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Rehydroxylation dating


Rehydroxylation [RHX] dating is a developing method for dating fired-clay ceramics. 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). 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. 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.

The concept of RHX dating was first stated in 2003 by Wilson and collaborators who noted that "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.

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

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. Moisture expansion and weight gain are known to be proportional to each other for a specified material at any specified firing temperature.

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. 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. 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.


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