Tempered glass is widely used in building facades, shower doors, and smartphone screens due to its high strength and safety fragmentation. However, a frustrating phenomenon known as spontaneous breakage can occur months or even years after installation. The primary culprit is a tiny impurity called nickel sulfide (NiS).

Nickel sulfide inclusions originate from raw materials or manufacturing equipment. During the glass tempering process, glass is heated to around 620–650°C and then rapidly cooled (quenched). This thermal shock creates compressive stress on the surface and tensile stress in the core, giving tempered glass its strength. At tempering temperatures, nickel sulfide exists in a high-temperature phase called α-NiS. When the glass cools quickly, the α-NiS inclusion does not have enough time to transform into the stable low-temperature phase β-NiS. Instead, it remains trapped as a metastable inclusion.

Over time, especially at room temperature or during temperature fluctuations, the trapped α-NiS slowly converts to β-NiS. This phase transition is accompanied by a volume expansion of approximately 2–4% over a period of months to years. Because the inclusion is locked within the highly stressed core of the glass, this volume increase generates enormous internal tensile stresses around the inclusion. When these stresses exceed the local strength of the glass, a typical "butterfly" or "hourglass" fracture pattern initiates from the inclusion point, leading to the sudden, non-impact shattering of the entire pane.

To mitigate this risk, manufacturers use a process called "heat soak testing." Panes are heated to about 290°C for several hours, which accelerates the α-to-β phase transformation and causes any flawed glass to break in the factory rather than in service. While not 100% foolproof, this procedure significantly reduces the probability of field spontaneous breakage. Understanding the NiS mechanism is crucial for architects, engineers, and consumers to specify safer glass products and manage expectations regarding this inherent, albeit rare, material behavior.