X-ray imaging shows how 17th century paintings lost their color

As beautiful as it may look, works of art are not immortal. For example, pigments and binders in oil paintings are inevitably degraded. Fluctuations in light, humidity, and temperature are common causes, but exposure to certain cleaning solvents during conservation and improper mixing of pigments by the artist can also destabilize the paint over time.

The task of conservation scientists is to understand the chemical reactions that cause degradation to answer three questions: How was the painting created, how did it first appear, and how did it change—both naturally and by intervention? The questions are not completely backwards. By reconstructing how the painting deteriorated, conservators may be able to prevent further damage and better preserve it.

Painting conservator and doctoral student Nouchka De Keyser (Rijksmuseum, University of Amsterdam, and University of Antwerp), his advisers Katrien Keune and Koen Janssens, and their colleagues scientifically answered all three questions in their analysis of the yellow rose of the mid-Abraham Mignon. 17th century painting Still Life with Flowers and Watches,¹1. N. De Keyser et al., science. Adv. 8, eabn6344 (2022). https://doi.org/10.1126/sciadv.abn6344 shown in the picture 1. Mignon painted his yellow roses with mineral orpiment (As₂S₃), used by artists since time immemorial to give a bright and vibrant appearance. But orpiment can be a problem. Over time, this mineral can discolor greatly, changing the look of painted orange curtains, lemons, yellow flowers, and gold metallics in old masterpieces.

Many artists, perhaps including Mignon, are aware of this and other mineral problems—it doesn’t dry well, doesn’t mix well with other pigments, and is highly toxic. However, it remained widely used until the 18th century. And orpiment isn’t the only troublesome pigment. In Vincent van Gogh’s 1888 painting Bedroom, for example, fading red pigment turns purple walls blue and pink floors brown. De Keyser and his colleagues wanted to understand what happened in the case of Mignon’s yellow roses. “The most exciting part of my job,” he says, “is playing detective, looking for evidence of certain chemical reactions, and retracing their steps to find out what’s really going on in an artist’s mind.”

Chemical analysis

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Most of the flowers in Mignon’s paintings remain brilliant. But roses stand out as flat, monochrome, and flavored with microcracks. De Keyser and his colleagues first analyzed the roses using x-ray fluorescence imaging. When an x-ray shines on a surface, it can knock out the nuclear electrons from the atoms in the paint. The emission of electrons, in turn, pushes the outermost valence electrons down from a higher orbital to a lower orbital and fluoresces. The wavelength of light is a characteristic of the chemical elements in the paint layer that absorb x-rays. And when an x-ray beam and photon detector are scanned rasterically over a painting, the resulting image reveals the spatial distribution of the elements.

The researchers mapped the location of arsenic, calcium, iron, sulfur, lead and copper in rose-bearing areas. Surprisingly, the analysis revealed features of the painting—the light and shadow that define the petals and stamens—that are optically invisible in the now-degraded rose image (Fig. 1a). But because the elements are still there, albeit in different molecular forms, the arsenic maps of their microscale distribution (Fig. 1b) uncovered the rose in most of its former glory. To compare the distribution of a particular element, see figure 2.

X-ray fluorescence cannot, however, resolve certain chemical compounds, in which the orpiment changes over the centuries. So the group turned to x-ray diffraction. Because the pigments were initially ground into a powder to make the paint mixture, the randomly oriented grains in Mignon’s canvas allowed the researchers to avoid the alignment difficulties associated with single-crystal diffraction. Indeed, obtaining the molecular specificity of powder diffraction is becoming an increasingly important technique for studying old paintings.²2. V. Gonzalez et al., Euro Chemistry. J. 26, 1703 (2020). https://doi.org/10.1002/chem.201903284

To resolve the molecular structure on the surface of the painting, the group used an instrument developed in the Janssens lab at the University of Antwerp. In reflection mode, the x-ray strikes the paint surface at a shallow 10° angle. De Keyser and his raster colleagues scanned the instrument across an area of ​​the rose in 1.5-millimeter steps with an exposure time of 10 seconds per pixel. Overall, the scan took 13 hours.

The powder diffraction map primarily identifies two lead-schultenite arsenates (PbHAsO₄) and mimetite [Pb₅(AsO₄)₃Cl]. The reaction leading to them begins with the photooxidation of the orpiment to arsenolite (As₂HI₃), a semi-soluble molecule that can diffuse throughout the multilayer paint system. When the oxide encounters lead ions, further reactions promote the precipitation of schultenite and mimetite. Each has a different spatial distribution in the painting.

The transparent and the visible

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Schultenite and mimetite do not have a bright yellow orpiment appearance; no, they are colorless and pale yellow crystals, respectively. And when mixed with calcite (CaCO₃), gypsum (CaSO₄·2H₂O), and quartz (SiO₂)—another mineral identified by powder diffraction and whose refractive index matches the oil—yellow paint used to make roses almost transparent. The orpiment crystals are still in the painting, but only along the rose border. The initial prevalence of the pigment is now lost, chemically turning into mostly transparent crystals.

The fluorescence map shows the results. Iron seeps into the rose’s surface, and diffraction maps identify it in the form of goethite, the main ingredient of yellow ocher. Like other 17th-century still life painters, Mignon is thought to have adopted the multistep method. He first blocked the position of the flowers with an ocher-based monochrome underpainting and then built the details by applying glaze for the shadows and orpiment for the sunlit parts.

In that approach, he marked the location of the roses using cheap ocher. Indeed, because the original orpiment has faded to transparency, the ocher undercoat is now the only optically visible remnant. Modern roses look dull, flat, and monochrome—the opposite of what Mignon wanted.

Reference

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1. 1. N. De Keyser et al., science. Adv. 8, eabn6344 (2022). https://doi.org/10.1126/sciadv.abn6344, Google scholarshipcross reference
2. 2. V. Gonzalez et al., Euro Chemistry. J. 26, 1703 (2020). https://doi.org/10.1002/chem.201903284, Google scholarshipcross reference

1. © 2022 American Institute of Physics.

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