Sand Grains and Chronology of Historical Buildings: Dating Mortars by Optically Stimulated Luminescence
Résumé
Dating of building materials is one of the essential means for the History of Construction. Previous researches
[1] on series of western European monuments showed that in the Middle-Ages the practice of re-using bricks
originating from older structures was a frequent phenomenon. Therefore, dating of bricks by luminescence or
archaeomagnetism is not always representative for the erection of the masonry, the moment to be dated. That is
why we have focused on mortar dating for the last 6 years.
Numerous attempts in mortar dating concentrated on the radiocarbon method which has, however, its limitations
[2], but also its potential and hopes [3]. Thanks to our 6-year experience, OSL has become today fully usable for
dating monuments of known ages. The basic premise in such analysis is that quartz in the sand used for making
mortar is optically zeroed during the preparation process (optical bleaching). The moment to be dated is the last
exposition of mortar to light, before being embedded within the masonry and hidden from light. But, one of the
problems encountered is the possible partial and heterogeneous bleaching of grains that led us to use a priori the
single grain technique (SG-OSL).
We will present some aspects of the methodology for dating mortars by SG-OSL, in comparison with standard
multi-grain OSL. A summary of the latest development of the theoretical approach that led us to imagine a new
way of statistical treatment (the so-called EED model, as exponential exposure distribution [4]) will be
presented and discussed. Finally, a good agreement between OSL and reference ages for a series of samples
from a variety of ages and situations was obtained even in case of poorly bleached material [5]. We will also
present cases for which we still encounter some difficulties in dating them by SG-OSL (low sensitivity mortar
samples, samples resulting from a mixture of sediment and calcareous stones …). Such cases imply that
characterization of dated mortars represents an essential step of the dating procedure. Anyway, the actual
situation of the OSL dating methodology justifies today the systematic use of SG-OSL in determining
chronologies of masonries imprecisely dated, as frequently encountered for the late Roman Antiquity and the
Early Medieval periods in Western Europe.
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