Effect of Na contamination on the chemical strengthening of soda-lime silicate float glass by ion-exchange in molten potassium nitrate

Commercial soda-lime silicate float glasses are chemically strengthened at 450 °C for 4 and 24 h in molten potassium nitrate containing small but controlled amount of NaNO3. The ion-exchange efficiency is determined from the measurement of surface compressive stress, potassium penetration and flexural strength. Chemical strengthening remains efficient also for NaNO3 additions up to 0.5 wt% (equivalent to ~1350 ppm Na) and the glass performance being identical to those obtained in pure KNO3, with no significative difference between “tin” and “air” side. For larger Na contamination residual stress and strength are reduced by an amount of about 25%.