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14 June 2009
Archaeological dating by re-firing ancient pots

Researchers in the UK have created a new way of dating archaeological artefacts that involves heating ancient pots to unlock their internal clocks. The relatively simple technique could become as important for dating ceramics as carbon dating is for organic materials, say the researchers at the Universities of Manchester and Edinburgh. The team has already dated ceramics from the Roman, medieval and modern periods to a high degree of accuracy, and they are now looking to establish a global research facility for the technique.
     The method relies on the fact that fired clay ceramics - like bricks, tile and pottery - start to chemically combine with water as soon as they are exposed to the atmosphere. A big breakthrough came when the researchers realized that this process has occurred at a predictable rate throughout history, related to temperatures. Now the researchers have turned their theory into a practical dating method.
     Moira Wilson of the University of Manchester and her team document how 'rehydroxlation dating' has so far dated objects up to 2000 years old, and they believe it could extend back as far as 10, 000 years. "Given the number and intensity of [dating] debates in archaeology, there is a huge gap in the field for this," Wilson said. Wilson was quick to point out that the water uptake in rehydroxlation is not the same as absorption - it is a much slower chemical process.
     The researchers calculated that the rate of reaction is independent of atmospheric moisture levels but is governed by the ambient temperature averaged over a ceramic's lifetime. The dating procedure involves measuring the mass of a sample of ceramic and then heating it to around 500 degrees Celsius in a furnace, which removes the water. The re-fired ceramic is then weighed immediately, using a highly accurate microbalance, to determine precisely the rate of water recombination. Once the rate is known, the age of the artefact can be extrapolated.
     At present, the most widely used alternative technique is thermoluminescence, which involves measuring the amount of light given off by a sample because this is related to the dose of radiation an artefact has received across its lifetime. One of the limitations is that it requires a lot of extra information about the archaeological site such as radiation levels, which may not be accessible if artefacts have already been sitting in a museum for many years.

Source: Physics World (8 June 2009)

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