A method that modifies the composition of a region adjacent the surface of an ion conducting glass body comprises applying an electric field, at an elevated temperature, between opposite surfaces of the glass body to remove substantially all monovalent and most divalent ions from the region. A desired ion-depleted region occurs adjacent the anode-contacted surface of the glass body if the anode is a blocking electrode of relatively nonreactive material with the components of the glass body. A preferred blocking anode is a porous layer of colloidal particles of graphite. A particulate graphite anode allows oxygen evolving from the glass to escape therethrough, or combine with the evolving oxygen to produce easily removable gaseous byproducts, and does not supply cations to the glass body.