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ART+Science CARBON-14 DATING How does it work? Beginning in the late 1940s, American researchers began to use the natural radioactive properties of carbon-14 to date organic materials. Soon after, in the 1950s, Willard Frank Libby began to date Egyptian samples with great success. He was rewarded for his work with the 1960 Nobel Prize for chemistry. Carbon-14 dating, also called radiocarbon dating, makes it possible to determine how much time has elapsed since an organism’s death (the felling of a tree or the death of an animal, for example). This technique, which has revolutionized archaeometry, allows for the dating of wood, ivory, bone, teeth, hair, textile, paper, and shell. The method is based on the measurement of the amount of carbon-14 remaining in the tested material. Carbon-14 is a radioactive carbon isotope, which is to say that it disappears over time. A living organism contains a constant quantity of carbon 138 14 as a result of its exchange with the atmosphere (respiration or photosynthesis). When it dies, this exchange with the atmosphere ceases and the quantity of FIG. 1: Carbon-14 exponential disintegration curve with chronological references. © CIRAM. By Olivier Bobin and Armel Bouvier carbon-14 diminishes at a known rate. Its concentration is divided in half every 5,730 years. The measurement of the amount of carbon-14 present in a sample compared to its total original content is what makes it possible to date materials. Measurements can be made on samples up to 50,000 years old. Beyond that age, the quantities of carbon-14 present are too small to be measured using the techniques we have at our disposal today. Measurements are made using a mass spectrometer in conjunction with a particle accelerator. Very little material is now needed to perform the test (today as little as 0.01 of a gram, although in the past 1 gram was needed), it takes very little time to perform (about an hour, where days or weeks were needed before), and results are much more accurate than they used to be using older methods. Dealing with Pollutants The sample is first subjected FIG. 2: Linear particle accelerator and associated mass spectrometer (AMS). FOR TRIBAL ART FIG. 3 (right): Salt container. Sapi-Portuguese. 16th century. Ivory. H: 23.1 cm. Private collection. Reproduced by kind permission of Entwistle, London. Photo: Ed Parrinello/SquareMoose. FIG. 4 (below): View of the back of a wooden mask and of the sample removal area (2 to 3 millimeters in diameter). © CIRAM.


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