Rehydroxylation dating

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Rehydroxylation [RHX] dating is a developing method for dating fired-clay ceramics.[1] It is based on the fact that after a ceramic specimen is removed from the kiln at the time of production, it immediately begins to recombine chemically with moisture from the environment. This reaction reincorporates hydroxyl (OH) groups into the ceramic material, and is described as rehydroxylation (RHX).[2] The RHX process produces an increase in specimen weight. This weight increase provides an accurate measure of the extent of rehydroxylation. The dating clock is provided by the experimental finding that the RHX reaction follows a precise kinetic law: the weight gain increases as the fourth root of the time which has elapsed since firing.[3] This so-called power law and the RHX method which follows from it were discovered by scientists from the University of Manchester and the University of Edinburgh.[4]

The concept of RHX dating was first stated in 2003 by Wilson and collaborators[3] who noted that "results ... suggest a new method for archaeological dating of ceramics". The RHX method was then described in detail in 2009[1] for brick and tile materials, and in relation to pottery in 2011.[5]

RHX dating is not yet routinely or commercially available. It is the subject of a number of research and validation studies in several countries.

Power-law kinetics

According to the RHX power-law, if the weight of a fired-clay ceramic increases as a result of RHX by 0.1% in 1 yr from firing, then the weight increase is 0.2% in 16 yr, 0.3% in 81 yr and 0.4% in 256 yr (and so on). The RHX method depends on the validity of this law for describing long-term RHX weight gain on archaeological timescales. There is now strong support for power-law behaviour from analyses of long-term moisture expansion data in brick ceramic, some of which now extends over more than 60 y.[6] Moisture expansion and weight gain are known to be proportional to each other for a specified material at any specified firing temperature.

Dating methodology

A small piece of the ceramic is first removed, weighed, and heated to 500°C, effectively dehydrating it completely. The amount of water lost in the dehydration process (and thus the amount of water gained since the ceramic was created) is measured with a microbalance.[1] Once removed from the furnace, the sample is monitored to determine the precise rate at which it combines with atmospheric moisture. Once that RHX rate is determined, it is possible to calculate exactly how long ago it was removed from the kiln.[4] If the date of firing of a certain ceramic were known from another source, the method could be used inversely to determine the average temperature of the object's environment since firing.[7]

Technical issues

The RHX rate is largely insensitive to the ambient humidity because the RHX reaction occurs extremely slowly, and only minute amounts of water are required to feed it. Sufficient water is available in virtually all terrestrial environments. Neither systematic nor transient changes in humidity have an effect on long-term rehydroxylation kinetics, though they do affect instantaneous gravimetric measurements or introduce systematic error (i.e. through capillary condensation).[8]

The rate of rehydroxylation is affected by the ambient temperature. Thus, when calculating dates, scientists must be able to estimate the temperature history of the sample. The method of calculation is based on temperature data for the location, with adjustments for burial depth and long-term temperature variation from historical records.[9] This information is used to estimate an effective lifetime temperature or ELT which is then used in the dating calculation.[5] The ELT is generally close to (but not exactly the same as) the long-term annual mean surface air temperature. For southern England this is about 11°C.

Any event involving exposure to extreme heat may reset the "clock" by dehydroxylating the specimen, as though it were just out of the kiln. For example, a medieval brick examined by Wilson and collaborators[1] produced a dating result of 66 years. In fact this brick had been dehydroxylated by the intense heat of incendiary bombing and fires during World War II.[10]

Research

The RHX technique was the product of a three-year study by a collaboration of University of Manchester and University of Edinburgh researchers, led by Moira Wilson. Though it has only been established on ceramics of up to 2,000 years of age, research is continuing to determine whether RHX can be accurately used on ceramics of up to 10,000 years of age. Research is also underway on earthenware, bone china and porcelain.[4]

Studies attempting to replicate the original work of Wilson et al. are limited. A 2011 attempt by Bowen et al. at Michigan Technological University did not yield data that were in good agreement with that generated by the Manchester and Edinburgh groups when the entire set of data was considered. Instead, the group proposed a generalized power law that seemed to provide a better fit to rehydroxylation mass-gain data collected from fired 18th century Davenport (Parowan, Utah) pottery.[11] Efforts to successfully replicate the original work are still underway at Michigan Tech.

References

  1. ↑ 1.0 1.1 1.2 1.3 Wilson, Moira A.; Carter, Margaret A.; Hall, Christopher; Hoff, William D.; Ince, Ceren; Wilson, Moira A.; Savage, Shaun D.; McKay, Bernard et al. (8 August 2009). "Dating fired-clay ceramics using long-term power law rehydroxylation kinetics". Proceedings of the Royal Society A 465 (2108): 2407–2415. 
  2. ↑ Hamilton, Andrea; Hall, Christopher (2012). "A review of rehydroxylation in fired-clay ceramics". Journal of the American Ceramic Society 95 (9): 2673–2678. doi:10.1111/j.1551-2916.2012.05298.x. 
  3. ↑ 3.0 3.1 Wilson, Moira A; Hoff, William D; Hall, Christopher; McKay, Bernard; Hiley, Anna (2003). "Kinetics of moisture expansion in fired clay ceramics: a (time)1/4 law". Physical Review Letters 90 (12): 125503. Bibcode:2003PhRvL..90l5503W. doi:10.1103/PhysRevLett.90.125503. 
  4. ↑ 4.0 4.1 4.2 "Fire and water reveal new archaeological dating method". ScienceDaily. May 25, 2009. 
  5. ↑ 5.0 5.1 Wilson, Moira A; Hamilton, Andrea; Ince, Ceren; Carter, Margaret A; Hall, Christopher (2012). "Rehydroxylation (RHX) dating of archaeological pottery". Proceedings of the Royal Society A 468 (2147): 3476–3493. 
  6. ↑ Hall, Christopher; Wilson, Moira A; Hoff, William D (2011). "Kinetics of long-term moisture expansion in fired-clay brick". Journal of the American Ceramic Society 94 (1): 3651–3654. doi:10.1111/j.1551-2916.2011.04831.x. 
  7. ↑ "Rehydroxylation dating for ceramic materials". Compute Scotland. 19 May 2009. 
  8. ↑ Drelich, J; Bowen, PK; Scarlett, TJ (March 2013). "Effect of humidity instability on rehydroxylation in fired clay ceramics". Journal of the American Ceramic Society. doi:10.1111/jace.12262. Retrieved 22 March 2013. 
  9. ↑ Hall, Christopher; Hamilton, Andrea; Wilson, Moira A (2013). "The influence of temperature on rehydroxylation (RHX) kinetics in archaeological pottery". Journal of Archaeological Science 40 (1): 305–312. doi:10.1016/j.jas.2012.06.040. 
  10. ↑ Dacey, James (June 8, 2009). "Archaeological dating by re-firing ancient pots". Physics World. 
  11. ↑ Bowen, Patrick K; Ranck, Helen J; Scarlett, Timothy J; Drelich, Jaroslaw W (Jaroslaw Drelich) (2011). "Rehydration/rehydroxylation kinetics of reheated XIX‐Century Davenport (Utah) ceramic". Journal of the American Ceramic Society 94 (8): 2585–2591. doi:10.1111/j.1551-2916.2011.04451.x. 
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