Ceramic-glass composites may be produced directly by the sintering of raw mixtures with high SiO2 content. The ionic conductivity is lowered by increasing the SiO2 content. With potassium content below 15 mol%, the activation energy of conduction decreases with the potassium content, which is attributable to a decreasing potassium jump distance. The activation energy of K2O-Ln(2)O(3)-nSiO(2) decreases with increasing radius of Ln from Dy to Sm and increases from Sm to La. This behavior is attributed to changes of the electrostatic binding and strain energies of mobile potassium. The elution of potassium with water is depressed by an increase in SiO2 content and by a decrease in radius of Ln.