Whether you enjoy Dutch masters or French impressionists, you hardly expect a common visit to the museum to entail questions like “Is yellow poisonous”, “How does red age”, or “Do the painter’s brush strokes correlate with the preferred orientation of the pigments”.
You might also assume that the old, young, and rebellious painters were not troubling their heads about these questions either. However, they were not all that naïve: ever since ancient times, artists paid particular attention to the choice of pigments. Their dedication to color has left them with a unique artistic footprint and us with something equally interesting: a scientific footprint. For a modern conservator or a restorer, every presence (or absence) of a pigment or a chemical compound is a clue that paves the way for successful restoration of old paintings. Such clues can also lead to something unexpected, such as the discovery of overpainted artworks, forgeries, or missing pieces of once grandiose paintings. Curious about the details of cultural heritage studies, we traveled to Brussels to meet the expert, Geert Van der Snickt, tenure track professor at the Conservation-Restoration department of the University of Antwerp.
“A restorer or conservator has more to do with interdisciplinary science than with art history”, says professor Van der Snickt. “A major part of our work is to define the exact problem of the restoration by collecting data on the paintings”, he explains as we walk through Brussels’ Royal Museum of Fine Arts.
In this case, material characterization is not as straightforward as you might assume. The obvious requirement, a nondestructive analysis, is usually fulfilled by using X-ray diffraction and spectroscopy techniques. The use of advanced laboratory and synchrotron sources has resulted in many successful restorations  and revealed hidden artworks from masters like Van Gogh and Van Eyck [2, 3]
However, there are other limiting factors. Paintings are usually very valuable, fragile, or both, so museums are not always keen on sending them to an external laboratory. Performing an analysis on-site is possible only with a portable XRD-XRF instrument, but these often come with unfortunate drawbacks: inflexibility, lack of speed, or low sensitivity. The investigated areas of paintings might be large, so the scanning process can be prohibitively long. To make matters more complicated, some objects cannot be removed from the frame, or they are covered with layers of paint or decay products. Therefore, an XRD analysis may require the combined use of a reflection and a transmission setup, both sensitive enough to detect low amounts of the sample in the object.
In order to meet all these requirements, the Antwerp X-ray Analysis, Electrochemistry & Speciation (AXES) department at the University of Antwerp decided to design a custom-made instrument. Lead by professor Koen Janssens, the group has constructed a mobile yet flexible solution for simultaneous XRF and XRD scanning. The diffraction setup can be easily switched between the reflection and transmission modes, and it features the PILATUS3 200K-A.
Figure 1. Simultaneous measurement of XRF and XRD data using the AXES setup. This setup allows up to 32° of diffraction data in 2θ to be collected in one shot.
Steven De Meyer, a PhD student in the AXES group, introduces us to the details on-site. The system is currently used in a studio at the Museum of Fine Arts, for the powder X-ray diffraction analysis of pigments on a triptych.
“We are scanning an area on an altar piece by Bouts, one of the Flemish Primitives. The mechanics are set to move in 1 mm steps in each direction. For each point, we are collecting the diffraction range between 13 and 45° in 2θ in ten seconds The measurement used to take an hour, so this is a huge improvement”, explains De Meyer. Using the XRDUA software, the collected Scherrer rings are automatically integrated in 1D patterns. Each pattern is assigned to the corresponding point and displayed on the computer screen in real time .
The PILATUS3 detector is a relatively large 2D detector, which helps to increase speed. “In order to scan the selected region we need to cover many points, so shortening the exposure time to only a couple of seconds can massively reduce the overall measurement time”, says Stijn Legrand, another PhD student at the department. “We also often notice preferred orientation effects, even sharp signals from single crystal species”, explains Legrand. “Our colleague Frederik Vanmeert is the best person to tell you about it.”
Our search after the preferred orientation takes us to Antwerp. Frederik Vanmeert greets us at the AXES department. “Indeed, some yellow pigments exhibit preferred orientation, and yes, some of them are poisonous”, smiles Vanmeert. “Chromium-based yellows were Van Gogh’s choice, and we wanted to know which one he used for the Sunflowers”, he continues. There are three chromium-based yellow pigments, which differ not only in hue, but also in composition, crystal structure, and their susceptibility to darkening due to the sensitivity to visible light.
Figure 2. The identification and distribution of chromium-based yellows on Van Gogh’s Sunflowers was carried out using XRD mapping. Courtesy of F. Vanmeert from Highly specific chemical imaging of Van Gogh’s Sunflowers with macroscopic X-ray powder diffraction (MA-XRPD) scanning, poster presented at Gordon Research Conference, Barcelona, 2018.
The shades of yellow were investigated using scanning XRD. However, as Vanmeert wanted to probe the deeper layers of the paint, the PILATUS3 was coupled with an Ag source and operated in transmission mode. The region was mapped with an exposure time of only 10 seconds per point. This revealed the presence of two yellows, of which one is light-sensitive . “Although both pigments are monoclinic and exhibit rod-shaped morphology, only the light-sensitive pigments show preferred orientation: the  orientation follows Van Gogh’s brush strokes”, concludes Vanmeert.
As we continue our tour of the department, Geert Van der Snickt shows us the selected posters, journal covers and group photographs. It becomes obvious that we have only scratched the surface of the rich scientific output of the group. In order to analyze and understand cultural heritage objects, the AXES group is moving between the lab, museums, and synchrotron sources, confidently using a variety of X-ray based techniques [6-8]. But the group doesn’t stop there.
“New projects are already planned”, reveals Geert Van der Snickt. “In the following years, we are planning to tackle oxalate films on the paintings, and we are also developing a new portable XRF-XRD machine!”
We thank the group for their wonderful hospitality and wish them all the best in their future projects. We hope to see them in Antwerp soon!