Prussian blue and X-rays
To understand how paint pigment deteriorates, researchers must focus on its infinitely small characteristics. By Philippe Morel
(From "Horizons" no. 107, December 2015)
Prussian blue can be found on Hokusai’s famous print The Wave as well as in the paintings of van Gogh and Picasso. The colour was discovered at the beginning of the 18th century and rapidly became commonplace in artists' workshops. Until that point blue had been difficult to produce, with painters habitually using ultramarine, a pigment of great expense created from lapis lazuli, or smalt, a fine powder made from cobalt which quickly lost its colour.
It was actually a chance contamination that led the paint manufacturer Johann Jacob Diesbach of Berlin to discover Prussian blue. It also turned out to be a very delicate paint. Although some artists did find it highly stable, others noted that it lost its colour very quickly when exposed to light.
A reversible degradation
Understanding why Prussian blue degrades became the focus of the work of Claire Gervais, a professor at the Bern University of the Arts. "These heritage materials are intriguing and lead to surprising knowledge", she explains. "They are heterogeneous and composite, and the mixture of organic and inorganic materials gives rise to sometimes unexpected properties. We still don’t know how to recreate the production methods, which are often complex and the result of long development processes. In fact, these materials have a long history of ageing that we cannot find anywhere else".
The chemist in Gervais sees Prussian blue as a ferric ferrocyanide, more precisely Fe7(CN)18·xH2O. It’s the transfer of electrons between the two ions FeII and FeIII that, by absorbing the red, gives rise to the bluecoloured pigment. But prolonged exposure to light subjects the pigment to the transformative process of photoreduction: the FeIII atoms gain an electron to become FeII. When the FeIII ions have all transformed, the transfer is no longer possible and the pigment loses its colour. This phenomenon is, however, partially reversible by exposing Prussian blue to oxygen in dark conditions.
Radiography of blue To better understand what’s happening, it’s necessary to delve underneath the surface, in other words to take X-rays. "X-ray absorption
spectroscopy allows us to see the atomic signature of iron atoms in the pigment, as well as their state of oxidation and the direct environment within the structure",
explains Gervais, who specialises in crystallography. "This way we can monitor both the changes in iron atoms during photoreduction and the subsequent loss of colour".
In the outskirts of Paris, the team uses a synchrotron – a ring-shaped particle accelerator which fires electrons along a curved path. The equipment can emit a powerful, stable and highly-focused electromagnetic array at a range of frequencies between infrared and X-ray.
Prussian blue is sensitive to visible light, and also to higher frequencies. "We knew
we’d have difficulty analysing it without damaging it", says Gervais. "But the precautions taken were not enough: the pigment lost its colour in the beam". Yet upon analysing the damage caused by the irradiation, the researchers realised it was also the result of photoreduction. Useful indeed: Xrays were therefore not only of help in the analysis, but also in the methodology. The Franco-Swiss team was naturally
not working on samples from works of art, but rather systematically examining
the influence of different artistic materials (paper, canvas, sizing, etc.) and the environment. They made sure to integrate variations of preservation strategies such
as humidification, anoxia (reducing levels of ambient oxygen) and even applying acid
to the paper.
Paper or pigment
The findings of these X-ray experiments cannot be directly translated to visible light, but they do demonstrate that Prussian blue degrades not only as a result of environmental factors but above all because of the material to which it is applied. Anoxia, humidity and potassium ions in surface fibres all cause the degradation
of Prussian blue to accelerate rapidly, whereas acids slow it down. This conclusion
is somewhat of a headache for museum curators, as anoxia is used to slow the
degradation of paper, but it now appears that it accelerates the degradation of Prussian blue at the same time.
At any rate, the laboratory and the museum remain very distinct environments. Gervais’s work has not yielded any miracle recipes for conserving or restoring works of art. What it can do, however, is to help identify works in need of specific conservation strategies because of the materials used or the conditions to which they have been exposed. For Verena Villiger, the Director of the Museum of Art and History of Fribourg, there is great interest in this kind of research. "Even without working directly within fundamental research projects, we can keep up to speed with developments through conferences and publications, although it’s not always as close as we?d like to be. It’s essential for scientists working in applied research to convert their new understanding of materials into preservation tools that we can then apply to our work".
Blue for biology
Prussian blue is not the preserve of great painters. Researchers use it as a biosensor
to study redox processes in living tissue. Light also modifies the magnetic properties of certain related materials, opening up interesting research paths in the field of preserving digital information in the form of magnetic bytes.
Philippe Morel is a science journalist who works for the magazine Tracés.