Though window protection and window cleaner skill are vital to ensuring a quality finished product, no amount of these can overcome low-quality windows.
In recent years, low-quality tempered glass has become a major source of scratched glass. Window manufacturers ultimately make the decision on the quality of the glass they install in their windows, and should therefore conduct constant quality inspections to insure only quality, blemish-free tempered glass is being used.
Note: Many of the topics covered on this page are also addressed in the video Tempered Glass Defects.
WHAT IS TEMPERED GLASS (COMMONLY KNOWN AS HEAT TREATED GLASS)?
Tempered glass is glass specifically designed for use in areas with a high risk of contact and breakage. When broken, tempered glass breaks into very small pieces, making it safer than annealed glass. It is 4-5 times stronger than annealed glass, and can withstand more wind, heat, and impact before breaking.
WHERE IS TEMPERED GLASS USED?
In residential applications, there are certain areas that require the use of tempered glass. The more common locations are listed below. For the specific requirements in your area, check your local building codes.
All doors (sliding-glass, French, and patio).
Side windows within 24 in. of an opening or door.
Stair landing windows within 4 ft. of the floor.
Bathroom windows, within 5 ft. of the floor.
All windows within 18 in. of the floor.
All tub and shower enclosures.
FACTS ABOUT TEMPERED GLASS
Tempered glass is no harder or softer than annealed (non-tempered) glass.
The entire glass pane is tempered from surface to surface and both sides are equal in hardness.
The exterior surfaces of tempered glass are in compression and the center is in tension.
Surfaces of tempered glass are not changed as a result of the tempering process.
Tempered glass is no more porous than annealed glass.
Every piece of tempered glass must have a permanent stamp or logo to signify it is tempered glass. However, there are no standards which regulate which surface of the glass is to receive the stamp or where the stamp is to be placed on the surface. Tempering stamps can face in or out on an insulated glass unit.
There is no standard or regulation requiring tempered glass to be washed or clean prior to tempering.
MYTHS ABOUT TEMPERED GLASS
Tempered glass scratches easier than non-tempered glass.
Tempered glass surfaces are different than annealed glass surfaces.
Tempered glass stamp or logo is located in the same location on all windows.
Tempered glass is only tempered on one side.
FABRICATING PROCEDURE FOR TEMPERED GLASS
The fabrication of tempered glass can be broken up into 4 major steps: sizing, edging, washing, and tempering. The details of each step are listed below.
SIZING THE GLASS
The first step in the process of manufacturing tempered glass is to cut a piece of annealed glass to the desired size. The sizing of the glass must take place before the tempering process because attempted cutting of tempered glass with result in breakage.
EDGING THE GLASS
Once the sample is cut to the desired size, it is necessary to seam the edges of the glass. This is normally done with a diamond wheel grinder or sander, and results in a piece of glass with squared off and smooth edges.
WASHING THE GLASS
The sizing and edging work done in the previous steps generates fabricating debris which is deposited over the entire surface of the glass. For this reason, all glass should be washed prior to entering the tempering furnace. If this debris is not completely washed off prior to the glass entering the tempering furnace, the remaining debris will be fused to the glass, resulting in a surface defect. This is by far the major cause of scratching on defetive tempered glass.
TEMPERING THE GLASS
With the piece of glass sized, ground, and washed clean, it is ready for the actual tempering procedure. In this step, the glass is heated in a tempering furnace to approximately 1200°F. The glass is then removed from the furnace and immediately quenched with cold air, reducing the temperature of the glass to 400-600°F. This quenching produces the temper.
