NIS IN HEAT STRENGTHENED GLASS

A technical paper about the breakage probability of nickel sulfide (NiS) inclusions in heat-strengthened glass, written by four experts: Dr. Andreas Kasper, John Colvin, Frank Rubbert, and Francis Serruys from Saint-Gobain and external collaborators.

This paper examines the spontaneous breakage due to nickel sulfide (NiS) inclusions that can occur in both toughened and heat-strengthened glass but highlights that the latter is less vulnerable with a very low breakage probability.

While rare, NiS-induced breakages in heat-strengthened laminated glass usually do not compromise safety due to the laminated glass's retention properties. The heat soak process remains mandatory for some applications, depending on local regulations.  

Spontaneous breakage of toughened glass in facades is under control today when the heat soak process is rigorously carried out according to EN 14179-1 (2005/2016).

Although the residual risk of heat soak tested thermally toughened safety glass is very low, heat-strengthened glass is often preferred under the assumption that it is less prone to spontaneous breakages due to NiS inclusions. However, nickel sulfide (NiS) breakages are not only observed in toughened glass but also in heat-strengthened glass.

Heat-strengthened glass has a lower surface compression and thus a lower central tensile stress when compared to toughened glass. Consequently, larger nickel sulfide inclusions are required to initiate a crack, as crack initiation is determined by the inclusion’s size and the surrounding stress field.

The lower tensile stress in heat-strengthened glass makes it much less vulnerable to spontaneous breakages compared to toughened glass (not heat-soak tested).

Critical size, timescale and probability of spontaneous breakage in heat-strengthened glass

The critical diameter “Dc” to cause spontaneous fracture of glass is determined. A good correlation is found with the diameter of reported nickel sulfide inclusions causing breakages on building facades. Knowing the distribution and diameter range of nickel sulfide inclusions in float glass, a conservative breakage probability of 1 breakage in 1,100 ± 200 tons of heat-strengthened glass can be estimated.

Furthermore, the timescale for breakages in service is examined. 90% of breakages normally occur within a period of 6 to 8 years after production, even in a cold climate. Although nickel sulfide breakages have been observed after more than 20 years, this remains an exception, and it may be concluded that breakages seldom happen more than 10 years after installation. 
 

Heat-Strengthened glass in practice

More data on broken heat-strengthened glass due to nickel sulfide inclusions is required to confirm the breakage probability, which in reality might be much lower. Due to this low breakage probability, it is not industry practice to carry out the heat soak process—it is done on demand, depending on regulation and project management risk.

Moreover, heat-strengthened glass does not satisfy code requirements for safety glazing. Therefore, it is most often used in the form of laminated safety glass. If a nickel sulfide inclusion causes a breakage in laminated heat-strengthened glass, it would stay in place due to its specific fragmentation, as required by EN 1863.

The low breakage probability, combined with its use as laminated heat-strengthened glass, explains why the heat soak test is usually not performed.