Glass is a brittle material whose strength is controlled almost completely by the character and distribution of flaws across the surface and edges of the glass. Some of the flaws are created in the manufacturing and fabrication, some are created during the handling and installation of the glass, and some are created during the in-service tenure of the glass. The in-service flaws are created by a wide variety of mechanical and environmental exposures that the glass surface comes into contact with while it is installed in the building. It is known that the normal in-service conditions associated with typical glass plates can reduce the strength of glass by as much as 50 percent over the period of a few years. This strength reduction occurs with time and is the result of washing, cleaning, and normal exposures.
If the glass is subjected to more severe exposures, even further strength reductions can result. These more severe exposures are usually the result of extreme mechanical contact that results in abrasion, scratching, scarring, or other visible surface degradations. Hence, the appearance of visible surface or edge damage indicates that the strength of the glass may have been significantly reduced by the in-service conditions to which it was exposed.
It is not recommended that window film be applied to glass with obvious signs of surface or edge damage.
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