I know many people are very excited about RHX dating (and also very skeptical). I was very excited about the proposed technique when it was published in 2009. This technique proposes to use the clock-like, nano-scale process by which water molecules bond with clay mineral crystals. By taking an old ceramic sherd, heating it to remove any mass caused by humidity (atmospheric water), measuring it's mass, then firing the sample at a higher temperature to drive off all the rehydrated (adhered) water and rehydroxylated (chemically bonded) water, one can measure the sample's mass without any water present. After that, carefully tracking the mass of the sample as it quickly starts to reabsorb water from the air allows you to generate an equation that models the time past, the water mass gained, and the rate at which this occurs. So long as you can match the temperature in the room to the average lifetime temperature of the object, a bit of math lets the lab technician calculate how long it took for the sample to reach the weight at which it was discovered by archaeologists. All my posts about RHX dating are all here.
In 2010, we applied for an NSF grant to study this process and see if we could replicate the findings of the UK researchers that had proposed it. We did win that grant, but we did some background work and tried to replicate their study. We published our results in 2011. With that publication under our belt, we reapplied to NSF for more funding to upgrade our lab equipment to match the quality of that being used by the UK team. At first, we were rejected, but then the NSF found a bit of money that allowed us to improve our instruments. Using that grant, we purchased a new Citizen CM11 Microbalance along with a Coy Humidity Control Glove Box. This microbalance allows us to measure 0.001 mg of variation in mass, fully two orders of resolution and sensitivity higher than the balance that was available at Michigan Tech before the NSF award! It's was a lesser piece of equipment than we'd hoped to purchase, but it has permitted us to take our experiments to the next level of research quality.
In November-December 2011, We tested the station to determine the stability of humidity and temperature within the box as well as the stability of microbalance while it is reading in the box chamber environment. Between December 2011 and August 2012, Jarek and Patrick collected and analyzed the mass-gain data corresponding to rehydration/rehydroxylation of Davenport sherds and some brick samples from Houghton, Michigan. After running tests on intact sherds, we also ran some pulverized samples. Jarek and Patrick worked over the data to examine the influence of relative humidity on mass gain and time curves. From that analysis, we just submitted an article for review and publication. This one deals with the question of which model (expressed in an equation) best describes the samples behavior as they rehydrate and rehydroxylate over time. We also raised some questions about the micropore structure of the sherds and how that influences the model.
Figure 2 is from our report to NSF. It shows the fractional mass versus time obtained for the Davenport pottery sample under constant temperature and humidity (plot on the left hand side).
|Figure 2. Compiled fitting results for intact Davenport sherd in both linear time (left column) and time1/n with n= 3.77 (right column). Results were obtained at 22oC and 20%RH.|
The right-hand plot in Figure 2 also shows the resulting theoretical fit using the following empirical equation describing the mass gain (m) (or fractional mass gain):
where t is the time, β is the term representing physically bonded water, α is the rehydration kinetics term, γ is the rehydroxylation rate, and n is the rehydroxylation exponent.
Agreement between experimental data and theoretical model is remarkable!
We found problems that present the researchers with challenges to making RHX a standard archaeometric tool for dating fired clay ceramic artifacts. I will write more about those in another post, but I can't say too much until our article is published. I'm also excited to report that our initial success led to a new award from NSF (BCS-1219540)!
Our new collaboration will allow our team at Michigan Technological University to coordinate an experiment involving five different international teams of faculty and students (Michigan Technological University, Arizona State University, California State University-Long Beach, Tel Aviv University, and The University of New England) working on new RHX experiments, advised by a team of UK scientists, many of whom initially proposed this technique, and continue to work on it's refinement (Universities of Manchester, Edinburgh, and Bradford). We will send a set of blind samples for testing to five labs including materials scientists and archaeologists from seven different universities in four countries.
I am very excited!