The science of such a survey, if undertaken, would depend on what the structure is made of. If it contains cement or mortar, radiocarbon dating may be possible on any organic material trapped inside it. 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.

Radiocarbon dating can also be done on paint, which contains a mix of organic and inorganic pigments. But if a rock artefact has not been painted on, it will require an art analyst to examine the sculpting or chiselling and put a date to it, based on existing samples from the same period.

The science of carbon dating

Radiocarbon dating is a technique developed in the late 1940s by University of Chicago professor Willard Libby, who would go on to win the Nobel Prize for Chemistry in 1960. 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 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.

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.

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.

Dating various materials

Radiocarbon dating can estimate the age of a sample taken from an animal, a human or a plant. It also works on objects made of organic matter, such as paper, leather or wood. Bricks and mortar, terracotta or ceramic objects, stucco, plaster and paint, too, can contain organic matter.

Although cement and mortar are not organic, radiocarbon dating of objects made of such materials has been the subject of several studies. This method is based on measuring the decay in organic material that has been deposited in cracks and trapped after the cement or mortar hardened. There is a rider here: this method will put a date to when cement or mortar was added to the structure, but the structure itself may have been shaped by human hands much earlier.

Because radiocarbon dating relies on the relative amounts of radioactive and non-radioactive carbon, which changes with time after the death of the organism, it does not work on something that was never alive in the first place.

For objects such as rock, there are other techniques. Under the broad umbrella of radiometric dating, which includes radiocarbon dating, various processes look at the decay of various isotopes. These include uranium (U238 and U240), potassium-40, and rubidium-87. As with C14 dating, the amount of decayed material is measured, the ratio between original and decayed material is calculated, and the sample’s age follows.

Radiometric dating, however, will only determine the geological age of the rock. “One of the limitations of this method is that it cannot date a sculpture,” said Sharada Srinivasan, an archaeologist with IISc’s National Institute of Advanced Sciences, and a Padma Shri.

Dating on the basis of a rock’s geochemistry would make sense only if the rock’s chemical profile was altered measurably at very high temperatures. Such processes were not generally practised in pre-modern times, Srinivasan said, while stressing that she was not familiar with the structure in Gyanvapi.

“If you chip on a rock, it’s just a mechanical thing, and does not alter its chemical profile. As it is not a pyrotechnological process, there are no geochemical markers of the event of working on stone to be able to calibrate to construct a relative chronology on that basis alone,” she said.

Dating a stone artefact

Sarada Natarjan, an art historian with Azim Premji University, works in the field of dating artefacts, including those made of stone, based on a stylistic analysis. While describing these processes, she too said that she was not familiar with the object found on the Gyanvapi complex.

“We art historians usually deploy a combination of formal-stylistic analysis of the artefact or architectural site, the geographical, local and immediate physical context of the object, literary references to the site and inscriptions in the vicinity if we are lucky, some hints from the rendering of stone, our prior knowledge of cultic practices and iconographical trends, and other extraneous elements to arrive at the dating of stone artefacts,” Natarjan said.

To date metal objects, she said, chemical analysis and spectrometry can be used, “provided we have some background information on metallurgical practices of different periods”.