Influence of salt bath calcium contamination on soda lime silicate glass chemical strengthening

Soda lime silicate float glass was ion exchanged in potassium nitrate baths systematically contaminated with calcium nitrate up to 0.01 mol%. The results show that surface compression and flexural strength are dramatically depressed if the treatment is carried out in salt containing calcium nitrate in excess of 0.0015 mol%, this being related to more limited sodium-potassium exchange on the surface. The presence of calcium in the salt accounts for a “blocking” effect of the conventional Na-K exchange which is shown to be thermodynamically less favoured than Na-Ca one, especially at higher temperature.