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 Cathedral

On-site monitoring of the performance of innovative treatments for marble conservation in architectural heritage

Innovative nanostructured treatments for stone conservation, based on dispersions of photoactive TiO2 nanoparticles, recently studied in our research group, provided interesting results after laboratory testing. Unfortunately, it is almost impossible to accurately reproduce in the lab the complexity of field exposure conditions. It is then of crucial importance to evaluate the behaviour and durability of new treatments in situ, once they are applied to real deteriorated surfaces of the architectural heritage exposed in outdoor. In the present research, the effectiveness of a TEOS-nano-TiO2 treatment and of two nanocomposites based on nano-TiO2 dispersion in organosiloxane and functionalized SiO2 for the protection of two specific marbles—Candoglia and Crevoladossola—has been evaluated on-site. The effect of the introduction of innovative titania nanoparticles in selected commercial products has been examined focusing on compatibility and protection efficacy in real exposure condition. An on-site testing protocol has been defined and carried out for 12 months after the application, considering colour and morphology changes and water absorption by capillarity. The nano-TiO2 addition does not hinder the performance of the protective treatments while it provides a positive contribution in soiling reduction. The results will be implemented in the operative framework of the ongoing conservation project of the considered historical façade

Surface–rain interactions: Differences in copper runoff for copper sheet of different inclination, orientation, and atmospheric exposure conditions

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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

"Mi Fuma il Cervello" self-portrait series of Alighiero Boetti: Evaluation of a conservation and maintenance strategy based on sacrificial coatings

"Mi Fuma il Cervello" ("steaming brain") is the iconic self-portrait of Alighiero Boetti, in which the artist is represented standing up while a copper hose lets water flow on his head. A hidden electric resistance heats the sculpture's head so that the splashing water forms a dense vapour mist as it comes in contact with the hot metal surface. Such system is normally active only during museum exhibitions and determines critical conservation issues due to the inevitable formation of a thick and adherent calcareous deposit. Recently, the Fonderia Artistica Battaglia (Milan, Italy) conducted an extensive conservative intervention aimed at retrieving the original aesthetic features of the bronze surface and its artificial patina finishing. A conservation strategy was proposed based on the use of a superficial coating, to protect the surface and to ease the removal of the calcareous deposits in view of future cleaning operations. Three siliconic commercial paints were selected and preliminarily applied to specimens that simulate the actual alloy. The efficacy, compatibility and durability of the treatments were tested by accelerated ageing test based on combined thermal and wetting cycles, representative of the real working conditions. A multi-analytical diagnostic approach was followed for the evaluation of the coatings characteristics and performances before and after aging: stereomicroscopy, ESEM-EDX, VIS-Light spectrophotometry, micro-FTIR, electrochemical impedance spectroscopy (EIS). The research showed that from the aesthetic point of view all coatings induce only limited and rather comparable initial colour variations. The permanence of the treatments upon ageing, used as durability indicator, was assessed in all cases but the protective layers appeared damaged and no longer continuous over the metal surface. The best performing treatment was identified and further tested with respect to re-treatability. The results provide indications for the general conservation and maintenance protocol

Imaging and micro-invasive analyses of black stains on the passepartout of Codex Atlanticus Folio 843 by Leonardo da Vinci

: This paper accounts for the diagnostic campaign aimed at understanding the phenomenon of black stains appeared on the passepartout close to the margins of Folio 843 of Leonardo da Vinci's Codex Atlanticus. Previous studies excluded microbiological deterioration processes. The study is based on a multi-analytical approach, including non-invasive imaging measurements of the folio, micro-imaging and synchrotron spectroscopy investigations of passepartout fragments at different magnifications and spectral ranges. Photoluminescence hyperspectral and lifetime imaging highlighted that black stains are not composed of fluorescent materials. μATR-FTIR imaging of fragments from the passepartout revealed the presence of a mixture of starch and PVAc glues localized only in the stained areas close to the margin of the folio. FE-SEM observations showed that the dark stains are localized inside cavities formed among cellulose fibers, where an accumulation of inorganic roundish particles (∅100-200 nm in diameter size), composed of Hg and S, was detected. Finally, by employing synchrotron μXRF, μXANES and HR-XRD analyses it was possible to identify these particles as metacinnabar (β-HgS). Further research is needed to assess the chemical process leading to the metacinnabar formation in the controlled conservation condition of Leonardo's Codex

The San Carlo Colossus: An Insight into the Mild Galvanic Coupling between Wrought Iron and Copper

: The San Carlo Colossus, known as San Carlone, is a monument constituted by an internal stone pillar support to which a wrought iron structure is attached. Embossed copper sheets are fixed to the iron structure to give the final shape to the monument. After more than 300 years of outdoor exposure, this statue represents an opportunity for an in-depth investigation of long-term galvanic coupling between wrought iron and copper. Most iron elements of the San Carlone appeared in good conservation conditions with scarce evidence of galvanic corrosion. In some cases, the same iron bars presented some portions in good conservation conditions and other nearby portions with active corrosion. The aim of the present study was to investigate the possible factors correlated with such mild galvanic corrosion of wrought iron elements despite the widespread direct contact with copper for more than 300 years. Optical and electronic microscopy and compositional analyses were carried out on representative samples. Furthermore, polarisation resistance measurements were performed both on-site and in a laboratory. The results revealed that the iron bulk composition showed a ferritic microstructure with coarse grains. On the other hand, the surface corrosion products were mainly composed of goethite and lepidocrocite. Electrochemical analyses showed good corrosion resistance of both the bulk and surface of the wrought iron, and galvanic corrosion is not occurring probably due to the iron's relatively noble corrosion potential. The few areas where iron corrosion was observed are apparently related to environmental factors, such as the presence of thick deposits and to the presence of hygroscopic deposits that create localized microclimatic conditions on the surface of the monument

Study and Characterization of Environmental Deposition on Marble and Surrogate Substrates at a Monumental Heritage Site

In this study, the results of the field exposure activity conducted between 2014 and 2017 on the façade of the Milano cathedral (Italy) are reported. The main research aim was to characterize environmental deposition in real exposure conditions and for this purpose, both stone substrates (Candoglia marble) and surrogate substrates (quartz fibre filters) were exposed on the cathedral façade in two sites at different heights. A complete chemical characterization has been performed on quartz filters and marble substrates, i.e., quantification of the deposited aerosol particulate matter (PM) and of the main ions. On quartz filters, the carbonaceous component of deposits was also investigated, as well as the color change induced by soiling, by means of colorimetric measurements. The combined approach exploiting marble and surrogate substrates seems to be a suitable monitoring strategy, although some aspects should be taken into account. In particular, differences in the deposits composition have been highlighted mainly depending on the type of substrate. The environmental data related to atmospheric pollution in Milan for the same period have also been considered but no direct correlations were found between some atmospheric precursors and their related ions in solid deposits