GLOBAL – In a significant breakthrough for art conservation, an international team of scientists has pinpointed why the vibrant emerald-green pigments used by 19th-century masters like Claude Monet and Vincent van Gogh degrade over time, causing iconic artworks to darken and crack. The study, published in the journal Science Advances on Wednesday, 20 November 2025, identifies light as the primary culprit, a finding with major implications for how museums worldwide, including the National Museums of Kenya (NMK), approach the preservation of cultural treasures.

The investigation focused on microsamples from Belgian painter James Ensor’s 1890 masterpiece, “The Intrigue,” housed at the Royal Museum of Fine Arts in Antwerp. Researchers from Italy’s National Research Council (CNR) and the University of Perugia led a collaborative effort utilizing powerful synchrotron radiation—a highly intense form of X-ray light—to analyze the chemical breakdown of the pigment. Their analysis revealed a dual degradation pathway: while high humidity can cause the paint to become brittle, it is prolonged exposure to light that triggers a chemical reaction, oxidizing the arsenic in the pigment and forming a whitish surface layer that dulls the original brilliant green.

The Science of Fading Colours

The pigment in question, known as emerald green or Paris green, is a copper acetoarsenite (Cu(C2H3O2)2·3Cu(AsO2)2). It was first commercially produced in Germany in 1814 and became immensely popular among artists for its vivid, intense hue, but its instability was suspected even by the artists themselves. The pigment is also highly toxic due to its arsenic content.

“It was already known that emerald green decays over time, but we wanted to understand exactly the role of light and humidity in this process,” explained Letizia Monico, a researcher at CNR-Scitec and a key author of the study, in a statement released around 20 November 2025. The team used a combination of non-invasive mobile instruments and advanced synchrotron X-ray techniques at facilities in Grenoble and Hamburg to map the chemical changes at a microscopic level. This allowed them to distinguish between two separate mechanisms: light exposure causing arsenic oxidation (As³⁺ → As⁵⁺) on the paint's surface, and high moisture (relative humidity ≥ 95%) promoting the formation of arsenolite crystals, which leads to flaking and brittleness. The study concluded that for “The Intrigue,” and likely other paintings using the pigment, light is the main risk factor.

Local Relevance and Global Lessons

While this research centers on European masterpieces, its findings underscore a universal challenge that resonates with conservation efforts in Kenya. The National Museums of Kenya is the custodian of the country's vast cultural and natural heritage, from priceless paleontological fossils to contemporary African art. According to a February 2024 report from the Office of the Auditor-General, NMK faces challenges in the conservation and restoration of heritage sites, sometimes attributed to a lack of prioritization and budgetary allocation for these activities.

The advanced scientific methods employed in the emerald green study highlight the critical role of technology in modern conservation. Although Kenya may not have a synchrotron facility, the principle of using scientific analysis to diagnose deterioration and inform preventive strategies is directly applicable. NMK has previously partnered with international institutions like the British Museum for conservation projects, such as the restoration of artworks from the Murumbi African Heritage Collection, which involved training for local staff on preventive care for delicate materials like skin, paper, and beads. Such collaborations are vital for knowledge exchange and building local capacity.

The European study's definitive conclusion about the damaging effects of light reinforces the importance of controlled environments in museums. For the NMK, which manages collections across the country, this scientific evidence provides a stronger basis for advocating for and implementing international best practices in gallery lighting, climate control, and storage to safeguard its diverse and irreplaceable collections for future generations. The research serves as a powerful reminder that the preservation of cultural heritage, whether in Antwerp or Nairobi, is an increasingly scientific endeavor that requires continuous investment in research, technology, and skilled personnel.