Radiocarbon dating for the detection of forgeries

The radioactive isotope carbon-14 (C-14) used for dating with a half-life of 5715 ±30 years is produced by the bombardment of atmospheric nitrogen-14 with cosmic neutrons. Therefore, in dynamic equilibrium, the atmosphere contains the tiny concentration of 1.25 × 10-10 percent C-14, which is assimilated by photosynthesizing plants like ordinary carbon and is taken up by herbivores and indirectly by carnivores. But after their death, the stored C-14 decays and none is taken up again for lack of metabolism. The now steadily weakening radioactivity of the sample is a measure of the time that has elapsed since the death of the organism under study.

Thanks to accelerator mass spectrometry (AMS), which measures the ratio of C-14 to the stable carbon isotope C-12, the sample quantity required for reliable dating has been reduced to a few milligrams. In the case of a painting canvas, a short snippet of thread is sufficient. Even tiny samples of the paints can be dated if they contain carbon-containing compounds, i.e. mostly polymerized oil.

To reach the sensitivity of AMS with an ordinary radioactivity measurement, one would have to be able to detect a single decaying atomic nucleus every two months, which is completely unthinkable. However, the sensitive AMS variant of C-14 dating is much more complex and therefore more expensive than conventional counting of decaying atomic nuclei. But this method can reliably date samples that are up to 60,000 years old. This corresponds to more than ten half-lives of C-14.

Recently, radiocarbon dating has also been used to detect painting forgeries. Recently, forensic analyses of artworks were carried out at the University of Paris-Saclay in the case of one suspicious painting each of the Impressionist and Pointillist styles. Accordingly, they had to date from the early years of the 20th century and be over 100 years old. Radiocarbon dating of the canvases, however, revealed an age of no more than 70 years, when the imitated artists had long since died.

Too fresh colors

In this way, the two paintings were clearly identified as forgeries. They came from the workshop of a Paris restorer, where numerous other paintings by well-known artists were stored. Based on stylistic criteria, they had been created between the mid-19th and early 20th centuries. However, experts found the colors too fresh for their age, so radiocarbon dating was performed using threads of canvas and a brush hair that had been operated out of one of the paintings.

Paradoxically, such studies are facilitated by the large, temporary increase in C-14 concentration in all fossil plant and animal tissues due to atmospheric nuclear weapons testing between the 1940s and the 1960s. If one finds such an increased concentration of the radioactive isotope compared to earlier times, the sample cannot be older than the bomb tests; it may even be younger. In any case, the above-mentioned forgeries could be detected purely physically without the chemical microanalyses of the colors on the canvas that were common until now.

The use of radiocarbon dating to detect painting forgeries is not entirely new. It was probably first used in 2014 on the basis of a suspect painting in the Peggy Guggenheim Collection in Venice. In this way, the painting was identified as a forgery. Five years later, scientists at the University of Fribourg measured the radiocarbon age of a painting already known to be a forgery.

Source: Carolyn Wilke, Nature 603, 374 (2022).