Deborah Slaton and David S. Patterson, AIA
The use of fully tempered (FT) glass is common in the building industry. While its increased strength (approximately four times stronger than annealed glass and twice as strong as heat strengthened) and distinctive breakage pattern that can reduce the potential for injury are regarded as benefits, its use should be carefully considered. A primary concern is FT glass may break without warning due to the presence of certain expansive inclusions, introduced to the material during initial fabrication. While the occurrences of ‘spontaneous breakage’ appeared to be waning over the past 20 years or so, an apparent escalation in the incidence of breakage attributed to this phenomenon has recently been noted, an upturn that may be the result of an increase in the use of nondomestic glass.
Breakage of FT glass from impact is often difficult to differentiate from spontaneous breakage attributed to an inclusion, as inclusions contributing to the latter are often too small to detect visually without the use of magnification. A distinguishing characteristic often used as an initial visual indication of spontaneous breakage is the ‘figure-eight’ or ‘cat’s eye’ pattern that is formed at the point of fracture origin. However, confirmation of the presence of an inclusion most often requires laboratory study.
Nickel sulfide (NiS) is the most common type of inclusion associated with spontaneous glass breakage. The NiS inclusion, which can be as large as 20 mils (0.020 in.) in diameter, exists in two crystalline forms:
- high temperature or ‘alpha’ form; and
- less dense, low temperature or ‘beta’ form.
During the slow cooling of glass while it is in production, as is typical with annealed glass, NiS (if present) generally undergoes a phase transformation from the alpha to beta stage and expands while the glass is still relatively fluid. In this case, the glass is able to withstand particle expansion without adverse consequence. However, during the production of heat-treated glass (especially tempered glass), the glass is cooled rapidly, and the NiS remains in its alpha phase. The NiS inclusion is then exposed to heat from the sun and from within the building while the glass is in service. The cumulative effect of the heat eventually triggers the phase transformation of the particle (from alpha to beta) and consequent expansion. When the particle is located within the center 60 percent of the glass thickness, which is in tension from the heat-treatment process, the expansion can cause a fracture to propagate and ‘spontaneously’ break the glass.
Most reports of spontaneous breakage occur within the first few years of service and the potential risk of incidence diminishes with time. For glass in exterior building façades, experience has shown failures from contaminated batches of raw glass will typically run their course in approximately eight to 10 years following installation. However, glass breakage due to the presence of NiS has been observed in buildings after more than 20 years of service.
Where safety or structural requirements mandate the use of FT glass, designers should be judicious with its use, and be aware of and educate their clients regarding the possibility of spontaneous breakage.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.
Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at email@example.com.
David S. Patterson, AIA, is an architect and senior principal with WJE’s office
in Princeton, New Jersey. He specializes in investigation and repair of the building envelope. He can be reached at firstname.lastname@example.org.