Spontaneous glass breakage

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Spontaneous glass breakage is a phenomenon associated with glass cookware, furniture, windows, and other architectural glass, in which the glass shatters for no apparent reason.

Tempered plate glass (also known as toughened glass) is used for balconies, doors, skylights, furniture, oven glass, and automobile windshields. Spontaneous breakage of tempered glass is most commonly caused by chipped or nicked edges during installation, stress caused by binding in the frame, internal defects such as nickel sulfide inclusions, thermal stresses in the glass, and inadequate thickness to resist high wind loads. Tempered glass breaks into many small pieces, avoiding the danger of injury caused by sharp edges and flying shards (Rupert 2013).

Glass cookware made with heat-strengthened or tempered glass can unexpectedly shatter due to thermal stress resulting from temperature changes during reportedly normal use. Fracturing produces shards in both types of glass.

Tempered glass fracture pattern. Image courtesy Wikimedia Commons user violetbonmua

Related Terms[edit | edit source]

exploding glass; shattering; chipping; inherent vice; tempered glass; toughened glass; heat-strengthened glass; Tuf-Flex [Libbey-Owens-Ford]; Herculite [PPG]; Pyrex [Corning, World Kitchens]

Translation[edit | edit source]

English spontaneous glass breakage
French bris de verre spontané
Spanish rotura de vidrio espontanea
Portuguese quebra de vidro espontânea
Italian rottura spontanea del vetro
German spontaner Glasbruch
Russian спонтанное разбитие стекла (spontannoye razbitiye stekla)
Chinese (Traditional) 自發性玻璃碎裂

Discussion[edit | edit source]

Spontaneous glass breakage or exploding glass has been the cause of highly publicized incidents involving domestic cookware, furniture, glass doors, and windows. All have a soda-lime-silicate composition, and are treated with various strengthening methods.

Glass cookware was originally made with borosilicate glass by Corning, which marketed their products under the brand Pyrex. As early as the 1950s, glass cookware was also made with soda-lime-silicate glass that was heat-strengthened to increase resistance to thermal stress. By 1998 when Corning licensed Pyrex to World Kitchen LLC, the popular cookware was made exclusively with heat-strengthened or tempered soda-lime-silicate glass. Borosilicate glass has a very high thermal stress resistance, whereas temperature changes within normal kitchen use can cause soda-lime-silicate glass to fracture. There is usually a delay between the initiating thermal shock and glass failure, causing the fracture event to seem "spontaneous" (Bradt 2012). Glass cookware fractures take the form of shards regardless of heat-strengthening or tempering, which indicates that the heat treatment is not sufficient for normal use; properly strengthened tempered glass fractures into small "dice" rather than shards. There are many compositions of soda-lime-silicate glass, each having different properties and thermal resistance. Statistically, only a small fraction of glassware sold experiences catastrophic breakage, but the nature of the failures has caused public outcry and scrutiny of the materials, manufacturing processes, and regulation of these consumer products.

Tempered plate glass is made by reheating panels of glass to about 650 C (1200 F), then quenching in streams of cold air. As the outer surfaces of the panel rapidly cool they are put into compression, while the slowly cooling center is in tension. Tempered glass is 3-5 times stronger than conventional glass of the same thickness, and gains the characteristic of breaking into small equiaxed pieces in a fracture process known as dicing (Konrad 1995,Bradt 2012). It was first used in 1942, and is also known as Tuf-Flex [Libbey-Owens-Ford] and Herculite [PPG].

During cutting, shipping or installation, tempered glass can be easily chipped or nicked on its edges. This may not cause immediate breakage, but it initiates internal stresses that increase over time with cycles of expansion and contraction. Similarly, poorly fit frames or gaskets can cause internal stress which exceeds the strength of the glass, especially when subjected to expansion and contraction or wind deflection. In large pieces of sealed insulating glass, thermal stress can cause breakage if the installation method is too rigid; when the center of the tempered glass pane is hotter than the edges, the center expands and the edges are put under tensile stress.

Nickel-sulphide (NiS) inclusions can be present in plate glass as a result of virtually non-preventable impurities introduced during the float glass manufacturing process, most likely from stainless steel machinery containing nickel and fuel containing sulphur. The inclusions usually range in size from 0.05-0.5 mm in diameter, and they do not always cause breakage when present. In tempered glass, NiS inclusions can expand over time because of an arrested phase transformation during quenching (Swain 1981). If the inclusion is located in the center tensile zone of the glass panel, its expansion may cause enough stress to shatter the glass, leaving a characteristic "figure eight" or "butterfly" pattern. The presence of NiS inclusions has been estimated to be 1 in every 500 glass panels, with a fracture rate of 0.8% (Napier and Blakely 2010). Failure has occurred most frequently 2-7 years after installation, and none appear to happen beyond 30 years' time (Karlsson 2017).

Heat-soaking has been introduced as a measure to initiate or accelerate phase change in NiS inclusions, causing the glass to break before shipping. Although manufacturers offer it, it is not a guarantee that spontaneous breakage will not occur, and it can reduce profits by destroying glass that may not have broken after installation.

In architectural uses, the risk of unexpected shattering has caused the building industries to reevaluate how the public is exposed to tempered glass. The most danger is caused by tempered glass overhead or in public areas. Many high-rise buildings use tempered glass, and greater scrutiny has been placed on safety measures to prevent pieces of glass from falling after breakage. The addition of safety window film has been recommended, and some building codes have been changed to require laminated or heat-soaked tempered glass in areas of risk.

References[edit | edit source]

Bradt, R. C., and R. L. Martens. 2012. “Shattering Glass Cookware.” American Ceramic Society Bulletin 91 (7): 33–38.

Karlsson, S. 2017. Spontaneous fracture in thermally strengthened glass ‐ a review and outlook. Ceram‐Silik. 61 (3):188–201.

Konrad, K., K. Wilson, W. Nugent, and F. Calabrese. 2014. "Plate Glass", in Twentieth-Century Building Materials. T. Jester (ed.). Los Angeles, California: Getty Conservation Institute. ISBN 1606063251.

Napier and Blakely. 2010. “The shattering truth about glass.” http://napierblakeley.info/maintenance/the-shattering-truth-about-glass/ (Accessed April 5, 2019).

Rupert, Michael L. 2013. "Spontaneous Glass Breakage: Why it happens and what to do about it." The Construction Specifier 66 (12): 10-19. United States: Construction Specifications Institute, Inc. ISSN 0010-6925.

Swain M.V. 1981. "Nickel sulphide inclusions in glass: an example of microcracking induced by a volumetric expanding phase change." Journal of Materials Science, 16 (1), 151-158. doi:10.1007/bf00552069.

Tempered glass. 2016. CAMEO (Conservation and Art Materials Encyclopedia Online). Museum of Fine Arts, Boston. http://cameo.mfa.org (accessed 2 April, 2019).

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