As promised yesterday, I'm back to discuss tempered glass. Tempered glass is glass that has been heated in a tempering oven, nearly to the point of melting, then rapidly cooled. This creates a significant amount of surface tension, which makes the glass far stronger. A piece of tempered glass can take roughly 5 times the force of annealed glass before breaking. For this reason, it is often used in situations where breaking glass would be most likely to cause injury, such as doors, picture windows, shelves, shower enclosures, and other such applications.
Another benefit to the tempering process is that it leaves glass far more capable of withstanding heat. That's why it is frequently used in oven doors, or windows that receive excessive sunlight.
Tempered glass is, however, still glass, and therefore susceptible to breaking. While the tensile strength can take significant impact on the face of the glass, a small amount of force applied to one of the edges will disrupt the surface tension, causing the entire piece to shatter. This brings us to another thing that sets tempered glass apart: when it breaks, it breaks into small, gravel-sized pieces, rather than the large shards of annealed glass. Those small pieces may scratch you, but they're too small to do any serious damage.
A drawback to tempered glass is once it has been put through the tempering process, it is no longer able to be cut or drilled. If it is found after the fact that the size is wrong, the only reasonable solution is to replace the piece.
One final, interesting fact about tempered glass. When viewed through polarized sunglasses, it is possible to see the wavy, almost oil-slick pattern left by the tempering process. Give it a try and see for yourself!
~Fanon