Giovanna Poggi

Hydroxide nanoparticles for cultural heritage: Consolidation and protection of wall paintings and carbonate materials

Colloids and Material Science are nowadays providing innovative and effective technological solutions in a wide range of applicative fields. In the last decade, nanomaterials have been specifically designed to ensure the long-term restoration and preservation of movable and immovable artworks. The main tasks to address by conservation scientists concern the cleaning, the deacidification and the consolidation of different kinds of artistic substrates. The aim of the present contribution is to provide an up-to-date overview on the synthesis and preparation of colloidal systems tailored to the consolidation and protection of wall paintings, plasters and stones, highlighting the most recent improvements. Two case studies, widely representative of typical consolidation problems, are presented, i.e. the preservation of wall paintings belonging to a Mesoamerican archeological site and the consolidation of two Italian Renaissance buildings.

Colloid and materials science for the conservation of cultural heritage: cleaning, consolidation, and deacidification.

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.

Hydroxide Nanoparticles for Deacidification and Concomitant Inhibition of Iron-Gall Ink Corrosion of Paper

This Article reports an investigation on the use of magnesium hydroxide nanoparticles dispersed in alcohols to inhibit two different and synergistic degradation processes usually affecting historically valuable manuscripts and, more generally, paper documents. We show that the preservation of paper from acid hydrolysis and oxidative ink corrosion can be achieved by stabilizing the final pH of deacidified paper around 6.5 to 7.5. Reactive magnesium hydroxide nanoparticles with a narrow size distribution, obtained by using a novel synthetic procedure, are very efficient in controlling papers pH to avoid further degradation of cellulose from acid hydrolysis, oxidative ink corrosion, or both. The deacidification and antioxidant actions of magnesium hydroxide nanoparticles are compared with magnesium oxide particles present in one of the best mass deacidification methods (Bookkeeper).

Interactions between nanostructured calcium hydroxide and acrylate copolymers: Implications in cultural heritage conservation

The interactions between an acrylic copolymer, poly ethylmethacrylate/methylacrylate (70:30) (Poly(EMA/MA), and Ca(OH)2 nanoparticles were investigated in order to establish the reciprocal influence of these two compounds on their peculiar properties. The carbonation kinetics of Ca(OH)2 nanoparticles by atmospheric CO2 was investigated by FTIR and SEM measurements and compared to that of a nanocomposite film. CaCO3 formation occurred even in the presence of the copolymer, but only after an induction period of ca. 200 h and with a lower reaction rate. Some implications in cultural heritage conservation dealing with application of nanolime on artifacts previously treated with acrylic copolymers were discussed. Contact angle measurements, mechanical cohesion properties, and water vapor permeability allowed us to conclude that the optimum behavior of nanolime with respect to transpiration was not compromised by the presence of the copolymer, and the behavior in terms of mechanical properties recovery by the application of Ca(OH)2 nanoparticles remained excellent even in the presence of poly(EMA/MA).

Calcium hydroxide nanoparticles from solvothermal reaction for the deacidification of degraded waterlogged wood

HYPOTHESIS A combination of acid and iron ions inside the wood has been corroding the cellulose matrix of the Swedish warship Vasa, imposing its deacidification. Past deacidification treatments displayed poor penetration inside the wood matrix with limited efficacy. A vacuum assisted treatment of wood using newly developed calcium hydroxide nanoparticle dispersions represents a possible candidate for the treatment of acidic waterlogged wood objects such as sculptures and decorative artifacts. EXPERIMENTS A solvothermal process was used for the synthesis of calcium hydroxide nanoparticle dispersions. Before the application on waterlogged wood, the physico-chemical characterization of these systems was carried out using several techniques. The efficacy of the deacidification treatment of wood samples from the Vasa was assessed by determination of pH and Differential Thermal Gravimetric (DTG) measurements. FINDINGS The proposed solvothermal reactions can be used to produce stable and highly concentrated calcium hydroxide nanoparticle dispersions in alcohols, needing no further purification before the application. This process has also the advantage to be upscalable to industrial level. Both pH and DTG measurements showed that the newly developed dispersions can homogenously penetrate inside the wood up to 20cm, neutralizing acidity and creating an alkaline buffer inside the wooden matrix, to hinder the degradation of residual cellulose.

Innovative chemical gels meet enzymes: A smart combination for cleaning paper artworks

HYPOTHESIS Due to their highly retentive properties, innovative recently developed, semi-interpenetrated hydrogels made up of poly(vinyl pyrrolidone) (PVP) chains embedded in a poly(2-hydroxyethyl methacrylate) (p(HEMA)) network should be efficiently used as cleaning material for fragile and degraded paper artworks. In restoration practice, indeed the wet cleaning of these artworks is usually performed by immersion of paper in water, a procedure which may lead to several drawbacks, including paper fibers swelling and dissolution of water-soluble original components. EXPERIMENTS This class of gels were yet presented in literature, but their interactions with paper materials and ability to be spiked with active enzymes (as cleaning agents), have not been analyzed. To establish the suitability of these hydrogels as paper cleaning materials, first, a rheological and microstructural characterization of the gels was performed. Moreover, diffusion of macromolecules inside gels was studied using fluorescence microscopy, to check if these innovative hydrogels can be used as carriers for hydrolytic enzymes. Indeed, pastes and glues are usually found in old paper artworks, and their removal is a very delicate operation that requires a selective action, which is granted by specific hydrolytic enzymes. At the same time, spectroscopic analyses on paper samples under investigation before and after cleaning treatment has been performed, thus assessing the capabilty of these gels as cleaning materials. FINDINGS With the aim of demonstrating the versatility of these hydrogels, several case studies, i.e., the removal of grime and water-soluble cellulose degradation byproducts, the removal of animal glue and the removal of starch paste from real samples, are presented. Results obtained with these gels have been compared to those obtained by using another gel used for paper artworks cleaning, i.e., Gellan gel.

