through the radical polymerization of acrylamidemonomer andN,N′-methylene bisacrylamide (cross-linker).42,43The resulting tridimensional network of covalent bondsproduces systems that can be loaded with water or micellarsolutions/microemulsions, exhibiting good retention properties.In recent work, acrylamide gels have been synthesized and loadedwith the EAPC amphiphile-based fluid for the selective removalof adhesives used for canvas relining.44

Model samples simulating the back side of canvases treatedwith adhesives have been relined using an acrylic vinyl copolymersuch as Mowilith DMC5 (a commercial copolymer consisting of65% vinyl acetate and 35% n-butyl acrylate).44 The removal ofthe adhesive from the surface was achieved by using two differentacrylamide/bisacrylamide hydrogels loaded with EAPC. Inparticular, the two gels have a constant cross-linker/monomerratio but differ for the polymer network concentration and werecharacterized by means of differential scanning calorimetry(DSC), scanning electron microscopy (SEM), and small-angle

Figure 6. (Left) Stratigraphy of a typical easel painting. (Right) Stratigraphy of an easel painting restored using a traditional approach. Usually, duringthe 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 usingneat 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 bythe application of an acrylamide/bisacrylamide hydrogel loaded with the EAPC system.44



X-ray scattering (SAXS). A higher concentration of polymerresulted in a more compact structure that allowed theconfinement of aqueous systems in smaller pores and thickerpore walls, resulting in different mechanical properties and waterretention capability. The hydrogel with the more compactstructure swelled the coating layer that was then removed bygentle mechanical action. The absence of any detectable gelresidues, determined via FT-IR ATR measurements, is alsoclearly shown by SEM images of the samples (Figure 7), wherethe excellent cleaning and removal of the adhesive are evident.44

The same acrylamide-based polymeric network has beenconsidered to formulate a magnetically responsive gel tailored toclean marble, mural, and easel paintings. This gel consists of anetwork functionalized with CoFe2O4 nanoparticles, which canbe loaded with an oil-in-water microemulsion for the selectiveremoval of Paraloid B72 coatings. Besides the control of thecleaning action, these responsive systems allow the minimizationof the mechanical action needed to remove them from thetreated substrate, as required in the cleaning of very preciousartifacts. In fact, these gels can be easily removed in the presenceof a magnetic field such as that of a simple permanentmagnet.43,45

Gels having a higher solvent retention capability are requiredwhen water-based cleaning fluids are used for the cleaning ofhydrophilic and highly water-sensitive works of art (manuscriptsand watercolor paintings). For these artifacts, properly designedgels were recently synthesized to prevent solvents from spreadingover the artifact surface and to release fluids at a slow rate so as toperform gradual, nonaggressive cleaning at the interface. Asystem with high control of water confinement can be obtainedwith 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 onthe balance between the mechanical strength, provided bypHEMA, and the hydrophilic properties, granted by PVP, toobtain a system that gradually releases the cleaning fluids whileavoiding gel residues on the treated surface. Moreover, these gelsare transparent, allowing the visual control of their cleaningaction, and can be realized as thin films (1 to 2 mm) and shapedas wished (Figure 8).46

Another important class of materials for the selective cleaningof synthetic coatings consists of high-viscosity polymericdispersions (HVPDs). These systems are not truly physicalgels according to their rheological behavior. It is well known thatpoly(vinyl alcohol) (PVA) forms HVPDs in the presence ofborate ions, which act as cross-linkers between polymer chains.The resulting 3D network is thermally reversible. PVA is

produced by the hydrolysis of poly(vinyl acetate), and dependingon its degree of hydrolysis, several HVPDs can be produced andloaded with polar solvents, enzymes, and amphiphilicsystems.37−39 The main feature of these systems relies on thepossibility of their removal by peeling without the use of neatsolvents, as would occur with solvent gels.In conclusion, by coupling highly retentive supporting gels

with nanostructured amphiphilic systems, a new palette ofcleaning tools has been offered to the conservator community.These new cleaning systems have tremendous advantages wheneasy, selective, and controlled removal of undesired layers (dirt,grime, polymers, etc.) is required.

