Properties of Tempered Glass
Tempered glass is thermally (heat) treated glass created to break into small, uniform fragments. Glass that has not been tempered has a tendency to travel when breaking, its shards taking on a weapon like shape. Sharp jagged edges are missing from tempered glass and so is the possibility of sure injury. Tempered glass has been made stronger through a heating process, and has to some extent become heat resistant so it does not shatter as easily when exposed to fire or pressure. Cracks in untempered glass are started by a defect, a point of weakness, that began in the making of the glass. Due to the compression and heating of tempered glass it is much more difficult to shatter and therefore the safer alternative in many situations.
Annealing The Glass
Tempered glass begins its transition as annealed glass. Annealing can be performed on glass even if it does not go on to become tempered glass. In fact, most glass has been annealed. Stress points are created during the making of glass and as a result glass responds easily to the slightest pressure and temperature changes by shattering. The annealing process strengthens the glass and prevents its easy destruction. The annealing process lowers the temperature of the glass slowly and by doing so relieves the stress points by relaxing them and making them disappear. There are two important points in the annealing process; the strain point and the annealing point. The annealing point comes first in the heating process. This is the point where the stresses relax. Heat soaking at this point allows the glass to take on an even temperature until the predetermined strain point is reached. Any stress left in the glass by the time it is heated to its strain point will always be there. The annealing process and its strain point is defined by the thickness and the type of glass being heated.
Tempering With Heat
Once the glass is annealed it can be cut and sized. It is then placed on a conveyor belt that rolls through a tempering oven or furnace. It is once again heated to the annealing temperature and slightly over it. This means the interior glass is not quite solid and it is at this point in the process that the exterior glass is forced to cool very quickly. This is achieved through blasting the glass with high pressured, cool air while it is just over the annealing temperature. This application of hot and cold temperatures is a controlled process. Untempered glass in a building fire is experiencing a tempering of sorts but it is dramatic and destructive as opposed to directed and controlled. In the tempering process, the outer glass tries to pull away from its edges setting up compression but the inner glass, which is hot, remains tense. This tension and compression is the strength of tempering.
Tempering With Chemicals
A chemical application to the annealed glass can set up a similar reaction. Placed in a bath of melted potassium nitrate, the glass is coated and the chemical creates compression by the exchange of ions on the surface of the glass. This process creates an even tougher tempered glass but is expensive compared to using a tempering oven. Chemical treatments are usually used on shaped glass whose surfaces do not temper evenly. The chemical process can be applied all over the shape and not change it. Heating a shape in a furnace will alter the object and produce an uneven tempering.