The critical cooling rate for 30Na₂O⋅10MgO⋅60SiO₂ glass, which exhibits low Rayleigh scattering, was evaluated based on viscosity. To this end, the viscosity and thermal expansion coefficient of this glass were measured using a thermomechanical analyzer. The beam bending and parallel plate methods were effective in measuring the viscosities in the range of interest of higher than 10⁴dPa·s. The critical cooling rate was determined as the cooling rate at which the density of the glass melt became identical to that of solid glass at the glass transition temperature when the melt was cooled from the liquidus to the glass transition temperature. The obtained value for this glass was 2.6°C/min.