Engineers at Manchester University have developed a new way of dating archaeological objects that promises to be as significant a technique for dating fired-clay ceramics such as bricks, tiles and pottery, as radiocarbon dating has become for organic materials such as bone or wood.
A team from Manchester and Edinburgh Universities discovered the new technique, and, working with the Museum of London, the team has been able to date brick samples from Roman, medieval and modern periods with remarkable accuracy.
The team has established that its technique can be used to determine the age of objects up to 2,000 years old – but believes it has the potential to be used to date objects around 10,000 years old.
The method relies on the fact that fired-clay ceramic material will start to chemically react with atmospheric moisture as soon as it is removed from the kiln after firing. This continues over its lifetime, causing it to increase in weight – the older the material, the greater the weight gain.
In 2003, the Manchester and Edinburgh team developed a new law that precisely defines how the rate of reaction between ceramic and water varies over time.
The application of this law underpins the new dating method because the amount of water that is chemically combined with a ceramic provides an ‘internal clock’ that can be accessed to determine its age.
The technique involves measuring the mass of a sample of ceramic and then heating it to around 500oC in a furnace, which removes the water.
The sample is then monitored in a microbalance to determine the precise rate at which the ceramic will combine with water over time.
Using the time law, it is then possible to extrapolate the information collected to calculate the time it will take to regain the mass lost on heating – revealing the sample’s age.
The researchers have calculated that a Roman brick sample with a known age of around 2,000 years was 2,001 years old. A further sample with a known age of between 708 and 758 years was calculated to have an age of 748 years.
They also tested a ’mystery brick’, with the real age only revealed to them once they had completed their process. This known age was between 339 and 344 years – and the new technique suggested the brick was 340 years old.
During the course of its research, the team also found that ceramic objects have their internal date clocks ’reset’ if they are exposed to temperatures of 500oC.
Used on medieval brick from Canterbury, the technique repeatedly dated a sample as being 66 years old. Further investigation revealed that Canterbury was devastated by incendiary bombs and fires during a Second World War blitz in 1942. The intense heat generated by the bombing had reset the dating clock by effectively refiring the bricks.
The results also proved accurate enough to show that a brick sample from the King Charles building in Greenwich came from reconstruction carried out in the 1690s and not from the original building, which was constructed between 1664 and 1669.
Dr Moira Wilson, senior lecturer at the School of Mechanical, Aerospace and Civil Engineering (MACE), said: ’We are extremely excited by the potential of this new technique, which could become an established way of determining the age of ceramic artefacts of archaeological interest.
’The method could also be turned on its head and used to establish the mean temperature of a material over its lifetime, if a precise date of firing were known. This could potentially be useful in climate-change studies.’
The three-year, £100,000 project was funded by the Leverhulme Trust, with the microbalance – which measures mass to 1/10th of a millionth of a gram – funded by a £66,000 grant from the Engineering and Physical Science Research Council (EPSRC).
Researchers are now planning to look at whether the new dating technique can be applied to earthenware, bone china and porcelain.