Consolidation and Deacidification: Alkaline EarthMetal Hydroxide Nanoparticles. Alkaline earth metalhydroxides can be used for a twofold task in the field of culturalheritage conservation. As reported in the previous sections, wallpaintings and carbonatic stones can degrade for several reasons.A valid alternative to the use of polymers for consolidation isconstituted by the use of inorganic consolidants. Among these,nanoparticles of alkaline earth metal hydroxides can beconsidered to be the most reliable and durable systems. In wallpainting consolidation, nanoparticles replace the original pig-ment’s binder lost during the degradation process, reconsolidat-ing the painting in a fully compatible way. In addition to that,calcium and magnesium hydroxides have proven to be excellentcompounds 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-chainalcohols.47−55 The main rationale behind the research effortsconcerning the synthesis of nanostructured materials for bothconsolidation and deacidification purposes is the control of thesize and shape of the particles that eases the penetration into thesubstrate (i.e., wall painting or paper) and leads to enhancedreactivity associated with the high surface area of the nano-particles.The use of hydroxide particle dispersions in short-chain

alcohols presents some advantages over the application ofsaturated aqueous solutions such as limewater (a saturatedCa(OH)2 solution), a widely used inorganic method forconsolidation. Calcium hydroxide has a low solubility in water,and a large amount of limewater is usually necessary for thetreatment; however, a large amount of water in the wall paintingfor porous matrices can favor the collapse of the pores related tofreeze−thaw cycles, the transport of salts, and the growth ofmicroorganisms. On the contrary, nanoparticles dispersed inshort-chain alcohols exhibit good penetration within the

Figure 8. pHEMA/PVP thin gel film application over a model sample simulating the Tibetan painting technique of Thang-Ka (tempera magra), which isvery 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 loadedwith water.46



painting’s porous matrix at concentrations sufficient for acomplete consolidation process, making hydroxide nanoparticlesan optimal system for wall painting consolidation.The second use of nanoparticles in conservation is related to

deacidification. Cellulose-based artifacts are usually threatenedby the concomitant action of hydrolysis and oxidation that leadsto the loss of the mechanical resistance of the fibers and todiscoloration phenomena of the substrate. As in the case ofconsolidation of classical wall paintings, traditional deacidifica-tion methodologies involve the use of highly alkaline aqueoussolutions. The ideal deacidifying agents are alkaline earth metalhydroxides and carbonates, which are very compatible with thesubstrate and its components and at the same time readilyneutralize acidity. However, the application of alkaline aqueoussolutions implies two significant drawbacks: (i) several papercomponents, such as inks and sizing, are water-sensitive and (ii)the high alkalinity of aqueous solutions may favor the alkalinedepolymerization of cellulose, which takes place at roomtemperature on oxidized substrates.56 Moreover, the traditionaldeacidification treatments are usually performed throughimmersion, and this implies poor control of the amount ofapplied deacidifying agent.The use of nonaqueous methods overcomes these drawbacks;

the commercial nonaqueous treatments that are nowadaysavailable are all based on solutions or dispersions of oxides andcarbonates precursors.57 For instance, in the case of paperpreservation, magnesium and calcium hydroxide nanoparticles inpropanol produce safe and stable deacidification, leaving a mildalkaline carbonate buffer reserve against reoccurring acidity onthe treated document.10,50,52,54 Recently, alkaline earth metalhydroxide nanoparticles dispersed in alcohols have been shownto be an efficient deacidification system for paper and haveproven to be promising tools for the deacidification ofarcheological wood. The application of these systems towaterlogged wood deacidification has been investigated in thecontext of the preservation of the Swedish 17th century warshipVasa, whose timbers contain high quantities of sulfuric acid,developed after salvaging from the oxidation of sulfur-reducedcompounds formed from bacterial activity.53,58

In the next sections, the main nanoparticles’ synthesispathways developed for application in the field of culturalheritage conservation are briefly reviewed.

