Rehydroxylation (RHX) dating is a developing method for dating fired-clay ceramics. This new concept relies on a key property of ceramic materials, in which they expand and gain mass over time. 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. The phenomenon has been well-documented over the past one hundred years (albeit more focused on limited timescales), and has now been proposed as a means to date fired-clay ceramics. The RHX process produces an increase in specimen weight, and 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. This power law and the RHX method which follows from it were discovered by scientists from the University of Manchester and the University of Edinburgh.
The concept of RHX dating was first stated in 2003 by Wilson and collaborators who noted that the "results … suggest a new method for archaeological dating of ceramics". The RHX method was then described in detail in 2009 for brick and tile materials, and in relation to pottery in 2011. The archaeological pottery first used to test the developing RHX method consisted of three categories for which the dates had already been calculated by other archaeological means. The first was an Anglo-Saxon loom-weight from 560 to 660 AD, a Samian-ware sherd from 45 to 75 AD, and three Werra earthenware sherds from 1605 AD. The types of samples used were deemed important for the experiment since they represented "three specific perceived issues associated with applying the RHX method to excavate archaeological pottery." These issues included potsherds found in waterlogged sites, low-firing-temperature ceramics, vitrified ceramics, and those containing a slip or a glaze. 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
The concept of clay ceramic expansion, post-firing, has been the object of discussion for a long time, first noted by Schurecht in 1928 to explain crazing in ceramic glazes, and confirmed in 1954 by McBurney that this and the expansion of ceramic bodies is due to the intake of moisture from the environment. Moisture expansion has since been an important property of clay ceramics to consider when using the material, such as clay bricks in construction. In 2003, it was proposed that moisture expansion could extend over much longer periods of time, contrasting with the previous research over more limited time scales. This was evaluated on bricks ranging from the Roman period to modern ones. It was ascertained that moisture expansion follows a power law: mass gain and expansion depend on time1/4 across archaeological timescales. For example, if the weight of a fired-clay ceramic increases as a result of RHX by 0.1% in 1 year from firing, then the weight increase is 0.2% in 16 years, 0.3% in 81 years, 0.4% in 256 years, and so on. The reason behind this quartic root dependence is uncertain; however, further research is being conducted to explain why, including NMR and IR spectroscopy. Despite the uncertainty, enough previous research and other more recent data have indicated that the law is valid, with moisture expansion and weight gain being proportional to each other for a specified material at any specified firing temperature. The basis of RHX dating depends on this power law.
Dating methodology First, a small sample of the material is obtained. To do so, the ceramic artifact is wet-cut using a water-cooled saw to avoid producing heat and consequently causing some dehydroxylation. After this, any loose debris must be removed; this can be done by thoroughly cleaning it under running water. The sample is then heated to 105 °C until constant weight to remove all capillary water and loosely adsorbed water. Next, the sample is conditioned in a controlled environment to the estimated effective lifetime temperature (ELT) and relative humidity to obtain the RHX constant (α). The ELT is generally close to (but not exactly the same as) the long-term annual mean surface air temperature. Finally, to completely remove all the water gained in the previous stage, the sample is heated to 500 °C for 4 hours until constant weight, which indicates that all the water has been lost and, therefore, has hypothetically returned to its original historical mass after removal from the kiln. After the preparation of the sample is complete, it is transferred to a microbalance chamber and exposed to water vapour at a controlled temperature (identical to the first ELT) and relative humidity to determine the kinetics of the mass gain through recombination with water. Once this process has been carried out for the desired length of time (on average one to two days), the mass data are recorded. Once the RHX rate is determined, it is possible to calculate how long ago it was removed from the kiln, and therefore assign a date to the material. To determine the date of the material, the rate of the mass gain needs to be calculated using the following equation:
y = α ⋅ T ⋅ t 1 / 4 , {\displaystyle y=\alpha \cdot T\cdot t^{1/4},}
where α {\displaystyle \alpha } is the mass gain rate constant, T {\displaystyle T} is temperature (the ELT), and t {\displaystyle t} is time. The older the sample, the greater the mass gained from combining chemically with water, since the initial mass of the sample is equal to the sum of that of the original fired material and the water combined with it over its lifetime. From there, using other data obtained, the age is calculated using:
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