The Crystallisation Relationships of a Soda-Lime-Magnesia-Silica Glass as Used for Drawn Sheet and the Process of Devitrification
JSGT 1940 V24 T139-T158
The gradient method has been employed for the investigation of the crystallisation-temperature-time relationships of a sheet glass of percentage composition 72.15 SiO2, 2.02 R203, 8.91 CaO, 2.05 MgO, 13.58 Na2O, 0.73 K2O and 0.04 BO3. The liquidus temperature was found to be 965° and the binary eutectic temperature 850°. The formation of the ternary eutectic was not observed. The temperature of maximum crystal growth was 910°. Examples were given illustrating the rate of crystal growth at different temperatures, the effect of time on crystallisation and the separation of the binary eutectic of devitrite and silica. The shape of the crystallisation-temperature curve was considered in relation to the equilibrium diagram and to the viscosity of the glass. It was shown that to a first approximation, above the temperature of maximum crystal growth, crystallisation was proportional to the temperature interval below the liquidus temperature; and below the temperature of maximum crystal growth, crystallisation was inversely proportional to the viscosity. The early stages of crystallisation could be represented by an equation of the type:-
r = Ce-A/Tabs.(TLiq. - T)
in which r is the rate of crystal growth and A is a constant deduced from the viscosity-temperature relationship of the glass. The process of devitrification was discussed from the viewpoint of the equilibrium diagram of the soda-lime-silica system by the aid of a perspective drawing of a portion of the equilibrium model relevant to commercial glass compositions. Correlation of the results obtained on commercial soda-lime-silica glasses with the known equilibria conditions for the pure glasses would ultimately lead to a representation of the effects of minor constituents on the crystallisation properties of such glasses. A plea was made that more attention should be given to this subject by all glass technologists.
Eric Preston, Ph.D., D.Sc., F.I.C., F.Inst.P.