Calcium Hydroxide Nanoparticles. The preparation ofcalcium hydroxide nanoparticles has been achieved eitherthrough a breakdown process or by a bottom-up strategy. Fairlystable dispersions of calcium hydroxide in alcohol were obtainedby grinding slaked lime (a putty lime produced bymixing calciumoxide with excess water) with a mill. The breakdown processleads to broad submicrometer/micrometer particle size dis-tributions that were applied, for the first time, to theconsolidation of degraded frescoes in the cathedral of SantaMaria Novella, Florence, Italy.47 Recently, an improvedpreparation method based on the thermomechanical treatmentof slaked lime has been proposed, where the completion of lime’shydration is favored by high temperature and pressure.55 As amatter of fact, the unreacted calcium oxide core in lime particlesundergoes a strongly exothermic reaction causing thefragmentation of agglomerates that produces nanoparticleswith an average size of up to 300 nm, which are stably dispersedin propanol. A different approach involves the building up ofparticles (bottom-up procedure) in aqueous solutions48 or inorganic solvents.49 Control of the nucleation of particles overtheir growth is achieved by a high degree of supersaturation thatis strongly dependent on temperature and pressure.59 Despitethe fact that these methodologies produce nanoparticles with asmaller and narrower size distribution (ranging from 20 to 60 nmfor the synthesis in organic solvents and from 50 to 400 nm forthe synthesis in water), the required purification steps necessaryto eliminate the NaCl byproduct make these procedures time-consuming. Nonetheless, stable dispersions of calcium hydroxidenanoparticles in alcohol obtained from an aqueous homoge-neous phase reaction have been applied in a variety of casestudies involving both consolidation48 and deacidificationpurposes.50,53,58

A different approach to the synthesis of small calciumhydroxide nanoparticles involves the use of alkoxides as reactionintermediates; highly concentrated stable dispersions in alcohol,not requiring any purification step after the preparation, can bedirectly obtained by a high-pressure alcohol thermal reactionstarting from calcium and short-chain alcohols. Because of theirphysicochemical characteristics, these particles are particularlysuitable for application on porous artistic substrates (Figure 9).Moreover, the synthesis pathway can be potentially scaled up tothe industrial level. In contrast, traditional deacidificationmethods (i.e., calcium hydroxide solutions) have been shown

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



to produce uneven depositions on cellulose fibers with particleshaving diameters of several micrometers.60

Barium Hydroxide Nanoparticles. Barium hydroxide hasbeen used as a consolidant for carbonaceous materials since theend of the 19th century.61 Ferroni proposed the use of bariumhydroxide aqueous solutions for the consolidation of frescoesthat were heavily damaged in the Florence flood of 1966.62,63 Itsuse is recommended when large amounts of sulfates are presentin the wall painting matrix. Typically, a two-step procedure isconsidered: (1) desulfation with an ammonium carbonatesolution loaded in a cellulose pulp poultice and (2) theapplication of a Ba(OH)2 solution that fixes the residual solublesulfates into insoluble barium sulfate. Furthermore, the presenceof excess barium hydroxide converts the powdery calciumcarbonate (formed in step 1) into calcium hydroxide that reactswith CO2 to reform a crystalline network of calcium carbonatethat acts as a binder, resulting in the consolidating action.Besides, barium hydroxide can be used together with calcium

hydroxide nanoparticle dispersions for the consolidation ofheavily degraded and sulfated wall paintings. In fact, in this case,Ca(OH)2 nanoparticles alone are not very efficient because theconsolidating action of calcium hydroxide is hindered by itspartial transformation to the more stable calcium sulfate. In thiscase, the most simple and elegant solution is the application ofmixed calcium and barium hydroxide nanoparticle dispersions inalcohol.The synthesis of Ba(OH)2 nanoparticles from the aqueous

homogeneous phase reaction is hindered by the low degree ofsupersaturation that can be achieved in water as a result of bariumhydroxide fair solubility (Ksp = 2.55 × 10−4). Instead, a

heterogeneous approach based on the breakdown of Ba(OH)2macrocrystals has been recently developed.55 Typically, themilling of commercial barium hydroxide in 1-propanol leads tofairly stable dispersions in alcohol for nanoparticles whose sizeranges from 200 to 400 nm (Figure 10).Recently, mixed calcium and barium hydroxide nanoparticle

dispersions have proven to be highly effective in theconsolidation of mural paintings heavily contaminated by salts(mainly sulfates and chlorides) in theMesoamerican area (Figure11).55,64 An additional advantage of the use of nanoparticledispersions of Ba(OH)2 in nonaqueous solvents is representedby the very low toxicity as compared to that of aqueous solutionsof barium salts.

