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Heat Strengthened or Tempered?

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

If you have a pair of polarizing sun glasses, 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. Using the polarized sunglasses, hold one lens close to one eye and slowly rotate it. If the glass appears to have a patchy, grid like, rainbow appearance and as you rotate the lens (this pattern will move and fluctuate during rotation), then you know that the glass is tempered or heat strengthened (it could be either). Below is a picture viewed by a camera through a polarizing filter of panes that were both heat strengthened and laminated. On many automobile rear windows, you will see this kind of grid like/checkerboard pattern as well. 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 these glass types will allow for film to be installed on them with substantially higher solar energy absorption values.  If you fail to see this obvious stress pattern, it does not necessarily mean that the pane is not tempered however the odds are that it is not.

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

How Heat Strengthened Glass Appears Through Polarized Sun Glasses. Viewed by a camera through a polarizing filter.

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