Saturday, October 10, 2009

Rehydroxylation Dating-- testing a new tool!

While we were digging in the field this summer, a team of seven materials scientists (led by Moira Wilson) from the University of Manchester and the University of Edinburgh and Ian M. Betts, an archaeologist with the Museum of London, published a paper in the Proceedings of the Royal Society A called "Dating fired-clay ceramics using long-term power law rehydroxylation kenetics."

Three Michigan Technological University students, Helen Ranck, Patrick Bowen, and Jessica Beck, are trying to replicate the technique.

Materials scientists and engineers have known that ceramic minerals slowly reabsorb water from the environment after they are fired. Dr. Wilson's team discovered that the rate at which environmental water recombines with clay minerals as hydroxyls is governed by a kinetic law at the nano-scale. They found that the rate was influenced by temperature, but was not changed by the quantity of water present in the environment.

Scientists could actually measure the rate of water mass gain for any given ceramic fragment by heating a sample in a kiln and then waiting and measuring the increase in mass as the clay molecules slowly recombined with environmental moisture at a known temperature. Lab researchers can then calculate the date of firing with these known measurements:

1. This determined rate of water mass gain.
2. The mass of the sample after excavation (when it contained all the re-bonded water).
3. The mass of the sample after the test firing (mass of the ceramic fragment minus the molecularly recombined water mass that it had absorbed since it was fired).
4. The average temperature through time the sample experienced since firing in the depositional environment.

Of course, including the +/- error, universal in archeometric dating.

If this technique works as well as the authors assert, it will add another powerful tool to archaeological techniques around the world. It will also revolutionize the Utah Pottery Project. Remember that one of the main goals of our archaeological research is to reconstruct the learning process through which these individual potters or potting groups, such as the Davenport family, adapted their skills and knowledge to Utah's new environment and raw materials.

One of the biggest problems we've had is that we can not use the ceramic fragments themselves to date the features full of broken kiln wasters, such as Andy's excavation of the cellar feature this summer. We rely upon stratigraphic clues (that waster pile is older than X, but younger than Y) or other artifacts found in the deposits, like stylish imported ceramic fragments, which can be dated. The features full of industrial wasters only rarely also contain other clues, however.

There are some other archaeometric techniques archaeologists use to date ceramics directly, such as Archaeomagnetic Dating and Optically Stimulated Luminescence. These techniques are useful also, but are either very specific to only work on kiln foundations (archaeomagnetic) or expensive and require expertise we don't have (OSL or TL). Either way, most of those techniques work better when applied to the distant past, and not the nineteenth century.

Historical Archaeology is like ethnoarchaeology in many ways. Given that we are studying people and sites in the historic period, we know a great deal more about accurate temperatures than archaeologists studying deep antiquity. I have the advantage of knowing the year, and even the month, during which some of these ceramics were fired. I hope we can test and refine the technique to higher levels of precision.

If this technique works as described, we will have an inexpensive tool that will allow us to build direct chronologies from the waster fragments. We will be able to sort the Davenports' waste into categories and know which ones reflect the steep learning curve from the 1850s and which pieces have clues about ongoing improvement and the training of the next generation of potters!

Moira Wilson corresponded with me over the summer while we were digging, and I appreciate her encouragement to consider application of her team's work. I am excited to be working with this puzzle with Jaroslaw W. Drelich, an associate professor in MTU's Department of Materials Science and Engineering. The original article appeared here. Useful discussions and interesting commentary about rehydroxylation dating appeared here, here, and particularly here.

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