Some say the only way to confirm tempered glass is by breaking it—if it shatters into small, blunt fragments, it’s tempered. While this is true, it’s a destructive method. Tempered glass undergoes a process of heating followed by rapid cooling, which creates internal stress, causing it to break into small, harmless pieces upon impact. However, breaking the glass isn’t always practical or desirable.
Fortunately, there is a non-destructive method to identify tempered glass:
A reliable way is to check for air marks, iridescence, or anisotropy—all of which indicate the presence of internal stress. To do this, place the glass under suitable lighting conditions. Then, either view it at an incident angle of 56 degrees or use polarized glasses. If the glass is tempered, you’ll observe irregular bands or spots of stress marks on the surface. Non-tempered glass will not display these patterns.
This method offers a practical and effective way to identify tempered glass without damaging it.