Medieval wall paintings can be dated with accuracy using nuclear techniques with carbon-14

Dating works of art

Researchers from the Carbon-14 Measurement Laboratory at the Paris-Saclay Center in the Alternative Energies and Atomic Energy Commission (CEA) in France have successfully dated samples of murals from the Château de Germolles (Burgundy, France) and fragments of wall plaster from an old rood screen, that has since disappeared, in Cordeliers Church in Fribourg (Switzerland). 

Separating organic from inorganic carbon

These paints are complex systems since they contain both carbon of organic origin, from lead white, for which radiocarbon dating has only recently been possible, and they also contain inorganic carbon from either another white pigment made from calcium carbonate or from building materials.

To achieve this discovery the researchers isolated the carbon from lead white using thermal separation, by heating the paint samples at low temperature. Under these conditions, only the carbon atoms from lead white are released as CO2 –a gas that is easy to recover– while the others remain bound to the calcium carbonate, which is stable up to 600 °C.

Determining the ¹⁴C age

The isotopic fraction of carbon-14 in the CO2 samples was then measured and, after statistical processing, the "¹⁴C ages" associated with these levels were determined using the carbon-14 calibration curve. These "ages" are made up of several time intervals that can sometimes be narrowed down using historical information.

For the Château de Germolles, for example, the acquisition date of the castle led researchers to limit the dating of the paintings to 1380-1400.

For Cordeliers Church, the results make it possible to differentiate between the two decors studied, the oldest of which dates back to 1426-1460. In both cases, the dates when the paintings were carried out are documented and are consistent with the results provided by the carbon-14 analysis.

Lead white

"Lead white is a pigment that was widely used by the greatest artists, particularly for painting skin tones," explains Lucile Beck, CEA researcher and head of the LMC14. It was also applied as a preparation layer in most paintings. As this layer often extends beyond the edge of the painting, one option could be to take sample from this area without damaging the work. We are now working to further reduce the mass of material required for the analysis in order to be able to perform absolute dating on easel paintings using this technique."

Carbon -14 dating

Carbon-14 dating requires organic materials from plants or animals that have taken in CO2 from the atmosphere during their lifetime. However, pigments are most often obtained by grinding inorganic minerals, which do not contain carbon-14, and the organic binders used by painters have degraded over time and now contain hardly any carbon. As for the material that is painted, wood for example, this can be much older than the painting and therefore cannot always be used to date the work accurately.

Carbon-14 dating of lead white started after the discovery of carbon-14 in lead carbonates used as cosmetics in Ancient Egypt and Greece. Researchers at LMC14 discovered that the carbon in the carbonates was of organic origin and confirmed that the pigment was produced by chemical synthesis, not by the grinding of minerals.

Thanks to the advances in this nuclear technique, we will gradually acquire better knowledge on the history and secrets of the great painting works of antiquity.

Source: CEA