From melt to fracture, four steps
Step 1, contamination : in the raw glass melt (typically float glass), nickel compounds appear as trace contaminants from nickel-bearing raw materials, stainless-steel contact in the furnace, nickel contamination in cullet. Sulphur is present as a fining agent or contaminant. Under the right conditions, nickel and sulphur combine into nickel sulphide, forming microscopic inclusions with variable density. Step 2, tempering : during thermal tempering (heating to roughly 620 °C, then rapid cooling), the NiS inclusion is stabilised in its smaller α-phase. Step 3, ambient transformation : at ambient temperatures over months to years, the inclusion slowly transforms to its larger β-phase. Step 4, fracture : when the expansion-induced tension in the surrounding glass matrix exceeds a critical threshold, governed by the size of the inclusion, its position (centre zone is critical), the residual stress of the tempering, and the glass thickness, the inclusion initiates a brittle fracture that propagates across the entire panel. Statistical incidence on tempered glass without mitigation is low but non-zero : industry estimates place the rate between 5 and 40 defects per 100 tonnes of tempered glass, heavily dependent on the source of the float glass. On a large tertiary project with tens of thousands of square metres of tempered glass, the expected number of spontaneous breakages over a 10-year horizon is non-trivial.
