If you have ever seen a window break, you can appreciate the tempering. When a normal piece of glass breaks, the shards are jagged and can be very dangerous. Consider what happens when a home is subject to a tornado or hurricane. When subject to high winds, windows will shatter. The result is sharp pieces of glass flying around at excessive speeds; this is an essential death trap and is an experience many people fear.
This is why tempered glass is made. This glass is treated via a thermal heating method so that when it breaks, the shards will be a uniform shape absent of sharp edges. Over the years, engineers have refined the tempering process, which has made this glass much more durable and resistant to heat and pressure (wind).
The process of making tempered glass
- Annealed glass
- Applying heat
- Applying chemicals
The glass will first be annealed. This is a counter measure to the pressure the glass is subject to when being made. When being manufactured, pressure points will occur on the surface of the glass. These points are essentially weak spots that can break much more easily when the glass comes into contact with heat or pressure. The annealing process works to strengthen these weak spots. This is done by gradually cooling the glass down to a specified degree. After getting cold enough, the pressure points will ultimately disappear.
The annealing process is critical and requires pinpoint accuracy, as it must take into account the strain point of the glass. This is the point at which the glass temperature must be met to eliminate the weak spots. The glass is then cut to the specified size for the appropriate end application (vehicle windshields, entrance doors, shower enclosures, etc). After this has been completed, the glass will be tempered.
The tempering process is comprised of placing the sheet of glass on a conveyor belt. It will be rolled into a furnace that heats the glass to about 630 degrees Celsius. The glass will reach the annealing point; the inner sections of the glass will not be completely hardened at this point. While this is happening, the outer edges of the glass are cooled in rapid bursts in a process called quenching.
This involves spraying the edges with controlled amounts of pressured air from various angles and degrees. As a result, the inner section will attempt to extend away from the outer edges, as it is cooling much more slowly. The tension between the two is what gives tempered glass its strength and durability. Tempering can also be achieved via the application of chemicals (melted potassium nitrate) that work to produce the same results at the molecular level. The bond of the glass is much stronger by this method, but is also much more costly.