Spontaneous Breakage in Fully Toughened Glass: Causes, Risk Reduction through Heat Soak Testing, and Alternatives for Building Facades

Spontaneous Breakage in Fully Toughened Glass: Causes, Risk Reduction through Heat Soak Testing, and Alternatives for Building Facades


Fully toughened glass, also known as tempered glass, is widely used in architectural applications due to its high strength and safety characteristics. However, one of the critical issues associated with fully toughened glass is the risk of spontaneous breakage. This article defines spontaneous breakage, discusses its causes, and explores how Heat Soak Testing (HST) can mitigate this risk. Furthermore, it argues that using heat-strengthened glass in building facades is more reliable and cost-effective than relying on HST for fully toughened glass.


Fully toughened glass is a popular choice in construction due to its enhanced strength and safety features. However, spontaneous breakage remains a significant concern. This paper discusses the phenomenon of spontaneous breakage, its causes, and methods to mitigate the risks, particularly through Heat Soak Testing (HST).

1. What is Spontaneous Breakage?

Spontaneous breakage refers to the unexpected shattering of fully toughened glass without any apparent external cause. Unlike breakage due to impacts or applied stresses, spontaneous breakage occurs internally, often without warning.

2. Reasons for Spontaneous Breakage

i. Nickel Sulphide Inclusions (NiS)

During manufacturing, small nickel sulphide particles can become trapped within the glass. Over time, these inclusions change phases and expand, leading to internal stresses that cause the glass to shatter, shown in Figure 1[1].

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Fig. 1. Nickel Sulphide Inclusions (NiS)

ii. Thermal Stresses

Temperature variations can induce thermal stresses within the glass, particularly if the heating or cooling is uneven. This can result in breakage [2].

iii. Edge Damage

Damage to the edges of the glass during handling or installation can serve as initiation points for cracks as shown in Figure 2, eventually leading to spontaneous breakage [3].

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Fig. 2. Edge Damage Example

3. Reducing the Risk through Heat Soak Testing

Heat Soak Testing (HST) is a process designed to reduce the risk of spontaneous breakage in fully toughened glass by exposing the glass to elevated temperatures for an extended period. This process aims to identify and break glass panes containing NiS inclusions before they are installed.

During HST, the glass is heated to around 290°C (±10°C) and maintained at this temperature for several hours. This accelerates the phase change in nickel sulphide inclusions, causing affected glass panes to break in a controlled environment rather than after installation [4].

4. Heat Soak Testing: Value for Money?

While HST effectively reduces the risk of spontaneous breakage, it is not always the most economical solution for all applications, particularly for building facades. HST adds significant costs to the glass production process, and despite its benefits, it does not entirely eliminate the risk of breakage.

For building facades, the use of heat-strengthened glass (HS glass) can be a more reliable and cost-effective alternative. Heat-strengthened glass undergoes a similar thermal treatment as toughened glass but at lower quenching pressure, resulting in a glass that is not prone to spontaneous breakage while still offering enhanced strength and safety compared to annealed glass [5].

Heat-strengthened glass offers the advantage of breaking into larger, less sharp edges that remain attached to the frame as shown in Figure 3, reducing the risk of injury compared to the small, granular pieces of fully toughened glass. This feature can be beneficial in minimizing the risk of injury during breakage. Additionally, the minimum in-process handling and the absence of Heat-Soak Testing make heat-strengthened glass a more cost-effective option for large-scale facade applications.

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Fig. 3. Heat-strengthened (HS) Glass remain attached


While spontaneous breakage is a significant concern for fully toughened glass, Heat Soak Testing offers a method to mitigate this risk. However, for building facades, opting for heat-strengthened glass may provide a more reliable and cost-effective solution, balancing safety, performance, and economic considerations.


[1] Avrami, M. "Kinetics of phase change." Journal of Chemical Physics, vol. 7, pp. 1103-1112, 1939. Available: https://link.springer.com/article/10.1007/s40940-018-0083-8

[2] Bishop, D.W., Thomas, P.S., Ray, A.S., Simon, P. "Two-stage model for the a–b phase recrystallisation in nickel sulphide." Journal of Thermal Analysis and Calorimetry, vol. 64, pp. 201-210, 2001. Available: https://www.sciencedirect.com/science/article/abs/pii/S0379711223001352

[3] Bordeaux, F., Kasper, A. "Nickelsulfid: Neue Ergebnisse zur Optimierung des HST." Glastechnische Berichte Glass Science and Technology, vol. 71, no. 3, pp. N27-N28, 1998. Available: https://ieeexplore.ieee.org/abstract/document/5411418

[4] Bordeaux, F., Kasper, A. "Optimized HST to eliminate dangerous NiS stones in heat strengthened and tempered glass." In: Proceedings of the ESG Annual Meeting 'Fundamentals of Glass Science and Technology', Vaxjö, Sweden, pp. 255-264, 1997. Available: https://link.springer.com/article/10.1007/s40940-018-0083-8

[5] Kim, J., Lee, H. "Thermal performance of heat-strengthened glass for facade applications." Glass Structures & Engineering,