We find that inward diffusion of network-modifying cations can occur in an iron-containing silicate glass when it is heat-treated in CO/CO2 (98/2 v/v) or H-2/N-2 (1/99 v/v) gases at temperatures around the glass transition temperature. The inward diffusion is caused by the reduction of ferric to ferrous ions and this diffusion leads to formation of a silica-rich surface layer with a thickness of 200-600 nm. The diffusion coefficients of the network-modifying divalent cations are calculated and they are different for the glasses treated in the CO and H-2 gases. At the applied partial pressures of CO and H-2, the H-2-bearing gas creates the silica-rich layer more effectively than the CO-bearing gas. The layer increases the hardness and chemical durability of the glass due to the silica network structure in the surface layer. (c) 2009 Elsevier B.V. All rights reserved.