Tempered glass is stronger than annealed glass due to uneven heating during production, creating layers that make it impact resistant. It’s used in construction, automotive and microwave glassware. Heating beyond 1,112°C and rapid cooling creates the layers. It’s more resistant to shocks but can completely explode if enough force is applied. It’s also more expensive to produce and has a softer surface prone to scratches.
Tempered glass is created with uneven heating which causes the cooling glass to form layers rather than a solid sheet, making it much more impact resistant than “annealed” glass. It’s not indestructible, but when it gives way, this glass shatters into a spray of cube-like pellets rather than jagged shards.
The patent for this type of glass was first filed in the early 20th century, but its popularity has increased in recent decades due to its use in the automotive and construction industries, as well as microwave glassware. Common current uses include automotive side and rear windows and windshields, storefronts, patio doors, and shower doors.
The technique used to create tempered glass involves heating glassware beyond the annealing point of 1,112°C (600°F). The beyond determines the various degrees of “temper” – it can range from twice as strong as annealed glass to six times the tensile strength. Once the glass is heated, the exterior is rapidly and artificially cooled, usually by jets of fresh air. This method solidifies only the outside, leaving the inside melted and fluid. This is what creates the various layers within the glass.
Due to its stratification, the surface of this glass is more resistant to shocks. The same thrown object that would punch a single hole in an annealed glass plate would likely bounce off a tempered glass comparison plate. The downside is that because glass has more units in its tensile strength, it would be more likely to completely explode if enough force was applied. This is sometimes a security concern, as once a window is breached, there are no shards of glass to deter entry. This glass is also more expensive to produce, due to the extra step needed to cool the outside. Curiously, it also has a softer surface which is more prone to scratches.
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