Why does Prussian blue fade? Understanding the role(s) of the substrate

Abstract

Prussian blue (PB) and its analogues are widely studied because of their interesting and promising magnetic and optical properties. The pigment Prussian blue, found in different types of artworks (paintings, watercolors and photographs), is also studied in the area of heritage science, where its capricious fading behavior under light or anoxia treatment poses problematic conservation issues. PB fading is due to the reduction of iron(III) to iron(II) and depends significantly on the artefact. This paper focuses on the roles of the substrate in affecting the PB structure and modifying the redox process. In particular, X-ray absorption experiments at the Fe K-edge of unfaded and faded PB-paper samples show that changes in the PB structure can happen by simple contact with the substrate, prior to the fading treatment. Spectrophotometric measurements on a series of model PB-paper samples further demonstrate the multiple influences of the substrate and show that not only its chemical composition but also its role as a dispersion and textured medium significantly alter the fading behavior of PB. A potential roadmap is proposed to rationally investigate the complex fading process of Prussian blue on a substrate.

Light and anoxia fading of Prussian blue dyed textiles

“…On the contrary, linen is a stable substrate and Prussian blue dyed on linen does not readily reduce under light. These findings corroborate those found for PB-paper systems [6,10] 6 . (1) Thus, if the substrate is able to supply cations and electrons (by oxidation or generation of radicals), it allows Eq.…”

“…The shape of the spectrum is typical of Prussian blue [6]. After the edge (7120-7122 eV), it exhibits a strong first oscillation (≈ 7140 eV) followed by a decrease in the amplitudes of the oscillations at higher energies (> 7180 eV) due to strong multiple scattering effects, a typical behavior for cyano-bridged organometallic compounds.…”

“…In her reference work on the history of artist's pigments [1], Berrie describes Prussian blue as "both permanent and impermanent". Indeed, those fading processes depend a lot on the object investigated [2,5,6]. Moreover, the faded pigment might partially recolor when stored in the dark under oxygen [7,8], puzzling conservators concerned with the preservation of Prussian blue objects and preventing them from choosing adequate conservation solutions.…”

“…Prussian blue fading as observed in cultural heritage objects is usually linked to this redox process, as demonstrated by recent studies dedicated to the photochemistry of Prussian blue. For instance, studies based on X-ray absorption spectroscopy at the Fe K-edge coupled with spectrophotometry investigated the reduction kinetics of Prussian blue in different environments [10], estimated the role(s) of the substrate [6] and evaluated the impact of impurities or other pigments on fading [11]. A list of intrinsic and environmental factors susceptible to influence Prussian blue fading could be underlined [6].…”

“…For instance, studies based on X-ray absorption spectroscopy at the Fe K-edge coupled with spectrophotometry investigated the reduction kinetics of Prussian blue in different environments [10], estimated the role(s) of the substrate [6] and evaluated the impact of impurities or other pigments on fading [11]. A list of intrinsic and environmental factors susceptible to influence Prussian blue fading could be underlined [6]. Nevertheless, considering the complexity of the pigment itself and its variety of use, much remains to be done to fully understand and mitigate the degradation of Prussian blue.…”