Called ‘rehydroxylation dating’, the method is based on the discovery that fired-clay materials, such as brick, tile and ceramic, recombine chemically with environmental moisture at a constant and measurable rate after firing. The water mass gained since the sample was first fired is calculated by measuring the mass of a ceramic sample before heating the sample to 500˚ºC and measuring the mass again. Next the precise rate at which the ceramic material recombines with atmospheric water is monitored using a device known as a microbalance, which measures mass to 1/10th of a millionth of a gram. Once the absorption rate for that sample is known, it is possible to calculate the time it will take to regain the mass lost on heating, revealing the sample’s age.
Complicating the method is the fact that the rehydroxylation rate depends on several variables, of which the mean lifetime temperature is the most critical. For this reason, the technique works best for determining the age of objects up to 2,000 years old whose recombination rate can be calibrated using historical weather data, but the Manchester team believe it has the potential to be used to date objects up to 10,000 years old. Dr Moira Wilson, a member of the development team, says the dating formula also works in reverse, in that ‘the mean temperature of the material over its lifetime can be calculated if the precise date of firing is known’ and says that ‘this could potentially be useful in climate change studies’.
Working with the Museum of London, the team has been able to date brick samples of known date from Roman, Medieval and modern contexts. They also blind-tested a brick and assessed its age as 340 years (its known age was then revealed to be 339 to 344 years old). A Medieval brick from Canterbury caused confusion when it was repeatedly assessed as 66 years old in tests; eventually the team realised that the brick had been, in effect, refired, and its water mass gain reset, when it was burned during the World War II blitz. •
Further reading: Dating fired-clay ceramics using long-term power law rehydroxylation kinetics in the Proceedings of the Royal Society A (http://rspa.royalsocietypublishing.org/content/465/2108/2407).
This article is an extract from the full article published in World Archaeology Issue 37. Click here to subscribe