How can glass be toughened to increase its strength?

Toughening glass to increase its strength is effectively achieved through a process known as tempering, which involves heating the glass to a high temperature and then rapidly cooling it. This method creates compressive stress on the surface of the glass, making it significantly stronger and more resistant to impacts and thermal shock.

The rapid cooling creates a state where the outer layers cool and solidify before the inner layers have a chance to cool down, resulting in a toughened structure that can withstand greater forces without breaking. This process is vital in applications where safety and durability are paramount, such as in automotive windshields and building glass.

Adding impurities during manufacturing, coating with a protective layer, or mixing glass with metals does not effectively strengthen glass in the same way that the heating and rapid cooling method does. Each of these alternatives may have their own benefits, such as altering optical properties or adding corrosion resistance, but they do not inherently toughen the glass to the extent achieved through tempering. Thus, heating and rapidly cooling the surface uniquely enhances the strength of glass through internal stresses that contribute to its overall toughness.