Electrochemical assessment of pigments-binding medium interactions in oil paint deterioration: a case study on indigo and Prussian blue

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, {KFe III [Fe II (CN) 6 ]} x {Fe III [Fe III (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.

Electrochemistry in Archaeology and Art Conservation

“…Then, the physicochemical methods can be viewed as analytical tools whose information (detection of Prussian blue), combined with historical data, can provide archaeometric information (authentication). Figure 2 shows an example of Prussian blue containing paint studied by electrochemical methods [12] …”

“…Figure 2 shows an example of Prussian blue containing paint studied by electrochemical methods. [12] The analysis of works of art and archaeological materials offers special difficulties, namely, the need of using nondestructive techniques or processing a minimal amount of sample as possible, the usual presence of multicomponent systems distributed in structured layers/stratified systems, accompanied by the possibility of the presence of interfering compounds and matrix effects resulting from the coexistence of analytes and alteration products.…”

“…The first one involves the mechanistic description of the solid state electrochemical processes. Although models for ion-assisted topotactic solid-to-solid reduction/oxidation [23][24][25][26] and reductive/ oxidative dissolution processes [27,28] are available, the reduction of metal compounds to metals and the complex interaction between pigments and binders in oil paintings [12] require more specific modeling.…”

“…The blue parts of the paint contain a mixture of indigo and Prussian blue displaying a peculiar degradation pattern described in. [12] Phot. Laura Osete-Cortina.…”

“… Photograph of the Sant Francesc de Paula painting by an anonymous artist in the 18 th century from the Tomàs Balvey Arxiu Museum (MATBC, Cardedeu, Spain). The blue parts of the paint contain a mixture of indigo and Prussian blue displaying a peculiar degradation pattern described in [12] . Phot.…”