Slowly cool the glass in a kiln. This process is called annealing, and it removes any stress points that may have formed in the glass during cooling. Glass that has not been annealed is significantly weaker. Once this process is completed, the glass can then be coated, laminated or otherwise treated to improve its strength and durability.
The precise temperature for annealing can vary based on the precise composition of the glass from as low as 750 degrees Fahrenheit to as high as 1000 degrees Fahrenheit. The rate at which the glass must cool may also change - generally, larger pieces of glass must cool more slowly than smaller pieces. Research proper annealing methods before beginning.
A related process is tempering, in which shaped and polished glass is placed in an oven heated to at least 600 degrees Celsius (1,112 degrees Fahrenheit) and then quick-cooled ("quenched") with blasts of air at high pressure. Annealed glass breaks into shards at 6,000 pounds per square inch (psi), while tempered glass breaks into small pieces at no less than 10,000 psi and usually at around 24,000 psi.