Isothermal and isochomal workability of glass melts - Influence of melting history: Minor composition changes of redox State and hydroxyl content - Αstudy of the sensitivity of the cylinder compression method

Abstract

The influence of redox State and hydroxyl concentration on the pure non-Newtonian flow behaviour, on the gross non-Newtonian viscosity, on the stress generation modulus, brittleness, high-temperature tensile strength and on the critical deformation rate of an iron oxide-containing laboratory glass melt with two different melting histories, but with the same main chemical composition, is investigated by means of the cylinder compression method. The comparison of these properties between the oxidized and the reduced glass melt is made not only at equal temperatures but also at equal viscosities in order to distinguish between isothermal and isochomal workability and to examine how sensitive the applied method is. The result is that the reduced melt with a somewhat larger Fe²⁺/F_total ratio and with a larger hydroxyl content has a smaller stiffness and brittleness and therefore, a somewhat better isothermal workability than the oxidized melt. This is valid only for the comparison at equal temperatures. When the comparison is made at equal viscosities, however, the workability - or better the isochomal workability - of the two melts is the same. The reason is that a shift of the Newtonian viscosity is produced by the redox shift and hydroxyl content of the reduced melt to lower values which diminishes with increasing temperature. It is possible to construct master curves with respect to the measured properdes at four temperatures, by which a transfer to the low viscosity range is possible, i.e., one can get the isothermal workability of the glass melts over the whole working range. The measured values of the above-mentioned properties are represented by fitting with recently developed relations which give rise to interesting practical and theoretical conclusions about industrial production optimization and about some structural aspects of stressed melts.