Two alkali borosilicate glass tubes, as used in pharmaceutical packaging, were subjected to chemical strengthening in four potassium nitrate salts, these mainly characterized by differing sodium and calcium content. The heat treatment was at 450 °C and 465 °C for 4 h. The mechanical strength of the tubes was measured by 4-point bending tests. The compressive stress build-up at the glass surface was checked by Vickers indentations. The composition of the glass surface after ion-exchange was studied by EDS analyses. The results show that treatment increases the strength to twice the initial value, and that this is similar in the two glasses. A greater sodium content in the salt slightly decreases the exchanged layer depth and, consequently, the bending strength. Conversely, even a very low calcium content in the salt dramatically decreases the efficiency of chemical strengthening: it is shown that calcium blocks the sodium-potassium exchange, this being attributed to the thermodynamically favoured Na-Ca exchange on the glass surface with respect to the expected Na-K.
