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.

X-ray Photochemistry of Prussian Blue Cellulosic Materials: Evidence for a Substrate-Mediated Redox Process

“…Ferrihydroxide is also found as an impurity in Prussian blue pigments synthesized with eighteenth century historical recipes. 29 Its presence has been attributed to an excess of iron salt (e.g. iron sulfate) not consumed during the synthesis of Prussian blue, which leaves Fe II ions in the system.…”

Identifying eighteenth century pigments at the Bodleian library using in situ Raman spectroscopy, XRF and hyperspectral imaging

“…M. Flinders to Australia. ' eighteenth century recipes by Samain et al [33]. 7 Raman spectroscopy carried out on the same blue in 2017 however, confirmed only the presence of ultramarine, indigo and an unknown but highly fluorescent blue in these paintings (Table 7).…”

“…Although not confirmed, the results provided by the Madrid colour chart, and characteristic absorption in the infrared region makes it seem likely that this was a copper blue such as azurite, bice or verditer. Since Bauer was required to paint almost 1300 paintings in colour at a rapid pace, the widespread use of indigo, which was considerably less expensive than azurite, smalt and particularly ultramarine, and a pigment that was very workable in watercolour would be an obvious choice [32,33]. For the volume of paintings that Bauer was commissioned to produce for Sibthorp between 1786 and 1794, it is understandable that he made as much use of it as he could.…”

Zur Photochemie von Künstlerfarben: Strategien zur Verhinderung von Farbveränderungen in Kunstwerken

“…Munoz-Garcia et al [41] beschrieben, dass PbCrO 4 und PbSO 4 zur Segregation in getrennte Phasen neigen, wobei in erster Linie die PbCrO 4 -reiche Phase dem Abbau unterliegt. [44,50] Die Gegenwart von Fe III -Oxyhydroxid als Nebenprodukt in aus PB synthetisierten Pulvern des 18. [45,46] Das Ausmaß des Phänomens ist variabel und hängt von der Gegenwart weißen Füllstoffs, [47,48] der Art der Umgebung und des Substrats [49] und dem Manufakturprozess des PB ab.…”

“…Amat et al [42] postulierten einen Abbau infolge der Freisetzung von CrO 4 2À -Anionen in die Farbmatrix, entsprechend einem extrinsischen Abbauweg.R ahemi et al [43] berichteten stattdessen über eine erhçhte Cr-Re- [44] Das Ausbleichen von PB gefährdet verschiedenartige Kulturgüter, darunter Te xtilien, Drucke,B laupausen, Wasserfarben und Ölgemälde (Abbildung 4a). [50] Studien an photochemisch gealterten Attrappen mit unterschiedlichen spektroskopischen Te chniken zeigten, dass die Ausbleichung von PB auf die Photoreduktion Fe III !Fe II zurückzuführen ist, [48][49][50] die hçchstwahrscheinlich durch den LMCT-Fe III -CN-Übergang aktiviert wird. [44,50] Die Gegenwart von Fe III -Oxyhydroxid als Nebenprodukt in aus PB synthetisierten Pulvern des 18.…”