Why synthetic emerald-green pigments degrade over time

The introduction of synthetic pigments in the 19th century transformed the art scene, especially with the advent of vibrant emerald-green hues. Esteemed artists such as Paul Cézanne, Edvard Munch, Vincent van Gogh, and Claude Monet embraced these brilliant colors. However, a significant drawback emerged: these synthetic pigments have a tendency to deteriorate over time, leading to cracks, uneven surfaces, and the formation of dark copper oxides, along with the potential release of arsenic compounds. This poses a major challenge for those dedicated to conserving these artistic treasures. Recent advancements in research have provided hope. European scientists have harnessed synchrotron radiation and a variety of analytical techniques to investigate the role of light and humidity in the degradation process, as detailed in a study published in the journal Science Advances. The application of scientific methods has become essential for art conservationists, particularly through the use of advanced X-ray imaging techniques. For example, in 2019, it was revealed that oil paintings at the Georgia O’Keeffe Museum in Santa Fe, New Mexico, had developed tiny blisters resembling acne over decades. Chemists determined these were metal carboxylate soaps, resulting from chemical interactions between metallic ions in lead and zinc pigments and fatty acids from the paint's binding medium. As these soaps coalesce, they create blisters that migrate through the paint layers. This issue of deterioration is not confined to contemporary works; it has been observed across various periods, including in pieces by the legendary Rembrandt. The Metropolitan Museum of Art in New York City is actively engaged in a project aimed at understanding the causes and mechanisms behind metal soap formation in traditional oil paintings. Furthermore, in 2023, researchers from the Rijksmuseum’s Operation Night Watch discovered rare traces of lead formate in a Rembrandt painting, adding another layer to the understanding of pigment degradation. In March 2022, scientists analyzed Jean-Baptiste-Camille Corot’s Gypsy Woman with Mandolin, employing multiple techniques to study the composition of the harmful metal carboxylate soaps present in the paint's upper layer.