X-rays show how a 17th-century painting lost its color

As beautiful as they are to look at, masterpieces of art do not last forever. For example, pigments and binders in oil paintings degrade inexorably. Light, humidity and temperature fluctuations are the usual culprits, but exposure to certain cleaning agents during preservation and the artist’s mixing of incompatible pigments can also render the color unstable over time.

Conservation scientists’ job is to understand the chemical reactions that cause degradation in order to answer three questions: how was the painting made, what did it originally look like, and how has it changed—either naturally or through intervention? These questions are not entirely backward-looking. By reconstructing a painting’s deterioration, restorers may be able to prevent further damage and better preserve it.

Painting restorer and PhD student Nouchka De Keyser (Rijksmuseum, University of Amsterdam and University of Antwerp), her advisors Katrien Keune and Koen Janssens, and their colleagues all have three questions in their analysis of a yellow rose in Abraham Mignon’s mid-17th-century painting Still life with flowers and a clock,¹1. N. De Keyser et al., Science. adult 8th, eabn6344 (2022). https://doi.org/10.1126/sciadv.abn6344 shown in figure 1. Mignon painted his yellow roses with the mineral orpiment (As₂S₃), which has been used by artists since ancient times to create a bright and vibrant appearance. But orpiment can be problematic. Over time, the mineral can severely discolor, changing the appearance of painted orange curtains, lemons, yellow flowers, and golden metal in ancient masterpieces.

Many artists, possibly including Mignon, were aware of this and other problems with the mineral—it dries poorly, is incompatible with other pigments, and extremely toxic. Nonetheless, it remained widespread into the 18th century. And Orpiment wasn’t the only troublesome pigment. In Vincent van Gogh’s 1888 painting The bedroom, for example, the fading of red pigments turned its violet walls blue and its pink floor brown. De Keyser and her colleagues wanted to understand what happened in the case of Mignon’s yellow rose. “The most interesting part of my job,” she says, “is playing detective, looking for evidence of certain chemical reactions and retracing its steps to find out what an artist really had in mind.”

Chemical analyzes

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Most of the flowers in Mignon’s painting remain brilliant. But the rose stands out as flat, monochromatic and riddled with microcracks. De Keyser and her colleagues first analyzed the rose using X-ray fluorescence imaging. When an X-ray shines on the surface, it can knock out a core electron from an atom in the paint. This electron emission, in turn, causes an outer valence electron to fall from a higher to a lower orbital and fluoresce. The wavelength of the light is characteristic of chemical elements in the paint layers that have absorbed the X-rays. And when the X-ray beam and photon detector are scanned across the painting, the resulting image reveals the spatial distribution of these elements.

Researchers mapped the locations of arsenic, calcium, iron, sulfur, lead, and copper in the area where the rose was located. Surprisingly, the analysis revealed painterly features—light and shadow that define the petals and stamens—that are optically invisible in the now degraded image of the rose (Figure 1a). However, since the elements are still there, albeit in a different molecular form, the arsenic map of their microscale distribution (Figure 1b) reveals the rose mostly in its former glory. To compare specific element distributions, see figure 2.

However, X-ray fluorescence cannot resolve certain chemical compounds that Orpiment has transformed into over the centuries. So the group turned to X-ray diffraction. Because pigments were originally ground into powder to create the color mixture, the randomly oriented grains in Mignon’s canvas allowed researchers to avoid alignment difficulties associated with single-crystal diffraction. In fact, determining molecular specificity by powder diffraction is becoming an increasingly important technique for studying ancient paintings.²2. V. Gonzales et al., Chem. EUR. J 26, 1703 (2020). https://doi.org/10.1002/chem.201903284

To resolve molecular structures on the painting’s surface, the group used an instrument developed in Janssens’ laboratory at the University of Antwerp. In reflection mode, X-rays strike the paint surface at a flat 10° angle. De Keyser and her colleagues scanned the instrument over the area of the rose in 1.5 millimeter increments with 10 second exposure times per pixel. The scan took 13 hours in total.

These powder diffraction maps identified two main lead arsenates – schultenite (PbHAsO₄) and mimetite [Pb₅(AsO₄)₃Cl]. The reactions leading to them begin with the photooxidation of orpiment to arsenolite (As₂O₃), a semi-soluble molecule that can diffuse through the multi-layer paint system. When the oxide encounters lead ions, subsequent reactions result in the precipitation of schultenite and mimetite. Each of them has a specific spatial distribution in the painting.

The transparent and the visible

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Schultenite and Mimelite do not have the light yellow appearance of Orpiment; Rather, they are colorless or pale yellow crystals. And when it is mixed with calcite (CaCO₃), gypsum (CaSO₄·2H₂O) and quartz (SiO₂)—other minerals identified by powder diffraction, whose refractive indices match those of oil—the yellow paint used to make the rose becomes virtually transparent. Orpiment crystals are still present in the painting, but only around the edge of the rose. The pigment’s early distribution is now over, chemically converted into largely transparent crystals.

The fluorescence map confirms the result. Iron is ubiquitous on the rose’s surface and the diffraction map identifies it in the form of goethite, a key component of yellow ochre. Like other still life painters of the 17th century, Mignon is said to have employed a multi-stage method. He first blocked the position of the flowers with a monochrome ochre-based underpainting, then built up the detail using glazes for shadows and orpiment for sunlit parts.

Using this approach, he marked the location of the rose with the inexpensive ochre. As the original orpiment has become transparent, the ocher underpainting is the only visible remnant. The modern rose looks matte, flat, and monochromatic—the opposite of what Mignon would have intended.

references

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1. N. De Keyser et al., Science. adult 8th, eabn6344 (2022). https://doi.org/10.1126/sciadv.abn6344, Google Scholarcross reference
2. V Gonzales et al., Chem. EUR. J 26, 1703 (2020). https://doi.org/10.1002/chem.201903284, Google Scholarcross reference

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X-rays show how a 17th-century painting lost its color

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