Police rely on radiocarbon dating to identify doctored paintings

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The researchers used radiocarbon dating to identify two works of art as fakes.Credit: Steve Russell/Toronto Star/Getty

Radiocarbon dating unmasked two doctored paintings in France – possibly the first time the technique has been used in a police investigation. The paintings were believed to be Impressionist and Pointillist works from around the turn of the 20th century. But a team led by heritage scientist Lucile Beck of the University of Paris-Saclay used radiocarbon levels in the fibers of their webs to date them to the past 70 years. The researchers concluded that the paintings are modern forgeries in a Feb. 4 report published in International Forensic ScienceI1.

The use of radiocarbon dating is gaining ground in the forensic analysis of works of art, thanks to advances that require smaller samples than ever before. Removing smaller samples from artwork is more acceptable to auction houses, museums, and painting owners. If there’s a chance a painting might be genuine – and therefore valuable – they don’t want the collection of larger samples to damage it, says art historian Anna Tummers of Leiden University in the Netherlands, which was not part of the new research.

The success of the technique could persuade more of the art world to seek radiocarbon dates, which can more accurately determine when a painting was made, Tummers says. Researchers typically use imaging and chemical analysis to detect art forgeries. These methods can look under brushstrokes to see how materials in a painting have aged, but cannot conclusively determine the date of a painting.

The consequences of forged works of art go beyond forgers lining their pockets in the global art market, which moves tens of billions of dollars every year. Counterfeits pollute people’s understanding of the meaning of artwork, says Tummers. “If we don’t carefully weed them out, it could really distort our understanding of our own heritage and our own history.”

Suspected counterfeits

The two paintings were part of a treasure trove of artworks that French investigators discovered in a restorer’s studio in 2019. Of some 600 paintings, dozens appeared to be mid-level masterpieces of the late 19th and early 20th centuries. But experts questioned the authenticity of the works because the paint appeared relatively fresh.

Close up of a sample of fiber removed from the back of a canvas with a pair of scissors

Beck’s team removed a fiber from the canvas of an impressionist painting, to test if the piece was a fake.Credit: L. Beck

To investigate potential counterfeits, the French government’s Central Office for the Fight against Illicit Trafficking in Cultural Property (OCBC) called on Beck. When she entered OCBC’s office, she was amused to see what would normally be a drab office with paintings displayed on its floor and tables.

Beck’s team selected a few works, including an impressionist garden scene and a pointillist harbor landscape, to test. In the office, the researchers used scalpels to remove samples, including a small piece of fiber, from the webs.

All living things absorb carbon, including radioactive carbon-14, from the atmosphere and from food. When a plant – such as flax or hemp, commonly used to make canvas – dies, the carbon-14 it has incorporated continues to decompose. Radiocarbon dating measures what’s left to estimate how much time has passed, says Mariaelena Fedi, a physicist at the National Institute of Nuclear Physics in Florence, Italy. The technique gives an absolute earliest date of a work of art, as it can take years from harvesting flax for a canvas to making the painting.

Atomic bomb testing, which began in the 1940s and took off in the 1950s, boosted the amount of carbon-14 in the atmosphere beyond naturally produced levels. Carbon-14 peaked around 1964 and declined after a partial nuclear test ban. Researchers can easily identify modern bomb-derived radiocarbon-containing materials because their carbon-14 concentrations are above pre-1950s levels.

Beck’s team tested their samples to see if they bore the signature of this bomb-derived radioactive carbon-14. In the lab, the researchers cleaned and dried the material, reducing several milligrams to about one milligram of carbon which was pressed into a graphite puck to be measured with accelerator mass spectrometry.

The canvas fibers of the Impressionist and Pointillist paintings clearly contained carbon from the mid-1950s or after the year 2000, the researchers reported. (That’s because the carbon-14 concentrations they measured could match either side of the atomic bomb’s peak.) Another fiber, taken from the varnish of pointillist painting and possibly from a paintbrush, also dated after 1950. Beck agrees that ideally the team would perform further chemical analysis to support their findings, but the researchers were limited by the tight schedule of the investigation.

Advances in the field

Although this appears to be the first report of radiocarbon dating being used to identify fake artwork in a police investigation, researchers have been laying the groundwork over the past decade.

Fedi and his colleagues made the first radiocarbon dating report to detect a forged painting in the Peggy Guggenheim collection in Venice, Italy, in 2014. The team collected snippets of the canvas and dated them after the death of the alleged artist – concluding that the painting had been forged2.

In 2019, Laura Hendriks and her colleagues used a known forgery of a village scene to test a radiocarbon dating method that used a much smaller sample size than previous techniques. Hendriks, a chemist at the University of Applied Sciences and Arts of Western Switzerland in Fribourg, converted a sample of the scene’s oil paint into carbon dioxide before feeding it into the mass spectrometer. The team was able to date the fake using only micrograms collected from the board3. The small sample was “just a few crumbs of dust, basically,” Hendriks says.

Such advances are good news for the field. There is a huge need for objective tools to detect forgeries, says Fedi, and radiocarbon dating is great when combined with other methods and the expertise of art specialists who can help interpret the history of these complex objects.

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