Investigation of eighteenth-century Prussian blue pigments by PDF analysis

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 1970's. The early preparative methods were rapidly recognized as a contributory factor in the fading of the pigment [1], a fading already known by the mid-eighteenth century. The eighteenth-century methods are based on the calcination of dried blood to produce a potassium hexacyanoferrate complex, which is the first of two essential reactants for synthesizing Prussian blue. The second reactant is an iron salt.
We successfully reproduced two typical eighteenth-century empirical recipes [2]. The resulting pigments were of variable color quality, ranging from intense blue to blue-gray or blue-green, and exhibit broadened or inexistent Bragg peaks. High-energy powder X-ray diffraction experiments were performed at the ID11 beamline at ESRF, Grenoble, France. The pair distribution function (PDF) of the pure Prussian blue pigments was refined with a three-phase model, in order to take into account the vacancy distribution in the unit cell of Prussian blue. In certain ancient Prussian blues, the PDF analysis revealed the presence of nanocrystalline ferrihydrite, Fe10O14(OH)2, and also identified the presence of alumina hydrate, Al10O14(OH)2, with a particle size of ca. 15 Å.
Paint layers prepared from these ancient pigments subjected to accelerated ageing 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 Prussian blue sample obtained from an eighteenth-century polychrome sculpture.

[1] Kirby, J.; Saunders, D. The National Gallery Technical Bulletin 2004, 25, 73. [2] Dossie, R. The Handmaid to the Arts; Nourse, J.: London, 1758; Le Pileur d'Apligny, M. Traité des couleurs matérielles et de la manière de colorer relativement aux différents arts et métiers; Saugrain et Lamy: Paris, 1779.