The dating technique, which earned a Nobel Prize in its discoverer Willard Frank Libby in 1960, “revolutionized archeology”, reminds AFP Lucile Beck in front of the imposing machine which occupies an entire play in the (CEA) complex in Saclay, in the Paris region.
The director of the C14 measurement laboratory (LMC14) recounts “the surprise” of the prehistorians when, in the 1990s, her colleagues managed to date the homes of the artists of the Chauvet cave (Southeast).
The masterpiece of parietal art, which could only be after Lascaux, according to experts, was in fact 36,000 years old!
50,000 years
The laboratory dates each year 3,000 to 4,000 samples for its own research and that of the French “national scientific community”: National Center for Scientific Research, Ministry of Culture, CEA, Research Institute for Development (IRD) and Nuclear and Radiation Protection Authority (ASNR).
Each sample is first examined to eliminate any traces of contamination. “”Typically, sweater fibers“From the archaeologist who manipulated the object, laughs Mrs. Beck.
It is cleaned in acid and basic and burnt baths at 800 ° C to recover the carbon dioxide (CO2). A gas then reduced to graphite and inserted in tiny capsules. Direction the particle accelerator, which will separate and measure carbon isotopes.
An isotope is a variant of the same chemical element, having a different number of neutrons, which changes its atomic mass.
Some isotopes are stable, such as carbon 12. Others are radioactive and disintegrate over time, such as carbon 14.
This is created in the upper atmosphere by bombing of cosmic and solar radiation on nitrogen.
In the atmosphere, this carbon oxidizes and becomes CO2 which is absorbed by plants during photosynthesis, then by animals that consume these plants.
All living organisms contain carbon 14. When they died, he disintegrated in an exponential way. There are only half left after 5,730 years. And almost nothing after 50,000 years, limit of the technique.
By comparing the number of carbon 12 and 14 particles at the exit of the accelerator, we obtain a relative age.
“”If carbon 14 production did not fluctuate over time, we could transform directly into the rate that remains in the sample“Explains Ms. Beck.
But the cosmic radiation “is not constant”. Just like the intensity of the magnetic field that protects us.
Scientists had to develop calibration curves, based on samples whose age is known with certainty.
This ultimately makes it possible to identify a false table, determining that the linen of the canvas was harvested after the death of the supposed painter.
Or establish a climate chronology, analyzing the Skeleton in Plankton calcite at the bottom of the oceans.
Lead white
If carbon 14 allows you to date bones, wood or fabrics, the LMC14 has developed advanced techniques to analyze materials that are not directly from the living.
For example, dating the carbon “trapped” in iron, when the ore was heated with charcoal. And thus show that the iron staples of Notre-Dame de Paris did not date of its restoration, but of its construction.
Or by using lead white, a pigment synthesized since the 4th century BC, which is found on many buildings and works of art.
To make it, we “put lead to corrode with vinegar and horse growing, which produces CO2 by fermentation”, says Ms. Beck. The pigment thus contains organic carbon.
“”I always say to archaeologists: do not clean the traces of corrosion, they also tell our past!“, She continues.
This is what made it possible to date the graves of a medieval abbey in which only small lead bottles had been found. By breaking down, the bodies had gone to CO2 that had corroded them.
“”This corrosion was ultimately the only witness who remained of the spirit of the monk“, Thinks of the researcher.