Spontaneous Breakage of Tempered Glass

All float glass contains some level of blemishes, including stones, seeds and bubbles, which are an unavoidable part of the glass manufacturing process. ASTM C1036, Standard Specification for Flat Glass, is the industry standard that provides the specific requirements for size, intensity and frequency of blemishes that are allowed.

One type of stone (crystalline blemish) is nickel sulfide (NiS). Nickel can be present in any particular batch of glass, derived from trace amounts of nickel in the sand, the fuel, the firebrick, or even the machinery used to mix the batch. Sulfur can come from the fuel or from sodium sulfate, one of the glass batch's ingredients. When NiS is present, the quantity is typically extremely small.

Most NiS stones are stable and cause no problems. However, there is a small quantity of very rare NiS stones formed, which, when cooled slowly from about 750°F down to room temperature, undergo a change in crystalline structure (a phase change) that results in an increase in volume. In annealed glass (standard float glass), this expansion takes place while the glass is at annealing temperatures, and so stress relaxation eliminates the tensile stress caused by the phase change.

NiS inclusions may undergo a phase transformation (shrinkage) in tempered glass due to the reheat/quench operations required to fabricate tempered glass. The rapid quench of the tempering process traps the inclusion in its small (Alpha) phase. Over a period of time, these inclusions may revert back (expand) to their original (Beta) phase state. When the inclusion is located in the center tension area of tempered glass, this increase in volume may cause a localized stress increase sufficient to break the glass.

Such inclusions can cause spontaneous breakage in tempered glass, without any load being applied, at any time, even five or ten years after the tempered glass has been fabricated. These inclusions are typically so small (about 0.010" in diameter) that they are virtually impossible to locate and identify in an individual unbroken lite of glass.

Glass manufacturers have recognized these details and have instituted programs of batch quality control, along with the elimination of any nickelcontaining materials from their raw material and glass handling systems. Automatic inspection of 100% of the float ribbon has also contributed to the reduction in the number of inclusions of all types present in float glass. Even though these improved quality measures are not capable of eliminating all such inclusions, the success of these programs is evident in the near elimination of spontaneous breakage reports related to current production float glass.

Heat soaking is a process that can uncover some NiS inclusions present in an individual lite of glass. But it is not 100% effective, and carries the risk of reducing the compressive stress in tempered glass.

Heat-strengthened glass is the preferred product of choice, rather than tempered glass, for applications where additional glass strength, compared to annealed glass, is required. The current low stress levels specified for heat-strengthened glass in the industry standard for heat-treated glass products, ASTM C1048, has been very successful in reducing the possibility of NiS spontaneous breakage in heat-strengthened glass.

Given the conditions listed above, the complete absence of NiS in glass cannot be guaranteed. Good design and engineering practices recognize this fact and accordingly only specify the use of tempered glass in applications where spontaneous breakage will not cause major problems. Suspended glazing, and all-glass walls using tempered glass, should be designed with redundant load paths so that the breakage of one individual lite will not lead to a progressive collapse.

Spandrel lites, which have to withstand high thermal stresses, should be specified as heat-strengthened glass to reduce the potential for NiS breakage.

Architects and engineers can minimize the chance of serious problems from NiS breakage by using heat-strengthened and laminated glass in critical applications.