Is it light or humidity? Scientists identify the culprits of emerald green degradation in masterpieces

Is it light or humidity? Scientists identify the culprits of emerald green degradation in masterpieces

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 synchrotron radiation techniques, they’ve unveiled how light and humidity affect the masterpieces over time, and have proposed a strategy for its mitigation and monitoring. The results are out now in 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.

Light and humidity

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.

Now a research team led by the Institute of Chemical Sciences and Technologies "Giulio Natta" (SCITEC) of CNR and the Department of Chemistry, Biology and Biotechnology of the University of Perugia, in collaboration with the ESRF, the European Synchrotron, and the University of Antwerp, has investigated what triggers the degradation of emerald green. The study[1] aims to improve strategies for preserving the masterpieces containing this pigment and to develop new methods to monitor their conservation state. “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 SCITEC-CNR, corresponding and first author of the publication, together with Sara Carboni Marri, a former PhD student from the same research group.

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, they carried out non-invasive, in situ analyses at the macro scale on James Ensor’s iconic oil painting, The Intrigue (1890), housed in the Royal Museum of Fine Arts in Antwerp (Belgium).

The goal was to assess the composition and conservation state of the green areas and to identify suitable points for microsampling. The measurements were performed using portable instruments from the University of Antwerp’s research groups, with the support from the MOLAB, the mobile laboratory platform of the

European Research Infrastructure for Heritage Science. These analyses were crucial for establishing preliminary insights into the conservation state of the green paint areas in a non-invasive manner, and for informing the subsequent sampling for synchrotron radiation X-ray measurements.

With these samples, the researchers came at the ESRF to use its bright X-rays and carried out micro-scale X-ray analyses at three ESRF’s beamlines, using X-ray diffraction and X-ray absorption spectroscopy. Other experiments were also carried out at the German synchrotron DESY. “Measurements at synchrotron are crucial for these kinds of studies because they offer the only way to obtain specific information on the nature of various arsenic compounds within the paintings' stratigraphy at the submicron scale. Materials are so complex that a single technique may not be enough to get the full picture. At the ESRF, we try to make such technique combination easier”, explains Marine Cotte, scientist in charge of the ID21 beamline at the ESRF.

The team 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 Edvard Munch oil paint tube. 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 (As₂O₃), a crystalline compound that makes the paint brittle and prone to flaking. Light, on the other hand, causes the original trivalent arsenic to oxidize into pentavalent arsenic compounds mainly at the surface, leading to a thin whitish layer that dulls the colour.

Strategies to preserve masterpieces and to monitor their conservation conditions

Overall, the arsenic and copper speciation results showed that the degradation behaviour of the light-aged paint mock-ups strongly resembles that detected in the two analysed cross-sections of The Intrigue. “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.

All in all, light and humidity affect the emerald green, but it is specifically light that poses the primary, ultimate threat to The Intrigue and potentially other masterpieces. So, researchers come up with a strategy for museums to detect and monitor the conservation status of emerald green paints in artworks.

Visual and colorimetric assessments alone are not enough to accurately determine the degradation state of emerald green paints. As this study shows, these methods need to be complemented by micro-X-ray or infrared analyses, which can detect degradation products such as pentavalent arsenic compounds.

The researchers also demonstrated a practical, non-invasive technique for assessing paintings directly in museums: external reflection infrared spectroscopy. This method is uniquely sensitive to the infrared signals of the degradation products containing pentavalent arsenic, allowing conservators to identify altered areas of paint on a macro scale.
“This technique is extremely valuable for guiding targeted micro-sampling and for subsequent X-ray analyses, which help determine the exact degradation state and enable early detection and monitoring of damaged paint layers,” concludes Costanza Miliani, co-author of the study, coordinator of the European MOLAB platform for heritage science access, and Director of the CNR Institute of Heritage Science.

Reference: Discovering the Dual Degradation Pathway of Emerald Green in Oil Paints: the Effects of Light and Humidity, Science Advances - DOI: 10.1126/sciadv.ady1807

Scientist contacts:

CNR: Letizia Monico (letizia.monico@cnr.it), corresponding and first author - Costanza Miliani (costanza.miliani@cnr.it)

ESRF: Marine Cotte (marine.cotte@esrf.fr)

University of Perugia: Aldo Romani (aldo.romani@unipg.it), corresponding author

University of Antwerp: Geert Van der Snickt (geert.vandersnickt@uantwerpen.be), Koen Janssens (koen.janssens@uantwerpen.be)

 

The study has been developed within the activities of the Next Generation EU - NRRP CHANGES, SPOKE 5 "Science and Technologies for Sustainable Diagnostics of Cultural Heritage", CUP B53C22003890006