Chemical Strengthening of Soda Lime Silicate Float Glass: Effect of Small Differences in the KNO ₃ Bath

Six slightly different KNO ₃ baths were considered in this work, for the chemical strengthening of soda lime silicate glass. Five of these were commercially available pure KNO ₃ salts for industrial or laboratory use and one was a salt from an unknown source that had been already used for at least 1000 h in Na–K ion exchange. Various amounts of sodium — the main impurity — calcium, and magnesium were measured in the six salts together with other more limited contaminants. Different strengthening effects associated with clearly dissimilar ion exchange efficiency were measured on glass treated in the different melts. The presence of Na — major impurity — in the salt was revealed to be not a critical aspect, at least up to concentration of about 0.5 wt%. Instead, the “blocking” effect caused by the presence of Ca and Mg in the KNO ₃ bath (also in amounts of few tens of ppm) is shown to be a major issue.