Electron attachment to the front side of an ion conducting glass which is in contact with a metal electrode at the back side induces transport of cations towards the front side of the sample. Contrary to conventional expectation charge transport is not blocked instantaneously but rather decays over an extended time of up to several days. Quantitative analysis of cation deficient zones at the interface between the glass and the back side electrode and their time evolution demonstrates that the electric field operative in this zone transiently exceeds the dielectric breakdown field strength, such that electrons escape into the metal electrode. This favors electro-neutrality of the cation deficient zone and consequently delays the evolution of blocking character. Charge transport will eventually stop once the electric potential applied (dc) drops across this zone. The physical reason for blocking of charge transport in this case is not that charge carriers cannot pass this zone but the absence of an electric field in the bulk sample. This conclusion is further supported by the observation that Cu+ ions escape from a sputtered copper electrode and enter into the cation deficient zone. As a consequence, usage of copper should be avoided when ion-blocking is an important goal.