RHX Experiments at Institut de Physique du Globe de Paris (IPGP) Maxime LeGoff and Yves Gallet, Equipe de Paléomagnétisme

Our equipment: A balance Mettler XP26 placed in a large wine cellar. 10 samples can automatically be weighed at a same time thanks to a rotating carrousel, to which small baskets are hanged. This device was designed to make possible relative RHX dating on samples having experienced the same environmental conditions during their lifetime AND during the weighing experiments. A furnace with temperature regulated and monitored during heating (in air) duration. (video follows)

time 1/4 (h 1/4 ) time (h) one of our weaker gradient after 105°C Baseline of our apparatus. - The variations in RH and T are really weak. - After about 1 day of “natural” drying inside our large climate chamber, the mass stability stands within about 10 ppm (after filtering the effect of small RH oscillations, not shown here).

15 groups of 10 samples of different origins and ages have been analyzed since January, 2012, leading to 470 temporal series (with weighing duration between several days and several weeks). Our measurements

Weighing series after drying at 105°C

Example of 7 series of 10 samples weighed after drying at 105°C during several hours (min. 5h) up to 2 weeks done before stabilization of room air temperature

a b With 4 specimens (a, b, c, d) studied per fragment, these experiments show that: 1) there is no mass gain when the samples are not heated 2) the heating duration (1d or 10d) has only a small influence on the “gradient” after drying twins c;d short series twins a;b

Weighing measurements after dehydroxylation at 500°C

Three examples of 10 long series of rehydroxylation measurements obtained after heating at 500°C. Twin samples often show different behaviors and normal, concave and convex behaviors are simultaneously observed. For the samian ware of Lezoux, with unstable climate conditions (breakdown of our conditioned air system), the related mass variations are practically negligible. 3 sites from France (500°C : 72h) 2 sites from Syria (500°C : 67h) 10 Lezoux (France) (500°C : 117h) 2 Ecouen 2 Hosp. de Beaune 3 x 2 Bois d’Epense

On the difficulty to test the 1/4 power law

Possibility for other exponent...

Deriving RHX dating

Example of calculation for 10 samples from 2 Syrian sites. The RHX age is calculated by extending the RHX regression line to its intersect with the horizontal line which is the presumed archeological mass (m A ). In this example, all samples share the same time range for the regression calculation. Of course, this is not a general rule and each sample should be individually analyzed. archeo : 860 / 970 y

These two animated figures show individual age computations using variable timespans (including the end of the series) for the linear regression calculation. They concern two samples from Syrian Lot40 One, in the top panel, is clearly convex, whereas the second, in the bottom panel is "normal". But both fail to give a realistic age.

This synthetic series, based on realistic parameters with an age of 2000 years, can be analyzed as a short (about 3 h 1/4 ) or as a long (about 5 h 1/4 ) time of weighing. The top and the left panels show the recalculation of the RHX age by a timespan analysis (see the arrows). This analysis provides important information on the reliability of the derived RHX dating: the latter is considered as reliable only if a plateau is observed. (Animated sequence of five different views, each corresponding to a new randomization)

Timespan analysis for 10 samples from 3 French sites, and 10 samples from 2 Syrian sites. The horizontal dashed lines show the known archeological dates. More or less convincing plateau ages are only observed for some of the Syrian samples

This figure illustrates the computations of RHX ages using 2 approaches: 1) The usual one, by extending the RHX regression line to (underestimated) m A (solid horiz. lines). 2) By extending the RHX regression line to the intersect with the regression line calculated from the weighing measurements after heating at 105°C (dotted lines). It is most probable that the "true" ages are between both calculation results since one can assume that the mass stabilization after drying should occur after a very long time, but when? (ppm)

CONCLUSIONS The numerous measurements carried out using the automatic weighing device constructed in our laboratory all fail in reproducing satisfactory RHX dating results, such as those in Wilson et al. (2009)’s study (we recall that for the latter study neither Energy Activation (Ea) nor Effective Lifetime Temperature (ELT) corrections were applied). Our ages are often too young, but in some cases they are much too old. Even if the ELT has a large effect on the RHX gradient, the correction for this effect would generally be insufficient to obtain good RHX dating results. The main difficulty is that twin samples (i.e. obtained from the same fragment!) show only rarely the same RHX behavior, and consequently give the same RHX age. Another difficulty is that each new heating (at low or high temperature) result in a significant loss of mass, as well as if we change by a few degrees the reference temperatures (105°C and 500°C) :Do these temperatures have physical meaning?