Tempered glass is a type of safety glass that is typically four to five times stronger than standard, or annealed, glass of the same thickness. This increased strength and its unique failure mode are why it is used in applications where human safety and durability are concerns, such as car side windows and shower doors. The fundamental difference lies in the internal stress profile engineered into the material, which transforms a fragile substance into a robust one.
Understanding Glass Weakness
Standard annealed glass is inherently vulnerable to breakage because of its surface condition. The manufacturing process inevitably leaves microscopic surface flaws, often called Griffith flaws, on the glass. These imperfections act as points of stress concentration.
When the glass is subjected to tension, the stress intensifies at the tip of one of these surface flaws. This concentrated stress causes the minute crack to rapidly propagate through the material. Since glass is brittle, it fails easily under tension, leading to the formation of large, jagged shards.
The Thermal Tempering Process
The tempering process begins by heating a sheet of annealed glass in a furnace. The glass is heated uniformly to an extremely high temperature, typically between 600°C and 700°C (1,112°F and 1,292°F), which is just above its softening point.
Once the glass reaches this state, it is rapidly cooled in a process known as quenching. High-pressure air jets are directed at both the top and bottom surfaces, causing the exterior surfaces to harden and solidify almost instantly.
The interior of the glass remains hotter and continues to cool and contract more slowly. This differential cooling locks in the internal stresses necessary for increased strength. All cutting, grinding, or drilling must be completed before tempering, as the finished product cannot be modified without shattering.
The Physics of Surface Compression
The strength of tempered glass stems from the permanent stress profile created during the quenching phase. As the interior cools and contracts, it pulls inward against the solidified outer layers. This action forces the glass’s exterior surfaces into a state of high compressive stress.
This outer compression is balanced by a corresponding layer of high tensile stress in the glass’s core. Standard specifications require fully tempered glass to have a minimum surface compression of 69 megapascals (10,000 pounds per square inch). Since glass is much stronger under compression than tension, this pre-compressed surface effectively shields the internal tensile layer.
For a crack to propagate and cause failure, an external force must first overcome this deep layer of compressive stress. The compressive force essentially closes the microscopic surface flaws that typically cause breakage in annealed glass. This mechanism makes the glass significantly more resistant to impact and bending forces.
The Safety Feature of Dicing
The engineered internal stress profile results in a distinct failure mode when the glass’s strength is exceeded. Once a crack penetrates the outer compressive layer and reaches the inner tensile core, the stored energy is instantly released. This sudden release causes the entire piece of glass to shatter simultaneously.
The material disintegrates into a myriad of small, blunt pieces, a characteristic known as dicing. Instead of producing large, sharp shards, the resulting fragments are small, pebble-like granules. This controlled failure pattern is why tempered glass is legally classified as safety glass, significantly reducing the risk of serious injury upon breakage.