Study of blue colour in ancient mosaic tesserae by means of thermoluminescence and reflectance measurements

The colouring effects of cobalt and copper ions in glass have been investigated by means of thermoluminescence and reflectance measurements. The two techniques have been used to characterise the colour centres of ancient blue glass mosaic tesserae, in comparison with standard modern colourless and blue glass. Thermoluminescence measurements allowed us to characterise the colour centres in the glass network and to evaluate the electron trap depth in ancient and modern samples. In ancient samples there is evidence of three electron traps, at the same temperatures of crystalline quartz, perhaps due to a partial re-crystallisation of the lattice. On the contrary, in modern samples, the continuum of trapping levels typical of amorphous silica is evident.