Fading Sunflowers: Not such a bright blaze

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  • Published: Nov 1, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Fading Sunflowers: Not such a bright blaze

Bye, bye...

the years are not being kind to poor old Vincent's legacy and McLean's almost prescient lyric

American singer Don McLean once penned an ode to tragic artist Vincent van Gogh (Vincent) in which he waxed lyrical of the master's paintings: "Flaming flowers that brightly blaze" he sang, presumably musing on the famous Sunflowers, painted by the Dutch master in Arles in 1888-1889, and protagonist of seven paintings.

But, the years are not being kind to poor old Vincent's legacy and McLean's almost prescient lyric "Colours changing hue" seems to portend the chemical changes being observed in Sunflowers as the years pass and pigments transform with age. New evidence from a spectroscopic study using mobile equipment and synchrotron radiation-based X-ray methods has now been published in the journal Angewandte Chemie.

Evidence for the pigment-changing processes comes from a new detailed, spectroscopic study of the version of Sunflowers housed in the collection of the Van Gogh Museum in Amsterdam, Netherlands and painted in 1889. A team led by Letizia Monico of the Institute of Molecular Science and Technology (CNR-ISTM) of Perugia, the University of Perugia and the University of Antwerp used X-rays from DESY’s lightsource PETRA III and from the European Synchrotron Radiation Facility (ESRF) to examine two micro samples - less than 1 millimetre - carefully sampled from different areas of the painting. The study not only revealed much about the chemistry of the pigments but points to areas of the painting that should be monitored more closely for any changes over time.

Beaming colours

Vincent van Gogh (1853-1890) is well known for his use of bright yellow colours in these series of paintings. The Dutch painter used what we know as chrome yellows, a class of compounds consisting of a range of lead chromates. “There are different shades of the pigment, and not all of them are photochemically stable over time,” explains Monico. “Lighter chrome yellows have sulfur mixed into them, and are susceptible to chemical degradation when exposed to light, which leads to a darkening of the pigment.” Lightfast chrome yellow has the chemical formula PbCrO4, whereas the light-sensitive type has the formula PbCr1-xSxO4, (where x is greater than approximately 0.4).

“The analysis shows that the yellow-orange hues mainly contain the lightfast version of chrome yellow, whereas the light-sensitive type is mainly found in the pale yellow areas,” explains team member Gerald Falkenberg, who runs DESY’s beamline P06, where the X-ray diffraction measurements were carried out on the Sunflowers samples.

Broad assessment

At the beamline ID21 of ESRF in Grenoble, the team examined the chemical state of the paint samples. When light sensitive chrome yellow darkens, the chromium is reduced from its highest oxidation state Cr(VI) to Cr(III). The scientists detected a relative proportion of 35 percent Cr(III) species on the surface of the paint. “At least at the two sites from which the paint samples were taken, a colour change has occurred in the Sunflowers as a result of the reduction of chrome yellow,” explains Monico. This suggests that the Sunflowers may originally have looked different from what we see today.

The team also used a series of portable spectroscopic instruments to identify those parts of the painting that might need ongoing monitoring for the sake of conservation. “Since chrome yellow pigments were widely used by late 19th-century painters, this study also has broader implications for assessing the colours of other works of art,” adds co-author Koen Janssens from the University of Antwerp.

"The identification and distribution of several chrome yellow varieties of different photochemical stability and the elucidation of the conservation state of these  pigments  are starting points to help conservators and museum curators to develop and improve  the storage and display strategies that might allow Sunflowers  to be preserved as long as possible over time," Monico told SpectroscopyNOW. "The availability of a high risk map, showing the regions that are composed of the light-sensitive type of the chrome yellow pigment, is very important, because calls conservators for a careful monitoring of the areas more exposed to risk over time. Moreover, taking into account  the findings described in our previous works (see for example: Monico et al. Anal Chem 2013, 85, 860-867; Monico et al. J Analyt Atomic Spectrom 2015, 30, 1500-1510), this study opens up the way to the elaboration of a more accurate, safe and controlled strategy for the light exposure of the painting (e.g., by minimizing the exposure to the green-blue light)."

Monico points out that given that in the version of Sunflowers they studied the chrome yellows were often encountered in mixtures with other pigments (for example: red lead, vermilion, zinc white, emerald green), further research is now needed to distinguish between the intentional colour contrasts obtained by Van Gogh by employing specific paint mixtures and the subsequent effects of colour change by pigment degradation. "We are currently working in order to explore how these added pigments and possibly the corresponding degradation compounds may influence the stability of lead chromates and contribute to visible chromatic alteration of the paint surface," Monico told us.

Related Links

Angew Chem Int Edn 2015, online: "Evidence for Degradation of the Chrome Yellows in Van Gogh's Sunflowers: A Study Using Noninvasive In Situ Methods and Synchrotron-Radiation-Based X-ray Techniques"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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