Pristine glasses have extremely high mechanical strength, but surface flaws reduce the strength of practical glass products. However, glasses can be made mechanically stronger by forming a compressive residual stress on the surface by thermal tempering or through ion-exchange.

Thermal tempering involves heating a glass product to near the glass softening temperature followed by rapid cooling.¹ This process produces a temperature gradient with a “cool” surface and a “hot” interior that contracts relative to the surface. The contraction induces a residual compressive stress on the surface at room temperature. Another method is ion-exchange, also called chemical tempering, in which glasses containing smaller alkali ions, e.g., Na⁺, are exchanged in a molten salt bath for larger alkali ions, e.g., K⁺.² Forcing larger ions into the glass structure creates a compressive residual stress.