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.Yet most archaeological material contains components which causes either addition mass gain or additional mass loss during the RHX measurement process.
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 quality of data generated by the Manchester and Edinburgh groups has been due to analysing fired-clay materials which do not contain these components.
Efforts to successfully replicate the original work and overcome the challenges presented by archaeological ceramics are underway in several academic institutions worldwide.
The RHX process produces an increase in specimen weight.
This weight increase provides an accurate measure of the extent of rehydroxylation.
Once that RHX rate is determined, it is possible to calculate exactly how long ago it was removed from the kiln.
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.
The ELT is generally close to (but not exactly the same as) the long-term annual mean surface air temperature. Any event involving exposure to extreme heat may reset the "clock" by dehydroxylating the specimen, as though it were just out of the kiln.