Nucleation in silicate glasses and effect of preliminary heat treatment on it

Time and temperature dependences of the nucleation rate of Li 2O · 2SiO 2 and 2Na 20 · CaO · 3SiO 2 crystals in glasses of the same composition have been studied by a “development” method. It has been found that nucleation kinetics are satisfactorily described by classical nucleation theory. A new phenomenon observed is the strong influence of preliminary isothermal exposure of a glass on nonstationary nucleation taking place at temperature higher than that of the preliminary heat treatment. The phenomenon is ascribed to heterophase fluctuations during the preliminary heat treatment.