Identifying the type of glass is necessary for safety and modification, as tempered glass cannot be cut or drilled after manufacturing. Standard, or annealed, glass is simply cooled slowly, resulting in a relatively weak structure. Tempered glass, also known as safety glass, undergoes a process of intense heating followed by rapid cooling, which dramatically increases its strength. Identifying the type of glass is necessary before attempting any alteration, such as cutting or drilling.
Visual Clues and Manufacturer Marks
The most direct way to identify tempered glass is by physically inspecting the surface for a manufacturer’s mark, often called a “bug.” This permanent stamp is typically etched or sandblasted into one of the corners of the glass panel. The mark usually includes the manufacturer’s name or logo and compliance standards, such as ANSI or CPSC.
While the presence of this stamp definitively confirms the glass is tempered, its absence does not automatically mean the glass is annealed. Some older or custom pieces may lack a clearly visible bug.
The heat-treatment process can sometimes leave behind slight surface imperfections that are visible upon close examination. These may appear as a gentle waviness or rippling, known as roller waves, caused by the glass moving over rollers in the furnace. These minor visual distortions are a sign of the thermal processing used to strengthen the material.
Identifying Internal Stress Patterns
A more reliable non-destructive method for identification involves exploiting the physical properties created during the tempering process. Tempered glass is intentionally structured with a high degree of internal stress. This is achieved by rapidly cooling the exterior surface after heating, causing the outer layer to solidify and enter a state of high compression.
As the interior cools more slowly, it attempts to shrink but is restrained by the rigid outer shell, placing the core in a state of high tension. These opposing forces—compression on the outside and tension on the inside—are what give the material its superior strength.
This internal stress structure can be made visible using polarized light, such as that emitted by an LCD screen or filtered through polarized sunglasses. When looking at the glass through the filter, the polarized light interacts with the stressed areas, an effect called stress birefringence.
The user will observe distinct dark lines, dots, or subtle checkerboard patterns, known as quench marks or strain patterns, across the glass surface. These visible patterns directly map the internal stress zones and confirm the glass has undergone the rapid-cooling process.
Understanding Failure: How Tempered Glass Breaks
The most dramatic difference between the two types of glass lies in their failure mode. When the internal tension in the core of tempered glass is breached, the stored energy is instantly released, causing the entire panel to shatter simultaneously into numerous pieces.
This failure results in thousands of small, relatively uniform, cube-like fragments. This process is commonly called “dicing,” and the small, blunt edges of the resulting pieces greatly reduce the risk of serious laceration injuries compared to standard glass. Tempered glass is four to five times stronger than its annealed counterpart.
In sharp contrast, when standard, annealed glass breaks, it fractures into large, irregularly shaped shards. These pieces often feature extremely sharp, jagged edges that pose a significant safety hazard. This is why annealed glass is not used in high-risk areas like shower enclosures or vehicle side windows.
Identifying the glass type is crucial before any modification. The act of attempting to cut, drill, or excessively score tempered glass will immediately trigger this dicing reaction. The structural integrity of tempered glass is dependent upon the compressed state of its outer surface. Disrupting this layer releases the internal stress, meaning modification attempts should be abandoned to prevent immediate destruction of the sheet.