How carbon dating works

Context: Radiocarbon dating is currently under the spotlight as the Supreme Court deferred the implementation of a direction given by the Allahabad High Court to conduct carbon dating and scientific survey of the ‘Shivling’ allegedly found on the Gyanvapi mosque premises in Varanasi.

Radiocarbon dating:

  • Radiocarbon dating is a technique developed to determine the age of certain archaeological artefactsof a biological origin (bone, cloth, wood and plant fibres).
    • It was developed in the late 1940s by University of Chicago professor Willard Libby (who won the Nobel Prize for Chemistry in 1960). 
  • The method utilises the properties of radiocarbon (carbon 14) which is an isotope of carbon that is unstable and weakly radioactive. The method is designed to measure residual radioactivity

The process: 

  • It is based on the amount of radioactive carbon and non-radioactive carbon measured in an organic sample. These amounts exist in a fixed ratio in all living organisms, but the ratio changes over time after death. 
  • Plants absorb carbon from the atmosphere as part of carbon dioxide and pass it on to other organisms across the food chain. The proportion between radioactive and non-radioactive carbon in all these organisms, therefore, is the same as their proportion in the atmosphere.
  • This ratio does not change throughout an organism’s lifetime. Although radioactive carbon decays over time, it gets replenished continuously as the organism regenerates tissue. After the organism dies, however, radioactive carbon decays without being replenished.
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  • For dating an organic sample, researchers measure both radioactive carbon (called C14) and the commonest form of non-radioactive carbon (C12). They calculate the ratio between the two amounts and compare this with the known ratio in living organisms. This tells them how much C14 has decayed.
  • Since, the rate at which C14 decays is already known. Having worked out how much has decayed in the sample, researchers can calculate the time it would have taken for that amount of decay – in effect, the age of the sample.
  • Radiocarbon dating does not, however, work with samples older than 55,000 years. By then, so much C14 would have decayed that it would no longer be possible to measure whatever remains.


  • Radiocarbon dating works only onorganic samples from the past, or something that was once alive and is now dead. That puts certain materials – such as rock – beyond its scope.
    • If the structure is made of rock, radiometric dating of various isotopes can determine the rock’s geological age – but not when it was sculpted, carved, or otherwise worked on by human hands
  • It may work on cement or mortar if there is an organic material trapped inside it, based on measuring the decay in the organic material. 
  • It can also be done on paint, which contains a mix of organic and inorganic pigments. 

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