Alkaline Nanoparticles for the Deacidification and pH Control of Books and Manuscripts

Manuscripts and books are susceptible to fast degradation owing to the presence of detrimental components used in the papermaking techniques, and to the action of environmental pollutants. As a result, the acidity of documents increases, promoting the acid-catalyzed depolymerization of cellulose. The latter process strongly reduces the mechanical properties of paper, reducing its long-term resistance to natural aging. The presence of inks concurs to degradation, making the conservation of manuscripts particularly demanding. In this chapter, the use of dispersions of alkaline earth metal hydroxide nanoparticles will be discussed as a method for counteracting the degradation of paper. These systems have proven efficient for the deacidification of cellulose-based artifacts, providing a mild alkaline buffer and maintaining a stable neutral environment. The palette of formulations nowadays available to conservators includes systems designed for the treatment of manuscripts featuring metal gall inks and modern industrial inks.

Poly(vinyl alcohol)/poly(vinyl pyrrolidone) hydrogels for the cleaning of art

The cleaning of modern and contemporary paintings is a delicate and challenging operation. Many contemporary paintings exhibit rough, clotted and pitted surfaces, where the removal of soil is difficult. Gels are among the most efficient tools to achieve controlled and efficient cleaning of works of art. However, most gels used in the conservation practice are too rigid to adapt rough surfaces, or too mechanically weak to be removed without leaving polymer residues. Several formulations of physically cross-linked poly(vinyl alcohol) (PVA)-based hydrogels, obtained by cast-drying or freeze-thawing of aqueous polymeric solutions, were formulated and characterized. The viscoelastic properties, porosity, and crystallinity of the gels were studied, along with the behavior of water inside the polymeric network. It was shown that the properties of the gels were improved through blending with poly(vinyl pyrrolidone) (PVP). The most promising gel formulation, in terms of mechanical properties and water retentiveness, was assessed for the removal of soil from an alkyd painting mock-up. A traditional gel, gellan gum, was also tested as a reference system. The effectiveness of soil removal was investigated by 2D Fourier transform infrared (FTIR) microscopy, using a Focal Plane Array (FPA) detector. In conclusion, it was shown that the newly developed formulation grants the residue-free removal of soil from rough and irregular artistic surfaces, overcoming the limits of traditional cleaning methods.

A Triton X-100-Based Microemulsion for the Removal of Hydrophobic Materials from Works of Art: SAXS Characterization and Application

The removal of hydrophobic materials from a porous support, such as wax stains on wall paintings, is particularly challenging. In this context, traditional methods display several drawbacks. The limitations of these methods can be overcome by amphiphile-based aqueous nanostructured fluids, such as micellar solutions and microemulsions. In this study, a microemulsion for the removal of wax spots from artistic surfaces was formulated. The nanostructured fluid includes a non-ionic surfactant, i.e., Triton X-100, and two apolar solvents, namely p-xylene and n-nonane. The solvents were selected on the basis of solubility tests of three waxes in several organic solvents. The nanostructured fluid was characterized by means of small-angle X-rays scattering (SAXS) and the information about micelle structure was used to understand the interaction between the microemulsion and the selected waxes. The microemulsion was then tested during the restoration of the frescoes in the Major Chapel of the Santa Croce Basilica in Florence, Italy. After some preliminary tests on fresco mockups reproduced in the laboratory, the nanostructured fluid was successfully used to clean some wax deposits from the real paintings, hardly removable with traditional physico-mechanical methods.

Structural, rheological and dynamics insights of hydroxypropyl guar gel-like systems

A dynamic, rheological, and structural characterization of aqueous gel-like systems containing hydroxypropyl guar gum (HPG), borax and glycerol is presented in this paper. The role of glycerol, which is introduced as a plasticizer in the formulation, is investigated by means of 11B NMR and 1H NMR PGSTE measurements in order to clarify its contribution to the gel network formation and its interaction with borax, with whom it forms a complex. The effect of gels components on the rheological behaviour and on the activation energy related to the relaxation process of the system was assessed by means of rheology. The results obtained suggest that the mechanical properties of these gels can be tuned and controlled by modulating the formulation in a wide range of compositions. Moreover, a structural characterisation has been also carried out by means of Small Angle X-ray Scattering (SAXS) to highlight the role of the various components on the mesh size of the network. The structural and mechanical characteristics of these systems suggest their potential use for applicative purposes. In this regard, one of the gel set up has been successfully tested as cleaning agent on the surface of a XIX stucco fragment coming from the La Fenice theatre (Venice, Italy) for the removal of a dirt layer composed by dust and particulated matter originated during a fire in 1996.