Modeling incomplete and heterogeneous bleaching of mobile grains partially exposed to the light: towards a new tool for single grain OSL dating of poorly bleached mortars
Résumé
Dating archaeological mortar by OSL is one of the challenging that allows to date the construction of ancient masonries. Amongst building materials, mortar is more convenient to date masonries than bricks, since the latter can be reused, salvaged from older buildings or structures. The event to be dated is the preparation of mortar when grains of sand (including quartz) were extracted, possibly sieved and added to lime and mixed together. In many cases, an incomplete bleaching of grains is reported in the literature and the only way to overcome this difficulty is to use the single grain technique (Jain et al., 2004; Urbanova et al., 2015…). Nevertheless, despite its potential, this technique needs to develop non-standard statistical tools in order to understand and describe the EDs distribution of poorly bleached materials.
The first step of this study is, to model the bleaching process of sand grains during their movement while being mixed with lime and water, in the view of obtaining a residual dose distribution. To do so, we base our calculation upon simple assumptions about the statistical behaviour of grains, and particularly the fact that individual grains can move independently during the making process. Assuming that the probability for a grain to be exposed to the light is memoryless, then we deduced that the time of exposition of a grain follows an exponential distribution. Coupling this exposition with both a bleaching equation of grains which is assumed to be exponential, and a growth function of OSL with dose, chosen in our case as a saturating exponential, we obtain the distribution of residual dose. This distribution is convoluted with the burial dose one (distribution of dose integrated since the mortar manufacture) to simulate distributions of EDs of archaeological materials.
The second step consists in checking the suitability of this simulation approach by comparing calculated and experimental distributions of EDs of mortar samples. The principle is to evaluate the parameters involved in the simulation process that best fits the experimental distribution of individual EDs and finally compare the Single Grain -OSL ages with the known ones. As the samples studied here are of known ages, that were determined according to archaeological or other physical dating approaches, it is then possible to evaluate the consistency of this data processing. A very satisfactory agreement is obtained with the known samples studied. We actually experience tens of SG studies that indicate the suitability of the procedure that, in a certain extent, could be generalized to other bleaching processes involving a mobility of grains as a major source of random exposition of grains to the light.
Jain, M., Thomsen, K. J., Bøtter-Jensen, L., Murray, A. S., 2004. Thermal transfer and apparent-dose distributions in poorly bleached mortar samples: results from single grains and small aliquots of quartz. Radiation Measurements, 38: 101-109.
Urbanova, P., Hourcade, D., Ney, C., Guibert, P., 2015. Sources of uncertainties in OSL dating of archaeological mortars: the case study of the Roman amphitheatre Palais-Gallien in Bordeaux. Radiation Measurements, 72: 100-110.