When flat glass is created, the molten silica mixture is slowly cooled in conditions that have been carefully controlled. This procedure means that any undesirable stresses have been removed from the glass itself. Cooling happens in a “lehr”, which is also known as an annealing device. Glass that has gone through this process is known as ordinary glass.
However, there is also something called “heat treated glass”, which is the type of glass for gas fireplace. This has been heated to the point of melting, after which it is cooled very rapidly. This type of heating and cooling induce stress on the glass, making it much stronger. It also allows the glass to endure heat and become more resistant overall. Heat tempered glass can be either heat strengthened or fully tempered. Fully tempered glass has to have at least 10,000 psi of surface compression, or 9,700 psi of edge compression at least. Heat strength glass has to have between 3,500 and 10,000 psi of surface compression, or between 5,500 and 9,700 psi edge compression. Anything below that is classed as ordinary glass. Furthermore, heat tempered glass has a number of specific fracture characteristics.
The Principle of Heat Treatment
When the edge or surface of glass is placed under tension, it will fracture. Any edge fissures or inherent surface will then propagate, becoming a visible crack or even a full break. The heat treating process utilized in tempered glass means that there already is a surface or edge compression in its initial condition. This is done by heating the glass firstly, and then cooling it very rapidly. The center of the glass, at the time, is then still hot compared to the actual surface of the glass. The center will then start to cool, compression the edges and surface. As a result, any type of impact, be that heat, wind pressure or even a missile impact, must then overcome this compression before it can fracture the glass itself.
The Process of Manufacturing Glass for Gas Fireplace
The most important procedure of heat treating glass is found in applying a very rapid air quench, which must happen as soon as the hot glass (it must be around 1200 degrees Fahrenheit at least) is removed from the furnace. It is the sustained and immediate application of the air quench that provides the glass with the necessary temper. The direction of the air against the hot glass should be mixed in nature, including reciprocation of blast nozzles that rotated and that are fixed. This means that the entire surface, on both sides, has had the heat extracted in a uniform manner. If it is uneven, the glass surface will heat back up again, as the center is still hot. The quenched condition of the glass is classed as stable when the surface temperature is reduced to around 500 degrees Fahrenheit. All of this can be achieved either through a vertical position, or through a horizontal position. There is no difference in strength between these two methods.