Blair's Texas Window Tinting and more

Heat Strengthened or Tempered?

How can I tell if my glass is heat strengthened or tempered?

If you have a pair of polarizing sunglasses, you can look through them at the window. It's best to have sunlight passing through or shining on the window when viewing it. You should also be looking at the window at an angle rather than straight-on. Hold one lens close to one eye and slowly rotate it. If the glass appears to have a patchy, quilted, rainbow appearance and as you rotate the lens (this pattern will fluctuate during rotation), then you know that the glass is tempered or heat strengthened (it could be either). Below is a picture of panes that were both heat strengthened and laminated, viewed by a camera through a polarizing filter. On many automobile rear windows, you will see this kind of rainbow/checkerboard pattern with the naked eye. This is a result of stress patterns being locked into the glass during the tempering process as jets of air quickly cool the glass surface in the tempering ovens. It is important to know whether panes are heat strengthened or fully tempered, since such panes allow for film to be installed on them with substantially higher solar energy absorption values.

 Remember two things:

1. If you fail to see this obvious stress pattern, it does not necessarily mean that the pane is not tempered (you can check with an electronic tempered glass indicator to be sure).

2. The presence of the pattern only indicates that pane has been heat treated. Whether it is heat strengthened or fully tempered can only be known by a label marker on the glass, knowing the source of the glass, or by breaking it to assess the fragmentation characteristics (a method a bit on the expensive and self-defeating side).

How Heat Strengthened Glass Appears through Polarizing Sunglasses. Viewed by a camera through a polarizing filter.