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GLASS QUALITY
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.
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 centre 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.
Most common types of tempering furnaces
There are three basic types of tempering furnaces most commonly used in the fabrication of tempered glass. The first and oldest of the three types positions the glass vertically (held by metal tongs) as it moves through the furnace. The second and most common furnace style in use today has the glass positioned horizontally on ceramic rollers. The third style is a gas hearth style, which transports the glass in a horizontal position on a bed of gas (at a 5° slope) and moves through the furnace with edge rollers.
Why does some tempered glass scratch?
The majority of the scratches found on tempered glass result from poor glass quality. The surface quality of tempered glass will have a direct effect on the possibility of scratching the glass during cleaning. Low-quality tempered glass has fabricating debris fused to its surface, which at the time of cleaning has a very high likelihood of being dislodged and dragged across the glass surface, resulting in scratching.
Notice in the following photo how each of the scratches has a clear point of origin. This point is the location where a glass particle was fused to the surface during the tempering process. This particle should have been washed from the glass before it was sent into the tempering furnace, but inadequate washing procedures allowed it to remain on the glass.

Defective tempered glass
The following photos are of an isolated defect on a tempered glass sample. The sample was a factory-direct piece of glass, and was not subject to a construction environment. The after photo shows the scratch that resulted from passing a new razor blade over the glass surface. Notice how the point of origin (and therefore the cause) of the scratch is the original surface defect. The black lines on the photos are felt pen markings used to identify the location of the defect.

Single glass defect, before cleaning
 
Resulting scratch from glass defect, after cleaning
The following photos show a new razor blade before and after it was passed over the surface of a piece of defective tempered glass. Notice the large amounts of fabricating debris present in the after photos.

New razor blade
 
New razor blade with fabricating debris from defective tempered glass surface

Cutting edge of new razor blade
 
Cutting edge of razor blade with fabricating debris taken from a defective tempered glass surface
The following photos are magnified images of the fabricating debris from the razor blade in the previous photos.

Isolated fabricating debris taken from a defective tempered glass surface  
Isolated fabricating debris taken from a defective tempered glass surface
Where does the fabricating debris come from?
Now that it's clear why some tempered glass scratches, the next question is "Where does the fabricating debris come from?" To answer this question, we need to look back at how the tempered glass is made. The third step in the process was to wash the glass after it had been sized and ground, but not all glass manufacturers do an adequate job at this step. Most problems in this area are related to the lack of maintenance to the washer and tempering furnace. If the glass washer and tempering furnace are not properly maintained, fabricating debris will build, making them less effective in cleaning and tempering the glass. Due to this, the glass will exit the washer with much of the fabricating debris still present as the glass is sent into the tempering furnace. Once the glass is inside the furnace, the debris will begin to liquefy and fuse itself to the roller side surface of the glass and to the furnace rollers. It is this fabricating debris that causes the poor quality surface and the scratching on defective tempered glass. When a window cleaner removes construction debris from the glass surface, they also remove these defects, which scratch the glass as they're moved across the glass surface.
Over time, the tempering furnace will also become contaminated with fabricating debris and must be serviced to clean the furnace rollers. To date, washing glass prior to tempering is not required or enforced by any regulation or standard.
Identifying and Tracking Defective Tempered Glass
Due to the severity of scratches related to defective tempered glass it is vital that one be able to identify a batch of defective tempered glass as quickly as possible. There are a few major signs that identify low-quality tempered glass. The first is that scratches related to defective tempering are always widespread on the glass surface, usually covering the entire pane. The second is that scratching will only occur on the side of the tempered glass that was in contact with the rollers in the tempering furnace. The side facing up in the tempering furnace is usually defect free, and will not scratch when cleaned with the same window cleaning tools and techniques.
Given the location of the furnace roller relative to the glass sample is so important, it is obvious that a method of determining which side was the roller side is vital. The key to this determination is the tempering stamp. Each tempered glass manufacturer has their own tempering stamp that they put on their glass samples (see photo of tempering stamp to the left). By identifying the location of the tempering stamp, the roller side of the glass can be determined. The important part in this determination is the type of tempering stamp.
There are two major types of stamps used: sand blasted and porcelain. A sand blasted stamp is located on the roller side of the glass and will identify the potentially defective surface. A porcelain stamp is located on the side opposite the rollers and will identify the quality surface.
This is a good time to point out that tempered glass is not always installed in the proper location in a building. Sometimes, with several windows being the same size, they are mistakenly installed in the wrong location. A check for the tempering stamp is the best way to determine if the glass is truly tempered.
How to determine if the tempering stamp is sandblasted or porcelain
As shown above, the tempering stamp holds the answer to which side is the potentially defective side of the glass. However, identifying the stamp side is not enough. The type of stamp must also be determined. The best way to determine the type of stamp is by passing a razor blade over it. If the blade passes over the stamp without any drag it is a sand blasted stamp. Otherwise, the stamp is a porcelain stamp. Keep in mind that some tempering stamps are applied in a mirror image to the surface, and can only be read through the glass, not from the surface on which they are applied.
IG (Insulated Glass) Units
The popularity of IG units has complicated the issues with defective glass as two of the surfaces are always hidden from contact. This often gives the false appearance that the tempering defects are not a consistent problem, as the defective surface is inside the IG unit.
Before discussing the details of IG units further, let's cover the standard numbering convention used for IG unit surfaces. The surfaces of IG units are numbered 1 through 4. Surface #1 is the surface facing the exterior of the building. Surfaces #2 and #3 are located inside the IG unit. Surface #4 is the surface facing the interior of the building.
Most insulated glass fabricators do not usually track stamps on tempered glass, unless requested for a specific reason. If an application required only high-quality tempered glass, tracking the stamp might then be required. Listed below are some examples of applications that require only defect-free glass, both tempered and annealed.
Mirrors - The silvering process will show any debris on the surface.
Laminated glass - The lamination will show any debris between the glass layers.
Low-E coatings - The coating will show any debris on the surface.
In recent years, Low-E coatings have become a very popular means of further insulating a building (usually the coatings are applied to surface #2 in hot areas and surface #3 in cold areas). In these applications, any surface that could contain defects should be used only as surface #1 or #4. The unfortunate side effect of this is that it forces the potentially defective glass surface to be exposed to the elements, and subjects the window cleaner to the risk of scratching the defective surface during normal construction window cleaning. 
