Introduction
Ion exchange has several advantages compared to traditional thermal tempering:
-    higher strengths;
-    complex shapes are easily tempered;
-    thin glass can be chemically tempered;
-    uniform compression;
-    constant glass geometry;
-    no optical distortion.
One easy way to strengthen glass is to submerge it in a molten alkali salt bath at a temperature below the transition temperature of the glass. The most recommended molten salt is KNO3 for a Na+ containing glass and NaNO3 for a Li+ containing glass. This basically consists in using salt bath alkali ions that are larger than those in the glass. The consequence of an ion-exchange between the salt bath alkali ions and the glass host alkali ions is a surface full of ions.
Examples of applications for chemically strengthened glass:
-    high-security applications, such as bullet resistant glass;
-    windows resistant to earthquakes and hurricanes;
-    windows for government buildings and monuments;
-    armoured defence vehicles, trains and airplanes;
-    high quality tableware and stemware.
Vidromecanica uses industrial methods to improve the mechanical properties of glass based on the introduction of residual stresses of compression on the glass surface. This method includes thermal tempering and chemical tempering.
Chemically tempered glass has the following advantages:
-    improved impact resistance;
-    improved flexibility strength;
-    improved scratch resistance;
-    improved resistance to temperature...

Introduction
Ion exchange has several advantages compared to traditional thermal tempering:
-    higher strengths;
-    complex shapes are easily tempered;
-    thin glass can be chemically tempered;
-    uniform compression;
-    constant glass geometry;
-    no optical distortion.
One easy way to strengthen glass is to submerge it in a molten alkali salt bath at a temperature below the transition temperature of the glass. The most recommended molten salt is KNO3 for a Na+ containing glass and NaNO3 for a Li+ containing glass. This basically consists in using salt bath alkali ions that are larger than those in the glass. The consequence of an ion-exchange between the salt bath alkali ions and the glass host alkali ions is a surface full of ions.
Examples of applications for chemically strengthened glass:
-    high-security applications, such as bullet resistant glass;
-    windows resistant to earthquakes and hurricanes;
-    windows for government buildings and monuments;
-    armoured defence vehicles, trains and airplanes;
-    high quality tableware and stemware.
Vidromecanica uses industrial methods to improve the mechanical properties of glass based on the introduction of residual stresses of compression on the glass surface. This method includes thermal tempering and chemical tempering.
Chemically tempered glass has the following advantages:
-    improved impact resistance;
-    improved flexibility strength;
-    improved scratch resistance;
-    improved resistance to temperature changes.

Thermal tempering processes can be insufficient
Glass is often only heat-strengthened or fully tempered for architectural window applications. The thermal-tempering process takes only a few minutes to introduce surface compression and creates a case depth on each side that is only a small part of the glass thickness. However, thermal tempered glasses are limited to achieving no more than 100 Mpa surface compression, rarely adequate for protection against high-speed rocks or strong hurricanes. Besides, because the
thermal-tempering process necessarily increases the temperature of the glass well above Ts during the manufacturing process, some optical distortion of the glass is unavoidable. The use of these kinds of windows in critical applications is not recommended. Even more so as significant thermal tempering cannot be obtained in glass thicknesses of less than about 3 millimetres.

The chemical strengthening process
The ion exchange process is most used to chemically temper glass which is too complex in shape, or very small or very large to be thermally toughened.
The process is carried out at temperatures below vitreous transition. The glass to be treated is put in a tank of molten potassium nitrate at a temperature of around 450°C. Ion exchange occurs during the submersion cycle. In this cycle the larger alkali potassium ions exchange places with the smaller alkali sodium ions in the surface of the glass. As the smaller sodium ions exit from glass surface, the larger alkali potassium ions fill those spaces.
Chemically strengthened glass is significantly stronger than annealed glass, depending on glass composition and strengthening process. The chemical tempering process does not affect the colour and light transmission of the glass.
An elastic and plastic increase of interstitial spots occurs incorporating the invading ions in the surface. The consequence will be the large surface compression and a balancing internal tension. Large ions force surface into compression, creating a closure stress on handling flaws and cracks.
Crack growth only occurs if the applied tensile strength is superior to surface compression. Therefore, the performance of chemically strengthened glass products depends on the importance of the surface compression introduced and on the “case depth”. Case depth is the depth of the compression below the surface, i.e., the depth at which the decreasing compression intensity basically reaches zero stress, changing to a tension at greater depths.
ASTM standard C1422-99, “Standard Specification for Chemically Strengthened Flat Glass”, classifies chemically strengthened flat-glass products based on the magnitude of surface compression and on the magnitude of case depth. Both should be as large as possible for protection against impacting projectiles. The ion-exchange process is atomic diffusion dependent and, consequently, extremely slow. Invading-ion penetration increases with time. Minutes of immersion in a molten salt bath could take place at temperatures just below strain point TS to avoid relaxation of the beneficial surface compression. A significant ion-exchange strengthening process requires hours or days of immersion to develop a sufficient case depth with high MOR (Modules of Rupture).
For a given glass composition, overall strengthening is a function of the type of invading ion (salt bath composition), bath temperature and submersion time. If the purpose is “fast exchange” with good MOR, an optimized treat-
ment would be at high temperature for a short time. However, if the objective is maximum MOR, the ion-exchange treatment would be at lower temperature for a long time.
In this tempering process, the physical properties that change are the ones referred to previously.  Specific gravity, hardness, softening point, expansion coefficient, stiffness and thermal conductivity remain unchanged. Tensile stress generated within the glass balances this stress.

The company, its know-how and mission
Vidromecanica’s know-how is the result of more than 30 years of dedication, research and development involving testing and experiments of a wide range of technical solutions to meet customer needs. We produce top performance machinery able to run 24 hours per day on long-term production runs.
Glassmechanics is a group company, which is responsible for the export of equipment manufactured by Vidromecanica.
Our mission is to develop, manufacture and supply equipment for the glass industry, with technology and par excellence quality, thus contributing in increasing the productivity of customers. We are engaged in the research and development of innovative technical solutions aimed at simplifying the use of equipment, reducing energy consumption and increasing their longevity.