The Effects of Various Oxides on the Viscosity of Glasses of the Soda–Lime–Silica Type
JSGT 1953 V37 T316-T372
Viscosity measurements have been made at high temperatures and at low temperatures on, a soda–lime–silica glass of the percentage composition SiO2 74, CaO 10, Na2O 16, and the changes of viscosity resulting from the replacement of silica by various oxides have been studied. The replacements were usually made on a cation-for-cation basis. At high temperatures the effects of substituting divalent cations are shown to be determined, in the main, by the field strengths of the substituted cations; the effects of monovalent cations are determined to a large extent by the size of the substituted cations; the effects of trivalent and quadrivalent cations are attributed to the strengths of the oxygen bonds linking the substituted cation to the oxygen with which it is associated as a glass structure-building unit. At low temperatures the curve showing the temperature corresponding to a viscosity of 1012 poises plotted against the ionic radii of the substituted monovalent
and divalent ions consists of three quite separate branches, with “breaks” corresponding approximately with ionic radii 0·31, 0·60, 1·00 and 1·60 Å. The first branch corresponds with tetrahedral co-ordination, i.e. with the ion surrounded by four oxygens, the second branch with octahedral coordination and the third with cubic co-ordination or a co-ordination of higher order. The viscosity is highest for the smallest ions represented by any one branch of the curve, and diminishes progressively with increase in ionic radius until a change to a higher co-ordination number can occur, when there is a sudden and large increase in the viscosity. Values have been calculated for the activation energies of the “units” involved in the viscous flow of the glasses at 1300° and at the temperatures corresponding to the viscosity value 1012 poises. The viscosities of glasses containing iron in different states of oxidation have also been studied, and indicate that: (a) The “blue” colour due to iron is due to ferrous ions. (b) The “grey” colour given by iron is probably due to a colloidal dispersion, presumably of Fe3O4. (c) The “colourless” form of iron is probably present as a “structure-building” unit, comparable with SiO4 in its contribution to the viscosity of the glass. No conclusive evidence could be obtained relating to the state in which yellow or brown “colouring” ferric iron exists in glass.
A. G. F. Dingwall & H. Moore
