Marie-Angélique Languille

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.

Strontium speciation in archaeological otoliths

Fish otoliths (“ear stones”) are major environmental indicators used in ecology and fisheries sciences. Otoliths consist of a biomineral material containing an organically-templated mineral calcium carbonate, normally aragonite, in which strontium is incorporated at trace to minor levels depending on water chemistry and individual physiology. Sr content and fluctuations inform on the life histories of ancient specimens and provide data for palaeoenvironmental reconstructions. Identifying the impact of post-mortem alteration is a critical question to assure the reliability of such work. A central parameter for the reliability of Sr content as a palaeoenvironmental proxy is whether the mode of incorporation can be considered as stable and homogenous at the microscale in otoliths over thousands of years. In addition, it is important to know whether a different kind of speciation of Sr is observed, especially at the outer surface of the sample in contact with the soil and local environment. Here, a novel combination of synchrotron microscale point analyses and raster-scanning X-ray absorption spectroscopy is implemented and used for the first time to study otoliths at different length scales, spanning from millimetres down to micrometres. Strontium is found in substitution for calcium in aragonite in all our analyses of five Holocene otoliths and their three modern counterparts; the first set of samples from the Peruvian coast, up to 11 000 years old, are studied for their potential as palaeoenvironmental proxies. The chemical environment of strontium in otoliths is independent of content of this element, location in the otolith, species, and archaeological age. This is shown with a high lateral resolution (about 10 μm) over wide fields of view, as a way to consolidate macro-scale approaches. To our best knowledge, this work is the first report of the chemical environment of strontium in ancient otoliths. Our work opens the way to new approaches to validate palaeoenvironmental studies of biocarbonate paleoproxies.