**4. Conclusions**

The presence of a small amount (up to 5 wt%) of NaNO3 in potassium nitride bath partially influences the strengthening process of soda-lime-silicate glass when the treatment is carried

out over a duration of 4 h. The interdiffusion coefficient and the penetration of K in the glass are substantially invariant while the surface concentration of potassium scales with the purity of the bath, and this is responsible for different compressive residual stress, which is higher when pure KNO3 is used. Nevertheless, the overall reinforcement associated to the final strength does not show clear dependence on the used bath mainly because of the large scatter in the surface flaws sizes, as typically observed in glass. It is clear that the quality of the surface of the bare glass in terms of flaws has a much higher importance than the salt purity in the final mechanical resistance.
