High-temperature viscoelastic properties of two different silicate glass melts have been measured with a new high-temperature rheometer between 700degreesC and 900degreesC. Our experiments demonstrate that the viscoelastic properties in the terminal zone, a low-frequency domain which has never been explored, are very sensitive to the chemical composition of the glass and to the thermal treatment applied during the preparation of the samples. The storage shear modulus G' and the loss shear modulus G" were measured at constant temperature between 10(-3) and 10(2) rad/s. The two chosen silicate glass melts have different chemical compositions and, moreover, each of them was prepared with two thermal treatments during the sample molding. In the experimental range of temperature, the viscoelastic properties of the two glass melts are characteristic of the ultimate rheological behavior at low frequencies called the terminal zone. We tested the possibility of building the rheological master curve for the two liquids over a wide frequency domain. Experiments show that one quenched glass melt has a Maxwell viscoelastic behavior and obeys time-temperature superposition, while the same glass annealed below the glass transition temperature presents changes in the viscoelastic behavior and does not obey time-temperature superposition. The second glass is non-Maxwellian and does not obey time-temperature superposition for the two thermal conditions of molding showing a phase transition, or a structural change transition, between 700degreesC and 900degreesC. We observed a great influence of the thermal treatment on the viscoelastic properties in the melted state for the two silicates. (C) 2003 American Institute of Physics.