How to identify tempered windows

We do not recommend using our window films on non-tempered Low-E glass. If you have done the Low-E flame test and your windows are Low-E, you can safely use our film ONLY if the glass is tempered. Here are a few ways you can easily identify if your windows in your home are tempered. 

Imperfections are visible on the surface of the glass.

Look for imperfections, if any. If you identify any distortions, slight bending on the glass, then most likely it is a tempered glass. These imperfections are a result of the intense heating procedure. When glass gets tempered, it becomes mendable when it is super-hot;, so when it is picked up using the tongs, the process leaves slight imperfections. Look closely and you will see for yourself.

Polarized glasses can help you to see the difference.

Believe it or not, one of the best was to identify tempered glass is  if you use polarized glass. View your glass in sunlight while using polarized glasses. We assure you that way you will see the difference. It looks like you are witnessing the tempered surface using a microscope. If you see darkened shady lines or spots spread across the glass surface, this is it. That glass is a toughened one. These lines are formed during the tempering process, as the machine rollers go over the glass.

Stamp identifier for authentic tempered glass.

Every tempered glass has a carved symbol, which works like an identifier for tempered glass. This stamp shows the manufacturer name and the CPSC standards, the stamp is a key symbol to give you proof whether the glass is tempered or not.
If the glass was bought directly from the manufacturer, you can check for a logo carved on the glass. Having a carved symbol on the glass is proof that you most likely have tempered glass.

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*Article Source: https://www.fabglassandmirror.com/blog/how-to-tell-if-glass-is-tempered/