How a 17th-century painted rose lost its color

As beautiful as they are to look at, art masterpieces are not eternal: Pigments and binders in oil paintings inexorably deteriorate. Light, humidity, and temperature fluctuations are the usual culprits, but exposure to some solvents or the mixing of incompatible pigments can also render paint unstable over time. In their effort to understand the chemical reactions that caused degradation in a particular painting, art conservators try to answer three questions: How was the painting made? How did it originally appear? And how did it change? The questions are not entirely backward-looking. By reconstructing how a painting deteriorated, conservators may be able to forestall further damage and better preserve it.

Paintings conservator and doctoral student Nouchka De Keyser (Rijksmuseum, University of Amsterdam, and University of Antwerp) and her colleagues have scientifically addressed all three questions while examining a yellow rose in Abraham Mignon’s painting Still Life with Flowers and a Watch (circa 1660–79), shown in panel a in the figure. Yellow roses have long been a problematic motif. To create the flowers’ bright and vibrant appearance, artists since antiquity have used orpiment (As₂S₃). But over time the mineral severely discolored the appearance of such items as orange draperies, lemons, yellow flowers, and the sheen of golden metal in several old masterworks. Although artists were aware of those and other problems, they continued to widely use orpiment until the 18th century.

In Mignon’s painting, most of the flowers retain beautifully preserved details. But the rose stands out as flat, monochrome, and peppered with microcracks (see panel b). De Keyser and her colleagues first analyzed the rose using x-ray fluorescence imaging. That technique mapped the spatial distribution of elements, particularly arsenic, calcium, iron, sulfur, lead, and copper. Their presence revealed painterly features—light and shadows defining the separate petals and stamens—that are optically invisible in the actual image. Because those elements still reside in the paint layers, the map of the microscale distribution of arsenic uncovers the rose in much of its former glory (see panel c).

Fluorescence cannot resolve the elements’ molecular phases, though. To identify them, the group used an in-house x-ray powder diffraction scanner. They noninvasively mapped the presence of crystalline pigment phases and degradation products and analyzed the paint surface in reflection mode. The diffraction map primarily identified two lead arsenates as degradation products—schultenite, or PbHAsO₄, and mimetite, or Pb₅(AsO₄)₃Cl. According to the authors, the presence of those lead arsenates accounts for the colorless appearance of the yellow rose. The original orpiment pigment still exists in the painting only along the border of the rose. The mineral’s bright, vivid yellow in other parts of the flower were transformed into reaction products that are largely transparent.

Like other 17th-century still-life painters, Mignon is thought to have adopted a multistep method to produce his work. He first blocked out the position of the flowers with a monochrome underpainting and then filled in details by applying glazes for the shadows and orpiment in lighter sections for sunlit parts. Accordingly, the position of the yellow rose was marked with a yellow ocher underpaint rich in iron. In its now degraded state, that underpaint is the only part that is optically visible. The fluorescence map reveals it as a layer rich in iron, and the diffraction map identifies its molecular source as goethite, a key ingredient in the yellow ocher. The result is dull, flat, and monochrome—the opposite of what Mignon would have intended for his lovely rose. (N. De Keyser et al., Sci. Adv. 8, eabn6344, 2022 .)