The dating game

Here we learn that Tony Robinson and his time team could have a new weapon in their archaeological arsenal, and it could be as useful as carbon dating… 

RESEARCHERS at the Universities of Manchester and Edinburgh have developed a new technique for dating ceramics, bricks and tiles that are thousands of years old. This technique, which could prove to be as useful as carbon dating, has been developed around a water sorption analyser (a CI Electronics Cisorp) that enables a sample's mass to be measured to within 0.1ug under tightly controlled conditions of temperature and relative humidity.

Not only does the technique make it possible to date artefacts with a high degree of confidence, which is useful for archaeology and for detecting forged artefacts, but it also offers the potential for studying specimens of a known age for investigating climate change. The research findings have been published in a paper - 'Dating fired-clay ceramics using long-term power law rehydration kinetics' - in the Proceedings of the Royal Society.

Key to the new dating technique is the fact that fired-clay items absorb moisture from the atmosphere throughout their lifetime, with the rate of absorption being related to the average ambient temperature and the characteristics of the clay. It has been established that a small (typically 3-5g) sample can be heated to 105°C so that capillary moisture is removed to give the 'initial as received' mass, then heating to 500°C for four hours removes all of the moisture that has accumulated over the sample's lifetime. The difference in mass between the 'initial as received' mass and the final mass represents the moisture that has been absorbed over the sample's lifetime.

Next, the sample's mass is monitored under conditions of controlled temperature and relative humidity while the sample cools and adsorbs moisture. This enables the kinetics of mass gain by recombination with water to be determined. Relative humidity is normally maintained at 30.0 +/- 0.1% RH and the temperature is maintained at the long-term mean temperature that the sample will have experienced (to within +/-0.2°C).

Absorption of moisture, which in this context is termed rehydroxylation, follows a 1/4-power law. Mass data is collected from the Cisorp every 30 seconds for a period of typically two to five days; the graph can then be extrapolated to the 'initial as received' weight, thereby enabling the sample's age to be determined. When a blind sample from the King Charles II building in Greenwich was provided by the Museum of London, the researchers determined the original date of firing to be 1691 +/- 22 years. In fact the building was constructed in 1664-1669 with alterations in the 1690s is a good fit with the age determined by the new dating technique. Other samples up to 2000 years old have been analysed successfully and the researchers believe the technique would be equally valid for samples up to 10,000 years old.

Several factors make the Cisorp well suited to this application. Clearly the ability to measure mass to 0.1ug is vital, as is the long-term stability, but so too is the tight control over the temperature and relative humidity in the sample chamber. Furthermore, the integral microbalance has dual weigh pans, which means that two samples can be analysed in parallel to increase the productivity.

Dr Moira Wilson, senior lecturer at the School of Mechanical, Aerospace and Civil

Engineering at the University of Manchester, says that the Cisorp produced much better data than an alternative technique: "At first we tried using a traditional top-loaded pan, but the results exhibited too much scatter. When we tried the Cisorp, with its microbalance head, we were very impressed by the clean line on the resultant graph."

While the Cisorp is not the only water sorption analyser on the market, Dr Wilson had no hesitation in selecting the Cisorp: "A colleague of mine had used a CI Microbalance System before and thought it was excellent, and colleagues in the University of Bristol also had nothing but praise for CI microbalances. CI Electronics offered to test samples for us to prove the Cisorp would fulfil our requirements and being UK-based was a distinct advantage."

Furthermore, Dr Wilson is very complimentary about the ongoing service and support she and her team received from CI Electronics while developing the new dating technique.

It has long been known that ceramics absorb moisture, but measuring the resultant very small strain (expansion) is extremely difficult. Changing to mass-based measurements not only creates opportunities for dating ceramics, but it also makes it easier to investigate problems associated with moisture absorption in modern ceramics - such as the cracking of glazes.

One of the beauties of the new dating technique is that the only apparatus required is a small furnace and the water sorption analyser - which can also be used to measure the 'initial as received' mass and the mass before rehydroxilation commences. This makes the technique simpler, quicker and less expensive than the existing method for dating ceramics known as thermal luminescence.

Following the successful development of the fired-clay dating technique, Dr Wilson is now preparing for further work for the Cisorp, such as measuring hydration and carbonation rates in cementitious materials, and investigating the influence of particle size in chemisorption kinetics in powdered ceramics.