Abstract
The degradation of laboratory oil paint film specimens containing indigo and Prussian blue pigments and pictorial samples from the Sant Francesc de Paula painting exhibited in the Tomàs Balvey Arxiu Museum (Cardedeu (Catalonia), Spain) has been studied by voltammetry of immobilized particles. This technique, combined with light microscopy, scanning electron microscopy-energy dispersive X-ray analysis, nanoindentation-atomic force microscopy, attenuated total reflectance-Fourier-transform infrared spectroscopy and gas chromatography–mass spectroscopy techniques permits the proposal of a dual scheme for the degradation of the pigments when naturally aged and submitted to accelerated UVA aging. Under conditions of moderate temperature, humidity and natural illumination, and low gradients of these parameters, Prussian blue acts as a radical scavenger moderating the production of reactive oxygen species produced in the oil binding medium by the action of ultraviolet radiation, resulting in the formation, in the solid state, of the solid-solution, {KFeIII[FeII(CN)6]}x{FeIII[FeIII(CN)6]}1–x, known as Berlin green, which then promotes the formation of indigo adducts with radicals. In several localized areas of the Sant Francesc de Paula paint showing strong degradation, Prussian blue acts as a promoter of the indigo oxidation to isatin, thus resulting in a considerable chromatic shift.
Highlights
The deterioration of oil paintings is an important problem in the field of conservation and restoration of cultural heritage
We present a solid state electrochemistry study, complemented by optical microscopy, scanning electron microscopy coupled to energy dispersive X-ray detection (SEM–energy dispersive X-ray spectrometric analyses (EDX)), nanoindentation-atomic force microscopy (NI-Nanoindentation-atomic force microscopy (AFM)), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-Fourier-transfor infrared spectroscopy (FTIR)), and gas chromatographymass spectrometry (GC–MS) on the interaction between two blue pigments, indigo and Prussian blue, and linseed oil in paintings, comparing the results from an old painting showing peculiar alteration features and a set of reconstructed paint specimens naturally and artificially aged
The degradation of oil paints containing indigo and Prussian blue pigments has been studied by solid state electrochemistry aided by light microscopy, scanning electron microscopy-energy dispersive X-ray analysis, nanoindentation-atomic force microscopy, attenuated total reflectance-Fourier-transform infrared spectroscopy and gas chromatography-mass spectroscopy
Summary
The deterioration of oil paintings is an important problem in the field of conservation and restoration of cultural heritage. This feature, like that found at 1703 cm−1 in the altered sample (Fig. 7), that is exclusively observed in reconstructed paint films containing Prussian blue (Additional file 1: Figures S3, S4), can be ascribed to stretching vibrations of free fatty acids released by the hydrolysis reaction undergone by the triacylglycerol molecules of the linseed oil.
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