Calcination was carried out in electric box furnaces with a carefully controlled heating cycle which burnt away the PET trays leaving the discs in their original positions on the refractory tray (Fig. ). The key element of this cycle was to limit the heating rate to 25 °C/h between 250 °C and 500 °C in order to avoid movement of the discs during the combustion of the PET trays thus preserving the layout and spacing of the samples.