Influence of salt bath calcium contamination on soda lime silicate glass chemical strengthening Vincenzo M. Sglavo a,b , Ali Talimian a, ⁎, Norbert Ocsko a a Department of Industrial Engineering, University of Trento, Trento, Italy b INSTM, Trento Research Unit, Florence, Italy abstract article info Article history: Received 28 October 2016 Received in revised form 17 December 2016 Accepted 19 December 2016 Available online xxxx Soda lime silicate float glass was ion exchanged in potassium nitrate baths systematically contaminated with cal- cium 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. © 2016 Elsevier B.V. All rights reserved. Keywords: Ion exchange Chemical tempering Soda lime silicate glass Residual stress Strength 1. Introduction Chemical strengthening is an effective technique used to improve the mechanical resistance of alkali-containing glass. It consists of an ion-exchange process involving small ions in the glass, replaced with larger ions contained in a molten salt where the glass article is im- mersed. If the process occurs below the glass transition temperature, the “stuffing” effect is responsible for a compressive stress on the sur- face layer of the glass, which accounts for higher strength and damage resistance. In a typical industrial situation, Li- or Na-containing glasses are subjected to ion-exchange in molten potassium nitrate, the inter- diffusion process being controlled by temperature, time and composi- tion of the glass: an important additional variable is the purity of the salt [1–8]. Previous works have shown that even small variations in the salt composition can sensibly influence the ion exchange process and, con- sequently, the final mechanical performance of the glass [9–13]. In some cases, a measurable increase in ion exchange efficiency has been reported [14–16]; in other works, the presence of extraneous alkali and alkaline earth in very small amounts is shown to be responsible for “poisoning” the bath and “blocking” the exchange [9,17–19]. One typical contaminant in a KNO 3 bath is calcium: this can be present in the original salt, it can be an impurity generated by the residues of washing process or it can be directly introduced by the industrial environment dust. In this work, the effect of a small amount of Ca in the potassium nitrate bath used for chemical strengthening soda lime silicate glass was systematically analysed to identify both the influence on ion-exchange process and the concentration ranges above which the strengthening process becomes ineffective. 2. Experimental procedure Commercial soda lime silicate glass from commercial source (Planibel®, AGC Glass Europe) was used in the present work. The glass transition temperature measured using Differential Scanning Calorimetry (DSC) [20] is shown in Table 1 together with chemical composition. Pure potassium nitrate, Sigma-Aldrich, ACS grade N 99.0, was used for the ion-exchange process. The salt was systematically contaminated using Ca(NO 3 ) 2 , which was prepared by calcination of calcium nitrate tetrahydrate, Sigma-Aldrich, ACS grade N 99.0, at 300 °C for 72 h. Initial- ly, a mixture containing 1 mol% Ca(NO 3 ) 2 in KNO 3 was prepared and then used to introduce specific amounts of Ca in the salt; several molten baths were successively produced adding from 0.0005 mol% to 0.01 mol% Ca(NO 3 ) 2 to the initial salt. Square samples, nominally 30 mm × 30 mm, were cut from the original sheet, which was obtained from one single plate, and subjected to ion exchange in the molten salts using stainless steel crucibles within a laboratory muffle; the glass-to-salt weight ratio was always equal to 1:5. Most of the ion-exchange processes were conducted at 450 °C for 4 h and in some cases at 420 °C, 440 °C and 460 °C or with duration of 24–96 h. Larger samples (50 mm × 50 mm), suitable for mechanical flex- ural tests, were also chemically strengthened using a semi-automatic Journal of Non-Crystalline Solids 458 (2017) 121–128 ⁎ Corresponding author. E-mail address: ali.talimian@unitn.it (A. Talimian). http://dx.doi.org/10.1016/j.jnoncrysol.2016.12.023 0022-3093/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol