We report the development of innovative embedding procedures for the analysis of paint cross sections by attenuated total reflection (ATR) Fourier transform IR microscopy. This technique was chosen because it is widely employed for the characterization and spatial location of organic and inorganic components in artistic samples. Moreover, the performance of the technique may be critically affected by sample preparation in terms of surface morphology and the presence of contamination. First, we evaluated the use of KBr as a barrier to contamination by the embedding synthetic medium. In this way, the sample cross section can be polished by means of a sample holder, which allows a controlled pressure to be applied to the sample, thus improving the reproducibility and quality of the surface cross section. In addition, argon ion milling was used for the polishing of samples embedded in KBr, and provided very promising results in terms of surface planarity and reduction of superficial contamination by KBr. Finally, the use of NaCl as an alternative to KBr was proposed thanks to its advantages in terms of hygroscopicity, cost, and toxicity. In addition, cross sections embedded in NaCl were characterized by greater hardness, a feature that allowed us to obtain improved contact with the ATR crystal.
The corrosion products usually found on outdoor bronzes are generated by the interaction between the metal alloy and the atmospheric pollutants. To protect the external surface of bronzes, different organic materials (natural or synthetic) can be applied, creating over time a patina consisting of a complex mixture of inorganic and organic degraded components. The correct chemical characterization of patina constituents is fundamental to define the state of conservation of a metal artwork and address proper restoration actions. In this paper, we evaluated the potentialities of near-infrared (NIR) reflectance microscopy (4000-7500 cm) as complementary method to mid-infrared (MIR) analyses for the characterization of bronze patinas. Although NIR spectroscopy has been already used in the field of heritage science, its application for the characterization of bronze patinas is almost unexplored. In this paper, several corrosion products usually found on the surface of outdoor bronze sculptures were synthesized, characterized, and submitted to the NIR-MIR total reflection analysis to build up a reference spectral database. We devoted particular attention to the NIR features of copper hydroxychlorides, such as atacamite and paratacamite, which have not been studied in detail up to now. A selection of organic-based formulations, commonly used by restorers to protect the bronze surface against the outdoor aggressive environment, were also considered as references. Successively, NIR-MIR reflectance microscopy was successfully employed for the analysis of patina micro-samples collected from the bronze statues of the Neptune Fountain (sixteenth century) located in Bologna. The obtained results demonstrate the ability of NIR spectroscopy to identify organic and inorganic patina constituents, even in mixtures. In addition, the study can be considered as a proof of concept for the possible future application of the technique for in situ diagnostic campaigns on bronze sculptures.
The application of hyperspectral imaging in the field of cultural heritage investigation is growing rapidly. In this study, short wavelength infrared hyperspectral imaging (960–2500 nm) has been explored as a potential non-invasive technique for in situ mapping of corrosion products on bronze sculptures. Two corrosion products, brochantite and antlerite, commonly found on the surfaces of outdoor bronze monuments, were considered. Their spatial distribution was investigated on the surface of the bronze sculpture The Man with the Key by Auguste Rodin in Oslo. The results demonstrate that hyperspectral imaging combined with image analysis algorithms can display the distribution of the two corrosion products in different areas (unsheltered and partially sheltered) of the sculpture.
In this research, we propose an advanced system for the cleaning of wax-based coatings applied on indoor bronzes. To this aim we developed a new kind of eco-friendly gel based on PHB (poly-3-hydroxybutyrate) used as thickening agent, biodiesel (BD) and dimethyl carbonate (DMC). BD is a mixture of methyl esters obtained from palm oil, which acts as cleaning agent while DMC was added as additional solvent to partially solubilize PHB and forming a gelly phase. For the first time a PHB-based gel obtained by mixing two solvents with different proprieties was proposed, expanding the range of possible formulations, that can be used according to the specific restoration purpose. After the preliminary characterization of chemical and physical properties of the gel, an ad hoc analytical protocol was implemented to evaluate both the cleaning efficiency and the release of residues on the treated surfaces. Standard samples were prepared following ancient recipes and submitted to spectroscopic and chromatographic analysis before and after the cleaning procedures. Finally, the performances of PHB-DMC/BD gel were assessed on a real case of study presenting a wax-based coating: the Pulpito della passione attributed to Donatello and dated back to 1460. In situ analysis demonstrated the high cleaning efficiency of the proposed systems also for the removal of aged coatings.
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