Fundamental and Practical Issues 549 errors (i.e., are not directly related to the ionic current flow in the glass). For short poling times, the current is seen to decrease rapidly in fused silica. The fast current decay component is associated with the movement of sodium and possi- bly lithium. Longer time constants of the current are associated with the move- ment of less mobile species. The drift of ions toward the cathode can result in charge accumulation or cancellation. The mechanisms taking place on the cathode side have been stud- ied to a much lesser extent than for the anode. From experiments with soda-lime glass (sodium content $16%) in which a white deposit is seen to appear on the cathodic surface of the sample, it is believed that cations reaching the cathodic interface are neutralized with the possible creation of oxide of the type Na 2 O [107]. Although the amount of such oxide expected in fused silica is small and difficult to observe, experiments indicate that all charges reaching the cathode are effectively neutralized. 12.5.4 Electrodes Thermal poling of glasses has been made with various low-resistivity elec- trodes. Thus, press-on stainless steel, silicon, and gold electrodes, as well as aluminum and gold evaporated films, have been successfully employed [34].