Francesca Gherardi

Highly transparent poly(2-ethyl-2-oxazoline)-TiO2 nanocomposite coatings for the conservation of matte painted artworks

A nanocomposite coating based on TiO2 nanoparticles and poly(2-ethyl-2-oxazoline) is used as consolidant of matte painted surfaces (temperas, watercolors, modern paintings). The aim of this work is to provide advances in the conservation of these works of art, while preserving their optical appearance, in terms of colour and gloss. Fiber Optic Reflectance Spectroscopy (FORS) measurements of a painting-model (an acrylic black monochrome) treated with the nanocomposite coatings revealed that it is possible to match the optical appearance of the painted surface by tuning the amount of nanoparticles in the polymeric matrix. The requirement of retreatability of the material has been verified by removing the nanocomposite cast on the painted surface with aqueous solutions. FTIR and SEM/EDX measurements showed that almost no traces of the nanocomposite remained on the painted surface, allowing its use for the treatment of real paintings. Test were performed using a contemporary studio-model on canvas attributed to Agostino Bonalumi (1935–2013).

Setup of a sustainable indoor cleaning methodology for the sculpted stone surfaces of the Duomo of Milan

Cleaning is a fundamental phase of the conservation and maintenance activity of the cultural heritage. It is required to be highly effective in the removal of undesired deposits, controllable at every stage and gradable, selective, and completely respectful of the substrate. Moreover, cleaning procedures which can also assure to be harmless to the environment and the operators are particularly valued nowadays. According to these general guidelines, in the present work a sustainable approach for the cleaning of indoor surfaces of the cultural heritage is presented. A methodology based on agar gel was setup and preliminary tested on pilot areas. It has been then applied to the stone sculpted surfaces of the “Fuga in Egitto” high-relief of the Duomo of Milan.A multi-analytical approach was carried out during the setup phase to compare the efficacy of different cleaning conditions in terms of agar concentration, application time and presence of additives. Hence, spectrophotometric measurements, optical observations, ESEM-EDX analyses, Fourier Transform Infrared Spectroscopy, and X-Ray Diffraction were performed. Specific attention was paid to the aesthetic features of the sculpted surfaces before and after the cleaning. For this purpose, spectrophotometric data have been analysed by multivariate analysis techniques such as Principal Component Analysis and Hierarchic Cluster Analysis. The overall intervention has been monitored to evaluate the cleaning results and to confirm the absence of any damage to the stone substrate.The application of agar gel proved to be effective in removing soluble salts and soot particles, as well as very respectful of the valuable sculpted surfaces. This methodology is also totally safe for both the operators and the visitors, it is based on a natural and low-cost raw material, and it is low time-consuming. It can be therefore considered as a sustainable alternative to the traditional procedures.

Photocatalytic Nanocomposites for the Protection of European Architectural Heritage

In the field of stone protection, the introduction of inorganic nanoparticles, such as TiO2, ZnO, and Ag in polymeric blends can enhance the protective action of pristine treatments, as well as confer additional properties (photocatalytic, antifouling, and antibacterial). In the framework of the “Nano-Cathedral” European project, nanostructured photocatalytic protective treatments were formulated by using different TiO2 nanoparticles, solvents, and silane/siloxane systems in the blends. The results about the characterization and application of two promising nano-TiO2 based products applied on Apuan marble and Ajarte limestone are here reported, aiming at investigating the complex system “treatment/stone-substrate”. The nanocomposites show better performances when compared to a commercial reference siloxane based protective treatment, resulting in different performances once applied on different carbonatic substrates, with very low and high open porosity, confirming the necessity of correlating precisely the characteristics of the stone material to those of the protective formulations. In particular, the TiO2 photocatalytic behavior is strictly linked to the amount of available nanoparticles and to the active surface area. The alkyl silane oligomers of the water-based formulation have a good penetration into the microstructure of Ajarte limestone, whereas the solvent-based and small size monomeric formulation shows better results for Apuan marble, granting a good coverage of the pores. The encouraging results obtained so far in lab will be confirmed by monitoring tests aiming at assessing the effectiveness of the treatments applied in pilot sites of historical Gothic Cathedrals.

A Photoluminescence Study of the Changes Induced in the Zinc White Pigment by Formation of Zinc Complexes

It is known that oil paintings containing zinc white are subject to rapid degradation. This is caused by the interaction between the active groups of binder and the metal ions of the pigment, which gives rise to the formation of new zinc complexes (metal soaps). Ongoing studies on zinc white paints have been limited to the chemical mechanisms that lead to the formation of zinc complexes. On the contrary, little is known of the photo-physical changes induced in the zinc oxide crystal structure following this interaction. Time-resolved photoluminescence spectroscopy has been applied to follow modifications in the luminescent zinc white pigment when mixed with binder. Significant changes in trap state photoluminescence emissions have been detected: the enhancement of a blue emission combined with a change of the decay kinetic of the well-known green emission. Complementary data from molecular analysis of paints using Fourier transform infrared spectroscopy confirms the formation of zinc carboxylates and corroborates the mechanism for zinc complexes formation. We support the hypothesis that zinc ions migrate into binder creating novel vacancies, affecting the photoluminescence intensity and lifetime properties of zinc oxide. Here, we further demonstrate the advantages of a time-resolved photoluminescence approach for studying defects in semiconductor pigments.

