title: Synthesis and fading of eighteenth-century Prussian blue pigments: a combined study by spectroscopic and diffractive techniques using laboratory and synchrotron radiation sources.
abstract: Prussian blue, a hydrated iron(III) hexacyanoferrate(II) complex, is a synthetic pigment discovered in Berlin in 1704. Because of both its highly intense color and its low cost, Prussian blue was widely used as a pigment in paintings until the 1970s. The early preparative methods were rapidly recognized as a contributory factor in the fading of the pigment, a fading already known by the mid-eighteenth century. Herein two typical eighteenth-century empirical recipes have been reproduced and the resulting pigment analyzed to better understand the reasons for this fading. X-ray absorption and Mössbauer spectroscopy indicated that the early syntheses lead to Prussian blue together with variable amounts of an undesirable iron(III) product. Pair distribution functional analysis confirmed the presence of nanocrystalline ferrihydrite, Fe10O14(OH)2, and also identified the presence of alumina hydrate, Al10O14(OH)2, with a particle size of ∼15 Å. Paint layers prepared from these pigments subjected to accelerated light exposure showed a tendency to turn green, a tendency that was often reported in eighteenth- and nineteenth-century books. The presence of particles of hydrous iron(III) oxides was also observed in a genuine eighteenth-century Prussian blue sample obtained from a polychrome sculpture.
authors.affiliation: Centre Européen d'Archéométrie, University of Liège, Sart Tilman B15, Liège, Belgium. louise.samain@ulg.ac.be
authors.forename: Louise
authors.initials: L
authors.lastname: Samain
authors.forename: Fernande
authors.initials: F
authors.lastname: Grandjean
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Long
authors.forename: Pauline
authors.initials: P
authors.lastname: Martinetto
authors.forename: Pierre
authors.initials: P
authors.lastname: Bordet
authors.forename: Jana
authors.initials: J
authors.lastname: Sanyova
authors.forename: David
authors.initials: D
authors.lastname: Strivay
mesh_terms: D003124:Colorimetry / Q000295:instrumentation; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005295:Ferrocyanides / Q000032:analysis* / Q000737:chemistry*; D008422:Materials Testing / Q000295:instrumentation*; D010150:Paint / Q000032:analysis*; D013052:Spectrometry, X-Ray Emission / Q000295:instrumentation*; D017356:Synchrotrons / Q000295:instrumentation*; D014961:X-Ray Diffraction / Q000295:instrumentation*
chemical_list: D005295:Ferrocyanides; C000170:ferric ferrocyanide
keywords: archaeometry; iron(III) hexacyanoferrate(II); light exposure fading; paint layers; pair distribution function