Tempered Glass and semi-Tempered glass are made by forming compressive stress on
the surface of the glass by heating the glass up to its intenerating temperature
and then fast well-proportioned cooling. During the process of cooling, the exterior
part of the glass is solidified because of the rapid cooling, while the interior
part of the glass shrinks because of slow cooling, forming compressive stress
on the surface of the glass and tensile stress inside the glass, thus improving
the intensity and heat stability of the glass.