Federal regulations prohibit the use of ordinary glass in certain locations where breakage might have life-threatening consequences, such as in sliding patio doors, doors on shower stalls and bathtubs, and glass panels near doors. Safety glass (or plastic) must be used for such purposes. There are several types of stronger glass:
After the glass has been cut to size, it is heated to about 1,200°F and then rapidly cooled. The rapid cooling freezes internal stresses into the glass that make it four to five times stronger than untreated float glass, and that cause it to shatter into many small pieces when it breaks. (The lenses in eyeglasses, if glass, are tempered.) Fully tempered glass cannot be cut, drilled or edged after tempering.
Heat-strengthened glass has been heat-treated at a lower temperature than fully tempered glass. It breaks in much the same way, but it is only about twice as strong as ordinary float glass. It is not considered a safety glass, but may be used where extra strength is needed.
Laminated glass is a sandwich, a layer of plastic between two sheets of glass. When it breaks, the glass fragments remain adhered to the plastic. Automobile windshields are laminated glass. Local dealers can cut laminated glass to size.
Wired glass is rolled glass with welded wire mesh laminated in the middle. It is mainly used in locations where the glass must remain in position even after it cracks, for example to block drafts during a fire. It was formerly very common in skylights. It is not as strong as float glass.
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Last revised: 15 October 2013.