An international team of French, Italian and Slovenian scientists have deciced to develop and new, more resistant king of coating for the bronze statues of our urban property.
In 2016 they came at SOLEIL, on the ANTARES beamline, to study their new coating in detail. As part of our series 'Next Chapter,' we reconnected with the scientists filmed in 2016 and asked them how their research has progressed since then.
In this video filmed in 2016, Luc Robbiola (TRACES, CNRS) shows the gradual deterioration of the bronze statue “Les Bourgeois de Calais” (Musée Rodin) due to weathering and pollution.
The bronze statues we sometimes admire in public spaces are inevitably deteriorating. This is visible on their streaked surfaces: rain gradually washes away the copper from the bronze, giving the masterpiece a greenish tint. As if that weren’t enough, urban pollution deposits a black crust on certain areas. If nothing is done to counter these processes, the statue could simply disappear over time.
There are waxes and resins that can be applied to the metal to protect it. However, their effectiveness only lasts about ten years and is not optimal. To better safeguard our heritage and develop an improved coating, we need to understand what happens at the molecular level, right at the surface of the artwork. This is a mission for... an “archaeometallurgist.” Luc Robbiola specializes in the physical chemistry of ancient metals, and in 2016, he turned to the SOLEIL synchrotron to investigate protective films.
He subjected a coated bronze fragment to the powerful X-rays of the ANTARES beamline. This allowed him to better understand the bonds formed between the atoms of the polymer and the copper, tin, zinc, or lead present in the bronze. The project was, in fact, carried out by an international team: France was responsible for characterization, Slovenia focused on fluorinated compounds, and Italy on organosilanes. “The product developed by the Italians showed good results—although it had one drawback: it smelled like garlic,” recalls Luc Robbiola with a smile.
No fewer than eight publications crowned the progress of this research, from 2017 to 2020, and PhD student Julia Masi was awarded a prize by the journal Coatings.
However, the complex international structure of such a project lacked funding to move to the pre-industrial phase. “We changed our strategy. Our method was well-developed, we had the tools and a good understanding of the phenomena. So we moved on to another metal—historical aluminum,” summarizes Luc Robbiola. There are indeed heritage objects made of this metal, such as airplanes from the 1920s onward. Some of these are historic wrecks recovered from the sea, which have been heavily corroded.
Luc immersed himself in “industrial archaeology” to understand the processes used, for example, during World War II to protect airplanes. He conducted analyses on the LUCIA beamline at the SOLEIL synchrotron to study the alterations involved in the corrosion process, this time working with a project involving French, Italian, and Czech teams.
Another avalanche of publications resulted from the progress of this research. “We are even studying an eco-friendly product, made with tomato cuticles, among other ingredients. It shows promise, but it has a slight undesirable orange tint.”
Many questions remain to be resolved before obtaining the ideal protective polymer and evaluating it on an industrial scale. But Luc Robbiola and his international team, like a durable alloy, are holding strong.