Magnesium Hydroxide Nanoparticles. Magnesium hydrox-ide microparticles are employed in many industrial applicationsas flame retardants and oxide precursors.65 In the past 10 years,several “classical” synthesis pathways were modified and tailoredto produce magnesium hydroxide nanoparticles for theconservation of cultural heritage. In particular, Mg(OH)2nanoparticles, obtained via aqueous homogeneous phasereactions and dispersed in alcohol, have been used for thedeacidification of paper, canvas, and wood.52,54,58,66,67 It wasreported that the particle size can be tuned from 50 to 300 nm bychanging the counterions’ nature52 and their concentration in thechemical reaction.54 Despite the fact that these particles areusually applied for deacidification purposes, they have beenrecently considered for the consolidation of dolomite stones(calcium−magnesium carbonate stones).64

By analogy to calcium hydroxide, magnesium hydroxidenanoparticles can also be obtained from an alcohol thermal

Figure 10. (A) Size distribution of Ca(OH)2 nanoparticles in 2-propanol obtained through high-temperature milling. (B) Size distribution ofMg(OH)2nanoparticles in 2-propanol obtained through a homogeneous phase reaction with excess magnesium ions. (C) Size distribution of Ba(OH)2nanoparticles 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) Thesame surface after the desulfation treatment with ammonium carbonate and the application of a mixed calcium and barium hydroxide nanoparticledispersion.



reaction starting from bulk metal. The conversion of amicrometer-sized alkoxide intermediate (Figure 12) into nano-metric magnesium hydroxide particles is favored by high pressureand temperature, leading to stable concentrated dispersions inshort-chained alcohols.Currently, magnesium hydroxide nanoparticles are mainly

used for the deacidification of paper and iron gall-inkedmanuscripts,54,67 one of the more important open problems inthe field of paper conservation.68,69 As can be seen in Figure 13,

the synergistic effect of acidity and cellulose oxidation mainly as aresult of the free metal ions introduced into the substrate with theapplication 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 enhancedby acidity: this process has a high rate at pH values below 4.5whereas the minimum catalytic activity of metal ions (iron andcopper) is in the 6.5−7.5 pH range.70 According to Strlic et al.70and Baty et al.,57 the ideal requirement for a conservationtreatment of manuscripts is to stabilize the paper pH aroundneutrality to hinder both acid-catalyzed hydrolysis and metal-catalyzed oxidation. The application of alkaline earth metalhydroxide nanoparticles dispersed in nonaqueous solventsprovides this feature, inhibiting the two different degradationmechanisms through a single, simple, safe treatment thatgradually takes to a neutral pH, which results in a significantincrease in the inked-paper resistance to weathering. Thenonaqueous treatment54,67 prevented the leaching of water-sensitive writing fluids and allowed an efficient distribution of thedeacidifying nanoparticles in the substrate. Moreover, the

reaction of nanoparticles with CO2 produces a carbonate bufferto protect cellulose from aging.The protective action of alkaline earth metal hydroxide

nanoparticles against ink corrosion was evaluated on papermodel samples featuring iron gall writing fluids. The degradationof these systems was monitored before and after acceleratedhydrothermal aging (T = 90 °C, RH = 75%) by measuring thecellulose degree of polymerization (via viscosimetric determi-nations) and pH.54,67 In Figure 14, the comparison between the

unprotected paper sample (i.e., not deacidified previous to agingand having a pH of 3) and the sample neutralized withmagnesium hydroxide nanoparticles clearly shows how nano-particle application protected the paper from ink corrosion. After48 h of artificial aging, the unprotected sample exhibited severedamage (e.g., brittleness) and could not be manipulated, whereassample 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 metal-catalyzed oxidation and the acid hydrolysis of paper, ensuring thelong-term preservation of iron gall-inked paper.

■ CONCLUSIONS AND OUTLOOKIn the past 15 years, colloids and material science generated anumber of innovative, functional, and fairly inexpensive tools forthe conservation of movable and immovable cultural heritage.The use of materials compatible with those constituting theworks or art leads to the long-term stability of the treated artifactwhose useful life can be extended with strong societal andeconomical benefits. Research in science of cultural heritage isstill far from being mature. The main possible future scenariosinclude (a) new “green” surfactant-based self-assembled systems,(b) water-in-oil, oil-in-water, and waterless cleaning micro-

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 hydroxideon {111} crystal planes during the hydration process of magnesiummethoxide. On the right, magnesium hydroxide nanoparticles of about 100 nm afterthe hydration reaction.

