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Fading mechanisms of the pigment Prussian Blue

Cl. Gervais, Institut d’art de Bern (Switzerland) and Smithsonian Institution (USA), 2011—2014}

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Prussian blue (iron hexacyanoferrate) is one of the first modern synthetic pigments and has been used in a wide variety of cultural artefacts, among them famous paintings from Watteau (1684-1721) or Van Gogh (1853-1890) and watercolors from Hokusai (1760-1849).

When exposed to strong illumination or submitted to anoxic treatment, some artifacts containing Prussian blue exhibit a discoloration of the pigment more or less pronounced. The general discoloration process implies the photoreduction of Prussian blue’s ferric ions. However, important issues for conservation purposes remain to be investigated, such as the role of the substrate (paper, textile) or the impact of several fading-coloration cycles on the crystalline and chemical structure of the pigment.

Our recent studies on PB model artefacts have shown that in the photoreduction process, the physico-chemical properties of the substrate play a critical role. For instance, X-ray absorption spectroscopy coupled to colorimetry of PB-paper samples showed that the kinetics and extent of the photoreduction process depends on the type of paper substrate (cellulose vs ligno-cellulose) and the presence of impurities (such as cations) within it [1].

The goal of the project aims at better characterizing the capacity of the substrate to modify the Prussian blue crystalline structure and alter its fading behavior. The project is a collaboration between the European research platform for ancient materials (IPANEMA, SOLEIL synchrotron, France), the Smithsonian Museum Conservation Institute (MCI, Washington DC, USA), the Centre de Recherche sur la Conservation des Collections (CRCC, Paris, France) and is further pursued at the Bern University of the Arts.


[1C. Gervais, M.-A. Languille, S. Reguer, M. Gillet, E. P. Vicenzi, S. Chagnot, F. Baudelet, and L. Bertrand. “Live” Prussian blue fading by time-resolved X-ray absorption spectroscopy. Appl. Phys. A, 111(1):15–22, Apr 2013.
C. Gervais, M.-A. Languille, S. Reguer, M. Gillet, S. Pelletier, C. Garnier, E. Vicenzi, and L. Bertrand. Why Does Prussian Blue Fade ? Understanding the Role(s) of the Substrate. J. Anal. At. Spectrom., 2013. In press.


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