Original article: Full‐field XRF instrument for cultural heritage: Application to the study of a Caillebotte painting

How can we ‘see’ hidden layers underneath a painting, or determine trace amounts of impurities in a pigment without taking a sample? The answer is with X-rays, but more specifically a phenomenon known as X-ray fluorescence (XRF). X-ray analysis is often used in a museum setting to answer questions related to an artist’s technique and materials, authenticity of an object, and conservation treatment history. Recent work described in this article demonstrates the use of a novel full-field XRF imaging system to collect non-invasive high spatial resolution elemental data from a painting in a museum exhibition space. This data provided insight on the identity of the pigments used and how that relates to the artist’s method.

When illuminated by an X-ray source, elements that make up a pigment will interact with incoming X-rays. If the energy is greater than the electron shell binding energy, the electron will be ejected and one from a higher shell will replace it, causing fluorescence as shown in Figure 1. These fluorescent X-rays are unique for each element. Pigments composed of elements with a high atomic number, like arsenic (As), will have secondary X-ray peaks at higher energies. Conversely, pigments composed of elements with a light atomic number, such as carbon (C), will have lower secondary X-ray energies, as shown by the spectra in Figure 1.

The past decade has seen significant developments in XRF instrumentation, which has opened doors for exciting studies in cultural heritage research. Specifically, scanning macro-XRF (MA-XRF) imaging has evolved from a technique that could only be done at a government-operated synchrotron facility to commercially available instrumentation found in museum laboratories worldwide. As XRF imaging becomes a standard technique to analyze artworks, researchers are improving data collection and processing, as well as alternative instrumental methods for performing XRF imaging.

One example of a new XRF imaging method, full-field XRF, is demonstrated in a recent publication from Philippe Walter and coworkers (DOI: 10.1002/xrs.2841). Full-field XRF was initially developed for use in future planetary exploration missions. The full-field XRF instrument (Cartix) uses a custom lens composed of a grid of square-shaped micropores (inspired by the structure of lobster eyes!) (Figure 2) to reflect and focus collected secondary X-rays on the detector. Image data is collected as a x-y-time “data cube” with x-y “slices” at a range of discrete X-ray energies. The data cube is then processed to create element-specific maps, such as those shown in Figure 3.

The Cartix instrument was taken into the exhibition galleries of the Museum of Art and History Baron Gérard in Bayeux, France to study the 1876 painting, Portraits in the Country by Gustave Caillebotte. Caillebotte was active during the Impressionist period and was well-known for his depiction of domestic and interior scenes. Figure 2 shows how the instrument was mounted on a tripod and XRF analysis was performed directly in the galleries. The authors mapped a small area of the painting focusing on one of the women’s faces, shown in Figure 3. The authors were able to characterize the pigments used. For example, the authors detected small brushstrokes of cobalt blue (Co), iron earth orange (Fe), and chrome yellow (Cr) pigments to create shadows in the woman’s face. This shadow effect accomplished by use of complementary colors, rather than using brown or black, was a common technique employed by Impressionist painters.

Full-field XRF is a promising and practical alternative to techniques like MA-XRF. The instrument allows for a fixed area image to be acquired with high spatial resolution in a relatively short period of time. A major advantage of full field XRF is its portability; being able to perform analysis in a relatively short period of time directly in a non-laboratory space is a major advantage when dealing with objects that are challenging to transport to labs for analysis. One avenue that full field XRF could be useful for in the future is for time-resolved measurements, such as monitoring pigment deterioration or assessing a new conservation treatment.