Original article: A Hidden Portrait by Edgar Degas

Many of us dream of becoming famous and leaving behind cultural heritage for the next generation. For artists, leaving behind paintings to be shown in galleries must be great, but what are the consequences? Here comes the downside of fame. Plenty of researchers will try to dig deeper and reveal some aspects of your life that you might not want to share… or expose the artworks that you would not consider to be great… or even reveal a painting that was hidden beneath the paint layers of a different painting!

That is exactly what happens in the world of restoration and conservation science. With modern analytical techniques, scientists are able to scan through the layers of paint and discover underpaintings that were painted over a long time ago (see this previous bite: Hidden Skeleton: Using scientific imaging tools to reveal the previous lives of art objects). This is exactly what happened with a painting by Edgar Degas! After his painting, Portrait of a Woman, experienced some degradation, traces of a portrait of a different model became visible through the uppermost paint layers (Figure 1).

The hidden portrait was first visualized using a conventional X-radiography technique, in which the thickness of the paint was used to obtain the image seen in Figure 1b. The hidden portrait was found to be oriented upside down relative to the final artwork, and it appeared to be unfinished. Following that, extensive X-ray fluorescence microscopy (XFM) was used to obtain the elemental composition of the hidden portrait at a high spatial resolution, which was then used to show the underpainting in greater detail.

X-ray fluorescence works by bombarding the atoms of heavy metals with an X-ray beam, which causes the ejection of core electrons in these atoms. This ejection also causes the emission of characteristic X-rays, which allow us to identify the element composition at each point of our painting.  Individual elemental maps are shown in Figure 2a, which show us how much of each element is present throughout our measured area based on brightness.

Unfortunately, the elemental maps do not explicitly reveal the identity of specific pigments, since the same element can be found in many different pigments. The researchers found a way to circumvent this issue by assigning  pigments based on the co-location of multiple elements that are known to be present in the same pigment.

For example, zinc (Zn), which is mostly visible on the face of the model, is usually associated with zinc white (ZnO). The cobalt (Co) that was detected likely corresponds to a blue pigment that was mixed with other pigments to achieve realistic flesh tones. Using this approach, they were able to identify the tones on the face of the hidden model by identifying the associated mixture of pigments suitable for representing the color of flesh.

Using the XFM elemental maps, the scientists were able to generate a false color reconstruction of the underpainting by assigning probable colors and transparencies to each of the elemental maps and then overlapping them. The result (2b) reveals a portrait of Emma Dobigny, one of Degas’ favorite models at the time the painting was made.

This extensive, non-invasive, analytical study of Degas’ painting, Portrait of a Woman, revealed a portrait of a different woman hiding in the paint layers. The resolution that was attained in this work is a result of recent developments in chemical imaging, and with them, we are able to even detect the brushstrokes of the underpainting. This case study demonstrates a promising methodology for the authentication of paintings, as well as the investigation of hidden paintings, revealing a whole new world of hidden heritage.