The fluidity (4) and the conductivity ( 0 ) of strongly supercooled solutions of sodium and potassium metasilicate and their mixtures, of general formula xK2SiO3-(l-x)NazSiO3-7Hz0, have been measured to -70 "C. The Arrhenius plots were curved, and both 4 and u data were fitted to the equation 4 (or a) = A exp[-B/(T -To)], where To is a "zero mobility temperature" obtained by an iterative least-squares method. The viscosity data were measured over nine orders of magnitude and were well described for all solutions by this three-parameter equation. When the conductivity was plotted as a function of composition at constant fluidity, a conductance minimum was found at low temperatures which was identified as the appearance of the "mixed-alkali" effect. The results offer quantitative support for the idea that the mixed-alkali effect is only found when the structural relaxation time, ( 7g), is much longer ( N 100 times) than the conductivity relaxation time, ( 7 c ) .

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