The characteristics of Ag+ diffusion during field assisted fabrication of a channel waveguide in glass substrates are analyzed using a numerical model. Differences between the results of the author's original model and the other typically used models are discussed. Experimental conditions have been chosen to clearly demonstrate the essential features of Ag+ concentration contours, particularly near the mask edges. Metallic and dielectric masks have been used in the experiment, and the results are similar for both mask materials. The shapes of Ag+ concentration contours reveal the presence of a thin polarized layer under the mask and seem to be consistent with the results predicted by the proposed numerical model. Some modifications of the model are suggested for a better fit of the numerical to the experimental results.