Comment on RHX DATING: MEASUREMENT OF THE ACTIVATION ENERGY OF
REHYDROXYLATION FOR FIRED-CLAY CERAMICS, by S.-J. CLELLAND, M. A. WILSON,
M. A. CARTER and C. M. BATT.
Maxime LeGoff and Yves Gallet
2014, October 28th
The method proposed for determining the activation energy (Ea), which is described in the
Clelland et al. (2014) paper, is really clever and useful (of course in conjunction with the
precise knowledge of the Effective Lifetime Temperature -ELT). However, after reading the
paper, our first remark is: Why did the authors not perform a RHX dating of the samples
listed in Table 4 (or at least for some to them)? We cannot imagine that the authors did not
measure the mass of the samples after drying at 105°C.
After a more careful reading, we think that some precisions are needed.
1) In the last paragraph of page 5, the discussion about Arrhenius equation is a bit confusing.
It would have been preferable to make a reference to Appendix A in Hall et al. (2013), where
the theoretical explanation appears more convincing, without the need to make a distinction
between r (which is a generic reaction rate coefficient, with units of the form "something"
versus time) and α. This would permit to avoid some mistakes, such as the choice of mg h-1
as an example for the unit of measurement for r, while here it is really in mg4 h-1. If the
authors keep the factor 4 in equ. 11, they must change r in α. Moreover, ln r and ln αm have
been mixed up in Figures 3 and 5 and their legends. This "factor 4" comes from the ¼ power
law. The authors most probably know that we are more and more skeptical about its
universality (Le Goff and Gallet, 2014a,b,c).
(Detail: the slash bar is also missing in equ. 10)
2) The authors develop some considerations on RH, SWVP and WVP, whose values are
summarized in Table 1, and the conclusion is:
whereas one can read in Wilson et al. (2014) about T0:
A clarification about this opposition between the two sentences would be appreciated.
3) In Table 4, are W1 and W2 the same samples as in Wilson et al. (2014)? One can see
that Ea for W1 (=82.7) has exactly the same value as W1, W2 in Table 2 of Wilson et al.
(2014). At that time, however, a different methodology was used…
4) About the Ea values in Table 4, it is stated that “There are no systematic differences between
the two methods and the activation energies determined are similar.” However, the differences
between the values derived from the two methods (new versus previous one) range from
+8%(MAN074) to -18% (MAN070). These differences are thus not negligible and would
deserve to be discussed (i.e. in terms of their influences on the RHX dating result)…
5) Finally, concerning the data and their fitting, we cannot be convinced by the sentence “the
good agreement between the experimental and theoretical data shown in Figure 4…". This
agreement is indeed far from being obvious, and this is what we show in the four panels of
the figure enclosed. From a precise digitalization of the data of Figure 2, we were able to redraw these data. The successive panels a, b, c show the different stages leading to panel d.
In this last panel, the different question marks highlight several “bad” agreements. Again we
would appreciate some comments about these problems, unfortunately completely ignored in
this paper.
References
Hall, C., Hamilton, A., and Wilson, M., 2013, The influence on rehydroxylation [RHX] kinetics
in archaeological pottery, J. Archaeo. Sci., 40, 305-312
Le Goff, M. and Gallet, Y. (2014a) Evaluation of the rehydroxylation dating method: Insights
from a new measurement device. Quaternary Geochronology, 20, 89–98.
doi:10.1016/j.quageo.2013.12.001
Le Goff, M. and Gallet, Y. (2014b) Evidence for complexity in the RHX dating method.
Archaeometry online August 26th 2014, doi:10.111/arcm.12137.
Le Goff, M. and Gallet, Y. (2014c) Experimental variability in kinetics of moisture expansion
and mass gain in ceramics. J. Am. Ceram. Soc. accepted for publication (Oct. 17th,
2014), doi: 10.1111/JACE13336.
Wilson, M.A., Clelland, S.J., Carter, M.A., Ince, C., Hall, C., Hamilton, A. and Batt, C.M.
(2014) Rehydroxylation of fired-clay ceramics: factors affecting early-stage mass gain
in dating experiments. Archaeometry, 56, 689-702.
1400
1400
slope αm3
from Fig 2b
1200
1200
(a) Figure 4 with superposed numerisation of figure 2
(b) fitted curves following Tables 2 and 3 parameters
M2 , t2
1000
1000
800
800
600
600
400
400
slope αm2
200
M1 , t1
slope αm1
200
from Fig 2a
t'2
0
t'3
0
0
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3
3.5
4
0
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1200
1200
0.5
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1.5
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2.5
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3.5
4
?
(c) Data (in green and orange) from (a)
with the fitted curves from (b)
1000
1000
800
800
600
600
400
400
200
200
(d) All together with the figure 4
?
?
?
0
0
0
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1.5
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2.5
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0
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4