Tempered glass is stronger than regular glass because it undergoes a specific heat treatment that makes it harder and more durable against impacts and temperature changes.
Tempered glass is ordinary glass subjected to a specific thermal treatment. First, the glass is heated to a very high temperature (around 650 to 700°C) until it becomes soft without melting. Then, the surface of the glass is cooled very quickly using jets of cold air. This creates a difference in cooling: the surface cools quickly and contracts, while the interior cools more slowly. As a result, the outside of the glass enters a state of permanent compression, while the inside is under tension. This compressive-tensile balance gives tempered glass its unique mechanical properties and exceptional strength compared to ordinary glass.
Ordinary glass has a generally homogeneous structure, with similar stress everywhere on the surface and inside. In contrast, tempered glass has a special structure: its surface is under compression (compressed inward), while its core is subjected to slight tension. These opposing stresses come directly from the specific manufacturing process of tempered glass, based on rapid cooling. This internal organization, like a sandwich, gives tempered glass its ability to better withstand shocks and impacts, unlike regular glass, which is very fragile.
The increased resistance of tempered glass mainly comes from a clever trick: a play of internal tensions. During the production process, the glass is heated to very high temperatures before being cooled very rapidly by blowing cold air on it. This hyper-rapid cooling creates an imbalance: the surface cools faster and contracts immediately, while the interior remains hot longer and cools slowly. As a result, the hardened surface ends up in a state of compression, while the core remains under tension. When you apply a shock to tempered glass, these complementary tensions make the material much more resistant: to crack the glass, the applied stress would first have to overcome this compressed surface zone. It's not easy, so your tempered glass can absorb impacts much better than ordinary glass.
Tempered glass withstands stress better because its outer surface is constantly under compression. This means it appears to be "tight" at all times, which allows it to better resist impacts or accidental bends. In comparison, ordinary glass, which does not have these balanced internal tensions, cracks quickly when subjected to too much force. On the thermal resistance side, the same observation applies: tempered glass tolerates abrupt temperature changes much better. It can easily withstand significant hot-cold fluctuations without cracking, while regular glass can sometimes shatter at the slightest high thermal shock.
In automobiles, side and rear windows typically use tempered glass, as it breaks into small, less sharp pieces than ordinary glass, thereby protecting passengers. Similarly, high-end mobile screens like the iPhone often benefit from tempered glass to limit cracks from accidental impacts, greatly reducing the risk of breakage. At home, the oven door and even the shower cabin also use it for its high resistance to temperature fluctuations and increased safety in case of impact. Finally, in various sports facilities—such as the windows of squash courts—tempered glass is preferred, as its strength easily withstands regular impacts and vibrations.
Although tempered glass is very resistant to shocks and thermal stresses, it remains vulnerable to damage on its edges. A small chip on a corner can significantly reduce its overall strength.
Some droplets, known as "Prince Rupert's drops," are formed by quenching glass and exhibit impressive resistance to shocks at their wide end, while being able to shatter completely if their thin end is damaged.
When tempered glass breaks, it shatters into small, less dangerous pieces, unlike ordinary glass which produces sharp shards. This makes it a safer option for both domestic and industrial use.
Thanks to its increased thermal resistance, tempered glass can withstand sudden and significant temperature variations, making it ideal for applications such as oven doors or cooktops.
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