The Protection of Marble Surfaces: The Challenge to Develop Suitable Nanostructured Treatments

Marbles have been extensively used in historical architecture owing to their good mineralogical and microstructural properties, durability, and aesthetic quality. Nevertheless, the protection of historical marbles in outdoor conditions is a difficult task, mainly because of their low open porosity. An overview of nanostructured protective treatments based on the use of SiO2, TiO2, ZnO, and Ag nanoparticles to confer superhydrophobic, self-cleaning, and antifouling properties to the surface is proposed. Particular attention is devoted to the development of photocatalytic nano-TiO2-based treatments. In this regard, advantages, drawbacks, and critical issues in the use of nanocomposites are covered. Recent advances using modified innovative TiO2 nanoparticles, in dispersion and as nanocomposites, are reported. Nanocomposites based on suitable TiO2 nanoparticles seem very promising, and a comparison of the results obtained in controlled lab conditions and on real deteriorated surfaces is also presented.

Use of nanocoatings for the restoration of matte paintings

Nanotechnology has been used with promising results for the conservation of cultural heritage. Nanotechnology completely modifies some properties of pure polymers, such as their gas permeability and mechanical or optical properties, by the introduction of apposite nanoparticles into the organic matrix to yield novel and innovative nanocomposite materials. For example, optical properties — such as ultraviolet (UV) and infrared absorption, light diffusion, and refractive index — can be improved and modulated by dispersing suitable nanoparticles in existing polymers, while preserving the transparency and flexibility of the material (Colombo et al., 2010). The present study reports on novel nanocoatings that are able to mimic the diffuse surface properties of matte painted surfaces. These have been obtained by exploiting the bulk scattering phenomenon created by the presence of nanoparticles with tailored optical properties (high refractive index and optical transparency in the visible range), suitably dispersed in a polymeric matrix. Three different coatings based on titanium dioxide (TiO2) nanoparticles dispersed in poly(2-ethyl-2-oxazoline) (Aquazol 200, CTS s.r.l.) were prepared, producing transparent nanocomposites that are notable for their tuneable optical properties (Colombo et al., 2012). In particular, the films showed ultraviolet (UV) filtering up to the edge of the visible range and tailored refractive indices, up to about 1.67, controlled by the increasing concentration of nanoparticles in the polymeric matrix. Moreover, although the nanocomposite films were optically transparent, they showed a loss in transmittance attributed to Rayleigh scattering, due to the presence of nanoparticles clusters in the matrix. As it was possible to tune the amount of diffused light by changing the nanoparticle concentration in the polymer, a novel application of this material has been found for the treatment of matte artworks (Colombo et al., 2015). The optical performance of this material has been tested on a black acrylic monochrome painting, in order to simulate its application in the conservation of artworks. Fibre-optic reflectance spectroscopy (FORS) was used to measure the optical appearance of replica paintings after treatment with the nanocoatings, revealing that it is possible to match the reflective properties of the pristine painted surface by tailoring the number of nanoparticles in the polymeric matrix to produce the appropriate bulk scattering (Fig. 1). The requirements of stability and reversibility of this material once cast onto painting models have been checked after two years of natural aging. Fourier transform infrared (FTIR) measurements showed a slight aging of the polymer revealing that, in natural indoor conditions, TiO2 nanoparticles do not cause extensive degradation. This has been demonstrated for the naturally aged samples by dissolving the material with aqueous solutions of a non-ionic surfactant (TWEEN 20, CTS s.r.l.) and a weak chelating agent (tribasic ammonium citrate, CTS s.r.l). The removal efficacy was ascertained using FTIR and scanning electron microscopy with energy dispersive X-ray analysis (SEM–EDX) to study micro-samples collected from the painted surfaces, showing that almost complete removal of both the polymer and TiO2 nanoparticles is possible. The encouraging performance of the material has been exploited in the restoration of a white, inverted monochrome sculpture, created in the 1960s and attributed to Agostino Bonalumi (1935–2013). This artwork was realized using a canvas made of cotton fibres and phthalate polyvinylchloride (PVC), assembled on a wooden frame measuring 35 × 35 × 11.5 cm. The coloured layer is characterized by the presence of titanium white as the pigment, kaolin as an extender and vinyl glue as the binder. The restoration process began with surface cleaning (Cremonesi & Signorini, 2004) and repair of an L-shaped tear (Borgioli & Cremonesi, 2005). Finally, the painting was retouched using Gamblin colours, particularly titanium white, Correspondence to: Carlotta Beccaria, Conservation studio Carlotta Beccaria, Via Conservatorio 30, 20122 Milan, Italy. Email: beccaria.restauro@gmail.com

Electrokinetic Characterization of Natural Stones Coated with Nanocomposites for the Protection of Cultural Heritage

Protective coatings, in recent years also from nanocomposite formulations, are commonly applied onto architectural stone and stone artefacts, mainly to prevent absorption into the porous stone structure of condensed water and dissolved atmospheric pollutants. While standard protocols are available to assess a coating’s performance, understanding the response of the coating-stone system is a complex task, due to the interplay of various factors determining the overall behaviour. Characterization techniques allowing to correlate the extent and nature of surface modification upon treatment with the most relevant physical properties (i.e water absorption and surface wettability) are thus of great interest. Electrokinetic analysis based on streaming current measurements, thanks to its sensitivity towards even minor changes in the surface chemical composition, may fulfil such requirement. Indeed, by involving the interaction with a testing aqueous electrolyte solution, this technique allows to probe not only the outer surface but also the outermost layer of the pore network, which plays a crucial role in the interaction of the stone with condensed atmospheric water. In this work a correlation was found between the extent of surface modification, as determined by streaming current measurements, surface wettability and capillary water absorption of 6 coating-lithotype combinations (3 lithotypes and 2 nanocomposites).