New insights towards the dating of Roman and medieval mortars by Optically Stimulated Luminescence (OSL): comparison of five case studies

Résumé En

Mortar is the only material of construction that cannot be, contrary to bricks or wood, replaced without being destroyed and its making is thus undoubtedly contemporary to the erection of a building. In theory, lime mortars composed from the mixture of lime and sand may be dated by optically stimulated luminescence. The basic premises in such an analysis is that quartz in the sand used for making mortar is optically zeroed during the process of quarrying and mixing. Several seconds of exposition to daylight are enough to set to zero the “chronometer”. Mortar is then embedded within the masonry to bind building elements, and thus it is hidden from light. The moment to be dated is this last exposition to photons. In case of heterogeneous bleaching the grains have to be analysed individually using a “single grain technique”. We propose a deeper insight to this promising technique demonstrating the results of the three year running study on mortar samples originating from well-dated Gallo-Roman and medieval reference structures. The obtained chronology is compared at every stage with the known information acquired by other physical and archaeological dating methods