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Toughened glass is created with uneven heating that causes the cooling glass to form layers instead of a solid sheet, making it much more resistant to impact than "annealed" glass. It isn't indestructible, but when it does give way, this glass comes apart in a spray of cube-like pellets rather than jagged shards.
The patent for this type of glass was first filed around the beginning of the 20th century, but its popularity has increased in recent decades because of its use in the automobile and construction industries, as well as in microwaveable glassware. Among the most common current uses are for side and rear windows and windshields in automobiles, display cases, patio doors, and shower doors.
The technique used to create toughened glass involves heating glass objects to beyond the annealing point of 1,112°F (600°C). How far beyond determines the varying grades of "toughening" — it can range from twice as strong as annealed glass to as much as six times the tensile strength. Once the glass is heated, the outside is rapidly and artificially cooled, usually by jets of cool air. This method solidifies only the outside, leaving the interior molten and fluid. That's what creates the various layers within the glass.
Because of its layering, the surface of this glass is more resistant to impact. The same thrown object that would create a single hole in a pane of annealed glass would likely bounce off a comparative pane of toughened glass. The downside is that, because the glass has more unity in its tensile strength, it would be more likely to explode completely if enough force was applied. This is sometimes a security concern, since once a window is breached, there are no glass shards to discourage entry. This glass is also more costly to produce, because of the extra step needed to cool the exterior. Curiously, it also has a softer surface that is more prone to scratches.