The scientific approach to materials is transchronological. There are two dating methods used by CIRAM laboratories:
- Carbon 14, reserved for organic materials such as wood, paper, ivory, bone, leather and textiles.
- Thermoluminescence, dedicated to terracotta and heated materials.
A global approach to precise dating
Please note that dating a work's support does not automatically correspond to its creation date. For the study of painted works, several types of imagery - natural light, grazing light, ultraviolet (UV) light or infrared (IR) reflectography - complement conventional dating techniques. However, one problem remains: how to analyze materials that cannot be dated, such as the pigments used in painting?
An anthropological and historical approach to complete the results
Historical knowledge of pigment manufacturing techniques will provide chronological data: the use of chromium oxide as a green pigment from 1840 onwards, for example, or the manufacture of titanium white (titanium oxide), which began in the 1920s. These investigations also have their limits: natural pigments provide no real chronological information, as in the case of ochre, which was used for the Lascaux cave paintings!
The principles of carbon-14 dating
Carbon-14 dating is based on the instability of the 14 isotope of carbon. Just as death interrupts human life, the life of flax or hemp ends when it is harvested. This is the moment that carbon 14 dates. A living organism contains a constant quantity of carbon 14, due to exchanges with the atmosphere (respiration or photosynthesis). When the organism dies, exchanges with the outside world cease, and the quantity of carbon 14 decreases according to a known exponential law. Its concentration is halved every 5,730 years. The dating limit is around 60,000 years. Beyond that, the quantity of carbon 14 is too low to be measured using current techniques.
This revolutionary technique earned its inventor, Willard Frank Libby, the Nobel Prize for Chemistry in 1960.
Carbon-14 variation over time, an important factor to take into account
Carbon-14 dates are expressed in "Before Present" or "BP" years. The "present" of carbon 14 was set at 1950 by Libby. Today, however, it is necessary to correct these values, as carbon-14 concentration has varied over time, depending on solar activity, climate change or industrial activity, for example. These results are calibrated using calibration curves. These curves transform the BP age into calibrated date intervals associated with a probability percentage (e.g. 450 ± 25 years BP corresponds, after calibration, to the interval 1422 - 1471 AD. - probability 95.4%).
The onset of industrial activity, for its part, led to a decline in Carbon 14 levels and imposed limits on the precision of measurements. Conversely, atmospheric nuclear testing has led to an artificial rise in Carbon 14 levels worldwide. This makes it possible to obtain very precise dates (up to one or two years) for the second half of the 20th century.
Some examples of paintings dated and authenticated by carbon 14
In the history of art, cases of falsification are common, and it is often thanks to a thorough analysis that inconsistencies have been detected. Here are two examples:
- A painting by a Russian Suprematist painter, dated 1920, had its canvas support finally dated post 1954.
- A Picasso composition proposed for dating in the early 20th century revealed, after dating the paper support, that the paper had been manufactured before 1954, and was therefore incompatible with the presumed period.
Generally speaking, it's much easier to prove that a painting is fake than to establish its authenticity:
- The dating of a painting attributed to Van Gogh was carried out in two stages. Dating of the support suggested a date compatible with the painter's activity, at the end of the 19th century. Pigment analysis led to a similar conclusion, since the pigments detected were known in the 19th century and had been found in other works by Van Gogh. These included vermilion red, copper arsenate green, zinc white and ochre. However, these pigments were known and used by all painters at the same time, and even today. The Van Gogh committee, made up of art historians, had to decide in favor of an original work.
- Conversely, analysis of a 14th-century illuminated parchment confirmed the dating of the support. As for the pigments, they proved to be modern... It was an "intelligent" copy made before 1950... on old paper. It is therefore essential to combine analytical techniques to detect forgeries.
Carbon-14 dating is the first essential step in authenticating a painting. But it must always be accompanied by chemical analysis of the pigments and scientific imaging. It is also necessary to complement these scientific investigations with a stylistic and historical analysis of the provenance and the opinion of art experts.
