Chemical Strengthening Process
Glass breaks from flaws on its surface when there is tension on the glass. Glass is strengthen through the an ion-exchange process. During this process glass is submersed in a bath containing a potassium salt (typically potassium nitrate) at 300 °C (572 °F). This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution. The Potassium ions in the solution migrate into the glass surface and replace the smaller sodium ions within the glass structure. The larger potassium ions then create a compressive stress layer that forms an extremely tough surface.
Chemical Strengthening enables glass to retain more of its strength even after a minor nick or scratch and protects it from more cracks and flaws.