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‘This process occurs at slower and slower rate with time and therefore the general practice is to not use bricks that are fresh out of the kiln,’ says materials chemist Christopher Hall of the University of Edinburgh, UK, and part of the original RHX dating team.
Hall says the key to their dating ‘clock’ was the discovery that the expansion occurs with defined kinetics.
‘We proposed in the last paragraph [of our 2003 paper] in an off-hand way that this could possibly be exploited as a means of dating and the media of the entire planet descended on us,’ Wilson recalls.
A new method for dating ceramics was an exciting prospect for archaeologists.
Being controlled only by the half-life of the isotope, these methods are insensitive to changes in temperature or the surrounding chemical environment.
Finding chemical processes that might provide reliable clocks is much more difficult – but perhaps not impossible.
Batt says many advances have been made since the initial work.
‘You start to destroy the hydroxyl groups that are associated with the clay,’ says Hall.
‘The idea came about by accident,’ explains Wilson, when researchers in Manchester found they could use heat to shrink samples of brick of all ages, from modern to Roman.
They found a relationship between the brick’s age and the amount of shrinkage; older bricks showing more shrinkage.
‘Two neighbouring hydroxyl groups react to eliminate a water molecule and leave one bridging oxygen.’ On cooling, the reverse process occurs and water becomes chemically associated with the clay minerals.
‘My own view is essentially it’s the beginning of the rock weathering process.Any scientific method for dating pottery is very attractive.’ Currently the most well developed method is thermoluminescence dating.