It was noted that the use of blocking electrodes for the measurement of the direct-current conductivity of oxide glasses seemed to be ruled out, by definition, even though it was an accepted technique. The procedure was usually defended by theorizing that such measurements could be carried out during a period of space-charge creation which preceded electro-depolarization. This theory was considered to be invalid because (a) the postulated ionic space charge could not exist in glasses with an appreciable ionic conductivity and because (b) so-called blocking electrodes did not actually block in the case of such glasses. It was concluded that electrochemical processes could be facilitated by water which was sorbed at the interface.

F.M.Ernsberger: Physics and Chemistry of Glasses, 1995, 36[3], 152-3