The invention relates to a mixture containing an alkali-activated aluminosilicate binder, said mixture, after hardening, containing at least 25% by weight of glass beads, based on the total mass. The medium includes a substrate having a color change layer, such as a direct thermal layer, and a patterned excitable layer placed over the color change layer. The present invention can also provide a light emitting device and its driving method in which a change with age in amount of current flowing between two electrodes of a light emitting element is reduced to obtain clear multi-gray scale display. The particulate collection surface is electrostatically charged in an opposite direction with respect to ground than the electrical charge of the ions. The upper burner is movable between a first collapsed position, wherein the upper burner is disposed within the lower burner, and a second operative position, wherein the upper burner is supported upright on the lower burner.