“Live” Prussian blue fading by time-resolved X-ray absorption spectroscopy
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Prussian blue (PB) is an artists’ pigment that has been frequently used in many artworks but poses several problems of conservation because of its fading under light and anoxia treatment. PB fading is due to the reduction of iron(III) into iron(II) and depends a lot on the object investigated. Due to the complexity of the structure, the precise physico-chemical mechanisms behind the redox process remain obscure. In this paper, we present a procedure to investigate light- and anoxia-induced fading of PB-paper samples by means of time resolved X-ray absorption spectroscopy performed at the Fe K-edge. A system composed of a visible light source and a flux-controlled environmental cell allowed light, gas and humidity to be modified in situ. The synchrotron X-ray beam was evidenced to induce a reduction of PB and to play a major role in the kinetics. The analysis of the PB fading kinetics of a sample submitted to various gas and light environments showed that both synchrotron beam and anoxia were influencing PB reduction in a correlated way. In comparison, light was found to play a minor role. Finally, we have demonstrated that the type of paper substrate could influence significantly the kinetics of reduction. Several hypotheses to explain the correlation between PB reduction mechanism and substrate are presented.
KeywordsPrussian Blue Cultural Object Visible Light Source Anoxia Treatment Into Iron
We would like to acknowledge Lucie Nataf of the ODE team and Frédéric Picca for his joyful and spontaneous help in python programming. Thanks also to the team of the Soleil Chemistry Laboratory for help in preparing PB-paper samples and Matjia Strlic for sharing with us interesting points on paper aging process. We thank Robert J. Koestler for suggesting us to investigate PB fading in cultural artefacts. Claire Gervais thanks the Swiss National Science Foundation for partial funding through the SNSF Professorship grant 138986. This work has been developed as part of the IPANEMA/Smithsonian Institution agreement on science cooperation.
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