Claire Gervais

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

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.

X-ray Photochemistry of Prussian Blue Cellulosic Materials: Evidence for a Substrate-Mediated Redox Process.

Beside its promising applications in the design of multifunctional materials, batteries and biosensors, the pigment Prussian blue is still studied in heritage science because of its capricious fading behavior due to a complex light-induced redox mechanism. We studied model heritage materials composed of Prussian blue embedded into a cellulosic fiber substrate by means of X-ray absorption near-edge spectroscopy. Significant X-ray radiation damage was observed and characterized. X-ray radiation induced first a reduction of Prussian blue, in a similar way to what visible light does, followed by a complete degradation of the pigment and the formation of iron(III) oxyhydroxide. We took advantage of this X-ray photochemistry to investigate in depth the redox behavior of Prussian blue. We could particularly demonstrate that the rate, extent, and quality of Prussian blue photoreduction can be tuned by modifying the pH and alkali cation content of the cellulosic substrate. The present study represents a step further in the understanding of Prussian blue heritage materials from an electrochemical viewpoint and provides evidence of substrate-mediated photochemistry applicable to a wider class of Prussian blue composite materials.

Light and Anoxia Fading of Prussian Blue Dyed Textiles

Although Prussian blue is a popular pigment, its stability has been questioned since its discovery in 1704. Its stability upon exposure to light and anoxia remains difficult to apprehend. The present paper focuses on the relative influences of light, anoxia and type of substrate on the discoloration of Prussian blue dyed textiles. Spectrophotometry and X-ray absorption spectroscopy measurements of samples artificially aged by light in air or anoxia show that both the extent of the reduction process at the origin of Prussian blue discoloration and the aging of the textile substrate are linked and strongly differ with the environment. The complex inter-relationship existing between Prussian blue discoloration and textile degradation and the final impact it may have on the conservation of the entire system is discussed.

Degradation mechanisms of reinforcing iron rebars in monuments: the role of multiscale porosity in the formation of corrosion products investigated by X-ray tomography

Monuments in stone often contain metallic reinforcements, the stability and degradation state of which determines the entire integrity of the edifice. Understanding their long-term corrosion process is thus an essential step towards a safe and efficient conservation strategy. In this study, we show that combining laboratory and synchrotron tomography at different scales is efficient for getting an overall three-dimensional picture of the iron corrosion products found in iron rebars from Orleans cathedral. We demonstrate that beside chemical characterization of the corrosion products, the study of the shape of the corrosion products, their spatial distribution within the stone binder and their relationship with the porosity of the binder can bring significant insights into the corrosion process, and particularly help in understanding the multiple roles that porosity plays in corrosion.

Time resolved XANES illustrates a substrate-mediated redox process in Prussian blue cultural heritage materials

The pigment Prussian blue is studied in heritage science because of its capricious fading behavior under light exposure. We show here that XANES can be used to study the photosensitivity of Prussian blue heritage materials despite X-ray radiation damage. We used an original approach based on X-ray photochemistry to investigate in depth the redox process of Prussian blue when it is associated with a cellulosic substrate, as in cyanotypes and watercolors. By modifying cation and proton contents of the paper substrate, we could tune both rate and extent of Prussian blue reduction. These results demonstrate that the photoreduction and fading of Prussian blue is principally mediated by the substrate and its interaction with the oxygen of the environment.