Applying the “pre-bleached with blue LEDs” protocol to date Umayyad mosaic tesserae by thermoluminescence

Abstract

This paper deals with a Thermoluminescence (TL) study of ancient mosaic tesserae from the qasr (winter residence) of Khirbat al-Mafjar, amazing palace of the Islamic caliphs located in the plain of Jericho. In literature, works dealing with the dating of mosaic tesserae usually rely on the dosimetric properties of the microcrystalline inclusions in the silica matrix rather than on the matrix itself. On the other hand, in the last decades, commercial glasses have been demonstrated to be suitable for retrospective accidental dosimetry. In this work, we applied a protocol widely used for the mobile phones’ glasses, the so-called “pre-bleached with blue LEDs” protocol, to evaluate the archeological dose absorbed by some Khirbat al-Mafjar tesserae in order to confirm their dating based on archaeological evidence. As for commercial glasses, the TL signal from ancient tesserae presents anomalous fading and it is light sensitive. The experimental protocol circumvents these problems isolating the thermally more stable TL signal with an optical pretreatment and allowing the determination of a fading curve for each analyzed sample. Using an integrative approach, we estimated the fading correction for these tesserae and we retrieved their ages. The results are partially in agreement with those hypothesized on historical ground and show good potentialities for the dating of amorphous archaeological glassy materials.