PHOTON SCIENCE

An international team of researchers have found what triggers degradation in one of the most popular pigments used by renowned 19th and 20th century painters. Using a multi-method approach, including advanced techniques at the light sources ESRF in France and PETRA III at DESY, they have unveiled how light and humidity affect the masterpieces over time and have proposed a strategy for mitigation and monitoring. The results are out now in the journal Science Advances.

During the 19th century, the Second Industrial Revolution sparked major advances in chemistry, giving rise to synthetic pigments that transformed art. Among them was “emerald green”, a vivid copper arsenite pigment admired for its brilliance and intensity. Emerald green was used by well-known late 19th and early 20th century painters, such as Paul Cézanne, Claude Monet, Vincent van Gogh, Edvard Munch and Robert Delaunay. Some of these painters, including Van Gogh, quickly realised that the paint would change over time, losing its original brilliant colour, cracking and triggering surface deformations. It was discovered later that it was also highly toxic.

Researchers believe emerald green degrades because its chemical composition is highly unstable under light, humidity and certain atmospheric gases. These conditions can cause the pigment to react and release arsenic compounds, alter its colour or form dark copper oxides. “It was already known that emerald green decays over time, but we wanted to understand exactly the role of light and humidity in this degradation”, explains Letizia Monico, senior researcher at the Institute of Chemical Sciences and Technologies "Giulio Natta" (SCITEC) of CNR in Italy, corresponding and first author of the publication, together with Sara Carboni Marri, a former PhD student from the same research group. The study aims to improve strategies for preserving the masterpieces containing this pigment and to develop new methods to monitor their conservation state.

With these objectives, the researchers used different methods to carry out a study of paint mock-ups, historical paintings and related microsamples across multiple length scales. First, the team carried out non-invasive, in situ analyses at the macro scale on the Belgian artist James Ensor’s iconic 1890 oil painting The Intrigue. The goal was to assess the composition and conservation state of the green areas and to identify suitable points for microsampling. From data acquired by University of Antwerp scientists, the team could identify the best spots to collect small samples of the green pigment. With these samples, the researchers then came to ESRF and PETRA III to use the bright X-rays produced at each facility. The team carried out micro-scale X-ray analyses at several ESRF beamlines and the PETRA III beamline P06. They then combined these findings with results obtained from macro- and micro-scale light-based analysis of artificially aged, laboratory-prepared oil paint mock-ups and historical paints, including an oil paint tube used by Norwegian painter Edvard Munch.

The mock-ups, which mimicked the composition of Ensor’s painting, were crucial in discovering that light and humidity activate distinct alteration pathways for emerald green in oil paints. The researchers found that humidity promotes the formation of arsenolite, a crystalline compound that makes the paint brittle and prone to flaking. Light, on the other hand, initiates oxidation of the arsenic compound, leading to a thin whitish layer that dulls the colour – and this most resembles the degraded pigments found in The Intrigue that were analysed at ESRF and PETRA III.

“These similarities support the conclusion that photo-oxidative degradation by light has altered the original emerald green of The Intrigue”, says Geert Van der Snickt, co-author and professor from the University of Antwerp. As a result, the team came up with a strategy for museums to detect and monitor the conservation status of emerald green paints in artworks. Micro-X-ray and infrared analyses can detect degradation products, complementing the more traditional colorimetric observations. The researchers also demonstrated a practical, non-invasive technique called external reflection infrared spectroscopy for assessing paintings directly in museums. This method is uniquely sensitive to the infrared signals of the degradation products containing the oxidised arsenic, allowing conservators to identify altered areas of paint on a macro scale.

The user team was led by SCITEC-CNR and the Department of Chemistry, Biology and Biotechnology of the University of Perugia in Italy and included scientists from ESRF, DESY and the University of Antwerp.

(from DESY News)


Reference:
Sara Carboni Marri et al., "Discovering the Dual Degradation Pathway of Emerald Green in Oil Paints: the Effects of Light and Humidity", Science Advances, 2025, DOI: 10.1126/sciadv.ady1807