in 3 M HCl at 100°C is investigated by the electron microscopic techniques. Glass samples in the form of 20
20
2-mm-sized polished plates with ~15 mol % R2O [where R = Na or (Na + K)] and the molar ratio B2O2/SiO2 = 0.3–0.6 are subjected to heat treatment at 550°C for a time sufficient for the phase composition to
attain the equilibrium value. It is shown that, when advancing from the porous glass sample periphery to the
center, the homogeneity of the structure is disturbed: the central regions of the samples exhibit dense zones with
microcrystalline boron-containing phases.