Figure 13. Corrosion of paper due to the presence of iron gall inks on ahistorical manuscript (adapted from ref 54).

Figure 14. Inked paper samples after artificial aging: (A) an unprotectedsample and (B) a sample treated with nanoparticles to protect it fromaging. Replica samples were made by the application of iron gall ink on99.9% cellulose unsized Whatman paper (adapted from ref 54).



emulsions/emulsions for the treatment of highly sensitivehydrophilic substrates (e.g., acrylic paintings), (c) organogels,as a complement to the already popular hydrogels, to be used assupport systems for the above-mentioned fluids, (d) gels that areresponsive to an external stimulus, (e) nanoparticle dispersionsin apolar solvents for the deacidification of water-sensitivesubstrates (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 andchemical reactivity of the nanomaterials play a leading role.However, the preservation and valorization of the culturalheritage legacy lead to the outcome of consistent economicalresources (i.e., tourism) and, from a different point of view,improve the image and perception of science and, in particular,chemistry.

■ AUTHOR INFORMATION

Corresponding Author*Tel: +39 0554573033. Fax: +39 0554573033. E-mail:[email protected].

NotesThe authors declare no competing financial interest.

Biographies

Michele Baglioni, photographer

Left to right: Giovanna Poggi, Rodorico Giorgi, David Chelazzi, andPiero Baglioni

Piero Baglioni has been the Chair of Physical Chemistry in theDepartment of Chemistry at the University of Florence since 1994 and isan MIT affiliate. He was appointed as visiting scientist/professor by theDepartment of Chemistry of the University of Houston, the WeizmannInstitute, the College de France, and MIT. He is the Director of theNational Center for Colloids and Nanosciences (CSGI), and he is onthe advisory boards of several international journals and amember of thescientific board of several national and international institutions andsocieties. He is the author of more than 350 publications in books andlargely diffused international journals. He is also the author of 21patents. In the field of conservation, he is a pioneer in the application ofcolloids and soft matter to the conservation of cultural heritage. He hasproduced several innovative methods for the consolidation and cleaningof paintings and the deacidification of historical documents.

David Chelazzi, Ph.D. in science for cultural heritage conservation at theUniversity of Florence in 2007 and Master’s in chemistry in 2003, iscurrently working as a postdoctoral fellow in the department ofchemistry at the University of Florence and CSGI. His main researchinterests are the development of methodologies for the consolidation,cleaning, and pH control of works of art such as wall and canvaspaintings, stone, paper, and archaeological wood. He is the author or co-

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

Rodorico Giorgi, Ph.D. in science for cultural heritage conservation atthe University of Florence and B.S. in chemistry, is currently a researchfellow in the department of chemistry at CSGI, University of Florence.Giorgi’s background is in colloids science. His main research interestsare the development of methodologies for the conservation of culturalheritage materials such as wall and easel paintings, stone, paper, andarchaeological wood. Giorgi is the author of about 80 publications in thefield of science for conservation.

Giovanna Poggi holds a Ph.D. in science for cultural heritageconservation from the University of Florence and obtained a Master’sin technology for the conservation of cultural heritage in 2007. She iscurrently working as a postdoctoral fellow at CSGI in the developmentand characterization of nanotechnology for conservation. Her researchmainly deals with the synthesis and characterization of nanoparticles andtheir application to paper and wood deacidification.

■ ACKNOWLEDGMENTS

We thank all of the conservators involved in the application of theconservation methodologies presented here to real case studiesfor their help and, in particular, Tiziana Dell’Omo and Lucia DiPaolo for the intervention in Nazareth (Israel) and FlorenceGorel for the experiments on Thang-Ka paintings. Moreover,Ramon Carrasco Vargas (Proyecto Arqueologico Calakmul,Mexico), Lilia Rivero Weber, Diana Medellin, Yareli JaidarBenavides, and Maria del Carmen Castro Barrera (CoordinacionNacional de Conservacion del Patrimonio Cultural, CNCPC,Mexico), and Diana Magaloni (UNAM and Museo National deAntropologia, Mexico) are acknowledged for the Mesoamericanarchaeological areas. CSGI, MIUR, and European Union(project NANOFORART, FP7-ENV-NMP-2011/282816) areacknowledged for financial support.

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