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

Dating archaeological lime mortars by single grain OSL is founded on the bleaching of grains during the mixing of lime and sand. However incomplete and heterogeneous optical zeroing of grains is often observed. In the view of evaluating reliable archaeological doses, we have developed non-standard statistical tools that describe residual dose distributions. For these purposes, a model of the bleaching process, based on simple physical assumptions leads to an exponential decreasing distribution of light exposure among sand grains. Coupling this exposure distribution with both decay of OSL grains with light exposition, which is assumed to be exponential, and dose response growth curve of OSL, chosen in our case as a single saturating exponential, distributions of residual doses are obtained. Such distributions are then added to burial dose distributions (defined as the doses absorbed by the grains since the mortar making) to simulate equivalent dose distributions of archaeological materials. A very satisfactory agreement is obtained with known age reference samples; in particular, our model leads to more accurate ages than the commonly employed Minimum Age Model. Finally, we estimate that this model, to some extent, could be generalized to other bleaching processes involving a mobility of grains as a major source of random exposure of grains to light.