Modeling Light Exposure of Quartz Grains During Mortar Making: Consequences for Optically Stimulated Luminescence Dating

Abstract

Dating lime mortar shows great potential for establishing the chronology of a construction. The basic premise of mortar dating by optically stimulated luminescence (OSL) is that quartz in the sand used for making mortar has been optically zeroed during the preparation process (optical bleaching). The moment to be dated is the last exposure of sand grains to light, before being embedded within the masonry and hidden from light. However, the main problem is the frequent partial and heterogeneous bleaching of grains, and this led us to use the single grain technique (SG-OSL) systematically. Some theoretical and experimental aspects of a new statistical treatment (the EED model, as exponential exposure distribution) are detailed and discussed. Our experience shows that SG-OSL dating of mortars is successful in a majority of situations. In a minority of cases (around 15%) difficulties originate when there is inappropriate OSL behavior of grains, and thus OSL dating is not possible. In the other cases, good agreement was obtained between OSL ages and the reference ones for a series of samples from a variety of ages and situations, even in the case of poorly bleached material. Anyway, the present situation of OSL dating methodology justifies the systematic use of SG-OSL in the dating of masonry today.

Chronology of the Late Roman Antiquity walls of Le Mans (France) by OSL, archaeomagnetism and radiocarbon

“…We have no 31 clear interpretation about such results. A hypothesis could be the fact that the mortar was prepared in very low light exposure conditions, for instance inside a pre-existing building (see Guibert et al, 2020 for a detailed discussion on this possible limitation of mortar dating by OSL).…”

“…Due to the forced movement of mixing, quartz grains from the sand are only shortly exposed to the ambient light when they reach the upper surface of the mixture and are thus bleached partially. In such conditions, the statistical distribution of light exposure of grains follows a decreasing exponential function (Guibert et al, 2017(Guibert et al, , 2020. In the case of crushed terracotta mortars, the interpretation of results depends on the origin of the quartz grains, either from the lime part or from the baked clay fragments.…”

“…Such high number of grains is necessary to analyse the dose distribution and obtain the significant chronological information carried by grains bleached during the mortar making. Specific tools are required to exploit the SG data and we used in this study the Exposure Exponential Distribution (EED) model, developed specifically for lime mortar dating (Guibert et al, 2017(Guibert et al, , 2020 (see supplementary material for details).…”