We report that inward diffusion of network-modifying divalent cations can occur in iron-containing silicate glasses when they are treated in a reducing atmosphere near the glass transition temperature. As a result of the inward diffusion, a silica-rich nanolayer forms on the glass surface, which increases the hardness and chemical durability. The thickness of the layer can be controlled by varying the heat-treatment conditions. We clarify the mechanism of the inward diffusion and calculate the diffusion coefficient for the network-modifying divalent cations. We demonstrate that the diffusion of these ions is the rate-limiting factor for the reduction process.