**1. Introduction**

The very first records of ion-exchange can be traced back to the Middle Ages, when glass artists used a mixture of silver salts, clay, and natural oil to obtain yellowish color in silicate glasses. The salts mixture was deposited on the glass surface and heated at about 600°C in reducing atmosphere, inducing the diffusion of silver ions into the glass and the formation of silver nanoparticles [1, 2]. Since then, the ion-exchange process was applied without much scientific

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knowledge. It was in the 20th century that scientist started to understand the surface of chemically tempered glasses and figured out an exchange between monovalent cations in glasses with silver and/or potassium cations in the molten salts [2, 3]. These investigations increased the industrial application of the ion-exchange process, especially with the aim of enhancing the optical and mechanical properties of glass [4].

Glass strength can be largely increased by the ion-exchange process, otherwise called chemical tempering. The exchange of small ions such as Li+ or Na+ in an alkali-containing glass, with larger ions such as K+ from a molten KNO3 bath at temperatures below the strain point of the glass, is responsible for the creation of bi-axial residual compressive stress in the surface layers of the material. Because glass products generally break due to excessive tension applied at a surface flaw, the introduction of surface compression strengthens the glass component.

During the ion-exchange treatment, the glass matrix is considered as a solid negatively charged structure where some mobile ions (Na+ in soda-lime-silicate glass) can be replaced by larger monovalent ions (K+ from molten KNO3) responsible for the generation of a compressive stress. The replacement takes place through an interdiffusion process, according to the Nernst–Planck equations. The flux of the ion species scales with the interdiffusion coefficient [2]:

$$\overline{D} = \frac{D\_{\rm Na} D\_{\rm K}}{D\_{\rm Na} N\_{\rm Na} + D\_{\rm K} N\_{\rm K}} \tag{1}$$

where N*<sup>i</sup>* is the fractional concentration of alkali ion *i* (Na or K) and D*<sup>i</sup>* the self-diffusion coefficient. One important parameter in the interdiffusion phenomenon is clearly the concen‐ tration of ions in the molten salt. Some studies have shown that the presence of poisoning elements (already present in the salt or coming out from the glass as in the case of Na) can hinder the diffusion of K+ into the glass and slow down the exchange process, thus reducing the strengthening efficiency. An important issue in industrial practice is also the replacement/ renewal of the molten salt which is time and money consuming [5]. Some researchers [3, 6], on the other hand, believe that the ion-exchange process is not affected in the presence of specific amount of poisoning elements.

In the present work, a systematic analysis of the effect of small amount of sodium as poisoning element in the molten bath on the performances of the strengthened soda-lime-silicate float glass was carried out.
