# RHX-Dating revisited…

Our latest publication on RHX-Dating is online!

Rehydroxylation-Dating is based on the observation that fired clay grows heavier and larger over time. This gain in mass is associated with water that is slowly moved very deeply into the material in a process called diffusion. The water is bound not as water molecule but as OH-groups – hence this process is called Hydroxylation. This process seems to follow a strict law over time, the mass gain $m_{OH}$ is [1]

$m_{OH}(t)=\alpha_m \times t^{1/4}$

This $t^{1/4}$ factor is very interesting, because in principle it should make it possible to date a fired piece of clay with very high precision!

To date the brick or ceramic artefact, we only need to measure its mass at the current time, remove the hydroxilated water and measure the mass again, in effect measuring $m_{OH}$. Afterwards, we could let the sample gain mass again for some time and measure $\alpha_m$.

This would indeed be a beautifully simple measurement to make but there’s always a catch, isn’t there?

In fact, there are two problems, one being that the constant $\alpha_m$ depends on the material and on the average temperature during the mass gain. Effectively this means that we have to measure $\alpha_m$ at at least two different temperatures and then use the Arrhenius equation to calculate $\alpha_m$ at our best estimate of the temperature during storage[2].

The other problem that we talk about in our article [3] is that not only are there numerous ways in which water can stick to the sample (plain “loosely bound” water sticking to the surface and others) but there’s also the chance that there is some mass in the sample that is lost when heating above 500°C that is not hydroxylated water. If this is the case, then the measurement for $m_{OH}$ will be too large, making the sample much older than it really is.

In the publication we try to identify this additional mass (called non-refractory compounds or NRC) and offer a few recipes how to remove any NRCs that might be in the sample.

## References

[1] M. A. Wilson, W. D. Hoff, C. Hall, B. McKay, and A. Hiley, “Kinetics of Moisture Expansion in Fired Clay Ceramics: A $(\mathrm{\text{Time}}{)}^{1/4}$ Law,” Physical review letters, vol. 90, p. 125503, 2003.
[Bibtex]
@Article{Wilson2003,
Title = {{K}inetics of {M}oisture {E}xpansion in {F}ired {C}lay {C}eramics: {A} $(\mathrm{\text{Time}}{)}^{1/4}$ {L}aw},
Author = {Wilson, Moira A. and Hoff, William D. and Hall, Christopher and McKay, Bernard and Hiley, Anna},
Journal = {Physical Review Letters},
Year = {2003},
Month = {Mar},
Pages = {125503},
Volume = {90},
Doi = {10.1103/PhysRevLett.90.125503},
File = {Wilson2003.pdf:Wilson2003.pdf:PDF},
Issue = {12},
Numpages = {4},
Publisher = {American Physical Society},
Spezifikation = {ReHydro},
}
[2] C. Hall, A. Hamilton, and M. A. Wilson, “The influence of temperature on rehydroxylation [rhx] kinetics in archaeological pottery,” Journal of archaeological science, vol. 40, iss. 1, p. 305–312, 2013.
[Bibtex]
@Article{Hall2013,
Title = {The influence of temperature on rehydroxylation [RHX] kinetics in archaeological pottery},
Author = {Hall, Christopher and Hamilton, Andrea and Wilson, Moira A.},
Journal = {Journal of Archaeological Science},
Year = {2013},
Month = jan,
Number = {1},
Pages = {305--312},
Volume = {40},
Abstract = {Almost all archaeological ceramics undergo slow, progressive rehydroxylation by chemical combination with environmental water. The reaction is accompanied by an expansion, and also by the small but measurable mass gain that provides the basis of the RHX dating method. The rate of the RHX reaction increases with increasing temperature. Here we describe comprehensively the effects of temperature on the RHX process in relation to the dating methodology. We deal in turn with the kinetic model of the RHX reaction, the temperature dependence of the RHX rate, and the influence of varying environmental temperature on the RHX mass gain. We define an effective lifetime temperature and show how this is calculated from an estimated lifetime temperature history. Historical meteorological temperature data are used to estimate the lifetime temperature history, and this can be adjusted for long-term climate variation. We show also how to allow for the effects of burial in archaeological sites on the temperature history. Finally we describe how the uncertainties in estimates of RHX age depend on the estimates of temperature history and effective lifetime temperature.},
Doi = {10.1016/j.jas.2012.06.040},
File = {Hall2013.pdf:Hall2013.pdf:PDF},
ISSN = {0305-4403},
Keywords = {Ceramics, Pottery, Rehydroxylation, Dating, Effective lifetime temperature},
Spezifikation = {archäa, keramik, ReHydro},
Url = {http://www.sciencedirect.com/science/article/pii/S030544031200307X}
}
[3] M. Numrich, W. Kutschera, P. Steier, J. H. Sterba, and R. Golser, “On the effect of organic carbon on rehydroxylation (rhx) dating,” Journal of archaeological science, vol. 57, p. 92–-97, 2015.
[Bibtex]
@Article{Numrich2015,
Title = {On the effect of organic carbon on rehydroxylation (RHX) dating},
Author = {Numrich, M. and Kutschera, W. and Steier, P. and Sterba, J.H. and Golser, R.},
Journal = {Journal of Archaeological Science},
Year = {2015},
Pages = {92---97},
Volume = {57},
Doi = {doi:10.1016/j.jas.2015.01.016},
File = {Numrich2015.pdf:Numrich2015.pdf:PDF},
Owner = {sindalf},
Spezifikation = {archäa, inhouse, ReHydro},
Timestamp = {2015.02.20}
}