Sebastiano Olindo Troja

Historical mortars dating from OSL signals of fine grain fraction enriched in quartz

In the last years the mortar dating through Optically Stimulated Luminescence (OSL) techniques has become a viable support for chronological estimations (date of construction or restoration episodes) of historical buildings. However, the dating of mortar has still open issues mainly regarding the assessment of the bleaching degree of quartz, the analysis of the OSL processes for this type of samples and the need to do appropriate tests for the most correct evaluation of the equivalent dose. This paper discusses the results obtained by OSL dating (blue diode stimulation) on the polymineral fine grain phase, enriched in quartz, extracted from lime mortar samples collected from different sites. Thermal transfer effects, through the behaviour of Equivalent Dose (ED) and recovery tests, degree and time of bleaching were studied. For each mortar sample the adjacent brick was collected; in some cases, sampling of the bricks bracketing a mortar layer was a possibility, thus obtaining a direct comparison with the standard thermoluminescence (TL) dating on the bricks.The results obtained show, for this set of samples, the possibility of dating the mortars through the use of the fine grain fraction provided of a suitable chemical-physical preparation procedure and the verification of the bleaching conditions.

Alpha and beta dose-rate determination using a gas proportional counter

The use of proportional gas detectors for determination of the alpha (α) and beta (β) dose rate (due to U, Th and K) of pottery and sediment samples in luminescence and ESR dating was investigated. Alpha and beta count-rate measurements on standard powders of U, Th and K at two different counter voltages, low voltage in which the system detects α-particles only and high voltage in which the system can detect and distinguish between both α- and β-particles using different pulse height regions, allow the determination of conversion coefficients that relate count rates to dose rates in samples. The conversion coefficients were applied to the alpha and beta count rates measured on different types of test samples. Analysis of the results shows a remarkable accuracy in the evaluation of the beta dose rates. However, under both measurement conditions (low and high voltage), the alpha dose rates were found to deviate from the expected values to the extent that this procedure must be considered unusable at present.

Different approaches to date bricks from historical buildings

The application of Thermally (TL) and Optically (OSL) Stimulated Luminescence on bricks used as building material has allowed solving an chronological issue in the field of historical building dating.The possibility to use one or more methodologies of dating is closely related to the luminescent and granulometric characteristics of the sample. Using some brick samples collected in the church of Sain Seurin in Bordeaux (France), this paper discusses the implications and the possibility to use different approaches and techniques for dating. With this aim luminescence measurements were performed on both polymineral fine grain and quartz inclusion phases extracted from each brick. For Equivalent Dose (ED) and consequently age determination, TL on mixed fine grain fraction (FG), OSL on quartz inclusions (QI) and on mixed fine grain (FG*) fraction, were used. The results obtained suggest the advantage of using OSL technique on fine grain fraction cleaned up by IR stimulation (FG*), but the use of quartz inclusion represents indeed a good alternative.