Nicole Bonelli

Innovative Hydrogels Based on Semi-Interpenetrating p(HEMA)/PVP Networks for the Cleaning of Water-Sensitive Cultural Heritage Artifacts

Water-based detergent systems offer several advantages, over organic solvents, for the cleaning of cultural heritage artifacts in terms of selectivity and gentle removal of grime materials or aged varnish, which are known to alter the readability of the painting. Unfortunately, easel paintings present specific characteristics that make the usage of water-based systems invasive. The interaction of water with wood or canvas support favors mechanical stresses between the substrate and the paint layers leading to the detachment of the pictorial layer. In order to avoid painting loss and to ensure a fine control (layer by layer) of grime removal, water-based cleaning systems have been confined into innovative chemical hydrogels, specifically designed for cleaning water-sensitive cultural heritage artifacts. The synthesized hydrogels are based on semi-interpenetrating chemical poly(2-hydroxyethyl methacrylate)/poly(vinylpyrrolidone) networks with suitable hydrophilicity, water retention properties, and required mechanical strength to avoid residues after the cleaning treatment. Three different compositions were selected. Water retention and release properties have been studied by quantifying the amount of free and bound water (from differential scanning calorimetry); mesoporosity was obtained from scanning electron microscopy; microstructure from small angle X-ray scattering. To demonstrate both the efficiency and versatility of the selected hydrogels in confining and modulating the properties of cleaning systems, a representative case study is presented.

Organogels for the cleaning of artifacts

Abstract The cleaning of artifacts must not alter the original properties of the objects. While the use of free solvents is risky, their confinement into polymeric networks can allow the safe removal of unwanted layers from artifacts. Recently, a methyl 2-methylprop-2-enoate (MMA)-based organogel was formulated as loaded with butan-2-one (MEK), and used to remove aged varnishes from canvas paintings. However, this formulation is not enough retentive to allow its use on paper, where higher retentiveness is needed to avoid the uncontrolled spreading of MEK and dissolved materials. Here, a new PMMA-MEK gel was designed to overcome this limitation. The amount of cross-linker and monomer used in the synthesis of the gel were tuned to achieve optimal retentiveness. Differential scanning calorimetry (DSC), differential thermogravimetry (DTG), small-angle X-ray scattering (SAXS) and attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) provided information on the solvent content, release rate, and mesoporosity of the gel as compared to the previous system. The lower solvent release rate of the new formulation allowed the safe removal of wax that jeopardized a 19th century paper document. The removal was confirmed through optical microscopy and ATR-FTIR, which also highlighted the absence of gel residues on the treated surface.

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.

Confined Aqueous Media for the Cleaning of Cultural Heritage: Innovative Gels and Amphiphile-Based Nanofluids

This chapter presents the applicative potentialities of gels for the cleaning of artworks surfaces. In particular, innovative physical and chemical gels, with high water retention capability, high responsiveness to external stimuli, and suitable mechanical properties, are described. The high solvent retention capability and the specific mechanical properties of these gels allow the safe cleaning of artifacts, even including water-sensitive substrates. In fact, the cleaning action is limited to the contact surface, and the complete removal of soil is achieved while avoiding solvent spreading and absorption within the substrate. In particular, the use of gels based on semi-interpenetrating (IPN) polymer networks provides great advantages because these gels are able to load water-based detergent systems, such as micellar solutions and microemulsions, which are effective in removing synthetic adhesives and highly hydrophobic detrimental materials. The combination of semi-IPN polymer networks with these detergents allows the cleaning of sensitive substrates such as canvas paintings and manuscripts.

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.

Film forming PVA-based cleaning systems for the removal of corrosion products from historical bronzes

Abstract This paper presents an innovative poly(vinyl)alcohol-based film forming system, specifically devised for the controllable and selective cleaning of copper-based artifacts. Traditional cleaning procedures are commonly performed using mechanical and/or chemical methods. Unfortunately, both these methods present some limitations related to both the poor selectivity and invasiveness in case of the mechanical procedure, and to the scarce control over the involved reactions when dealing with a chemical approach. The innovative system proposed in this work allows combining the advantages of chemical and mechanical treatments thanks to the confinement of a complexing agent (EDTA) within a fluid, polymeric matrix, that is able to form a solid thin film upon drying. After treatment, the polymeric film can be completely removed from the artwork through a gentle peeling action. In this contribution, the film formation mechanism was investigated by means of thermal analysis and rheology; the role of plasticizers, volatile solvent fraction, and quantity of loaded EDTA is also discussed. Finally, the results of cleaning tests performed on artificially aged samples, and on a real case study, the “Fontana dei Mostri Marini” by Pietro Tacca in Florence, are presented.

Restoration of paper artworks with microemulsions confined in hydrogels for safe and efficient removal of adhesive tapes

Significance From Dead Sea Scrolls to Federico Fellini and Lucio Fontana drawings, pressure-sensitive tapes (PSTs) have been used as adhesive fasteners or as part of temporary conservation treatments that frequently became permanent. Their safe and efficient removal poses ethical and aesthetic questions: Adhesive tape residues damage the paper substrate and, due to discoloring, prejudice the artwork enjoyment and conservation. Selective removal without affecting the underlying support is challenging and often impossible. We tackled this issue from a physicochemical and a colloidal perspective, by designing a system where nanosized solvent droplets are confined within a hydrogel. This method has the potential to revolutionize the approaches used so far in the removal of PSTs and coatings from a plurality of materials. The presence of pressure-sensitive tapes (PSTs) on paper artworks, either fortuitous or specifically applied for conservation purposes, is one of the most frequent and difficult issues encountered during restoration. Aged PSTs can damage or disfigure artworks, compromising structural integrity, readability, and enjoyment. Current procedures are often inherently hazardous for artistic media and paper support. Challenged by the necessity to remove PSTs from a contemporary and an ancient drawing (20th century, by artists da Silva and Hayter, and a 16th-century drawing of one figure from the Sistine Chapel by Michelangelo), we addressed this issue from a physicochemical perspective, leveraging colloid and interface science. After a characterization of the specific PSTs present on the artifact, we selected a highly water-retentive hydrogel as the host of 23% wt/wt of “green” organic solvents uniformly dispersed within the gel in the form of nanosized droplets. The double confinement of the organic solvent in the nanodroplets and into the gel network promotes a tailored, controlled removal of PSTs of different natures, with virtually no interaction with the solvent-sensitive artwork. This noninvasive procedure allows complete retrieval of artwork readability. For instance, in the ancient drawing, the PST totally concealed the inscription, “di mano di Michelangelo” (“from Michelangelo’s hand”), a possibly false attribution hidden by a collector, which is now perfectly visible and whose origin is currently under investigation. Remarkably, the same methodology was successful for the removal of aged PST adhesive penetrated inside paper fibers of a drawing from the celebrated artist Lucio Fontana.