Nickel Sulfide Inclusion and Spontaneous Glass Breakage

The sudden, inexplicable shattering of tempered glass can be a shocking and unsettling event. Whether it’s a shower door, a window, or a glass tabletop, the sight of glass disintegrating into a pile of hundreds of small cubes often leaves property owners bewildered. This phenomenon, known as spontaneous tempered glass breakage, is frequently attributed to a microscopic flaw within the glass itself: a nickel sulfide (NiS) inclusion.

Understanding Tempered Glass and Its Vulnerability

To grasp why a speck can cause such destruction, it’s essential to understand the nature of tempered glass. This “safety glass” undergoes a specialized heating and rapid cooling process that creates a permanent internal state of tension and compression.

The outer layers are compressed, while the inner core remains under tension, an internal tug-of-war exerting at least 10,000 pounds per square inch (psi). This unique stress profile is what gives tempered glass its strength and its characteristic breakage pattern – shattering into small, cubic pieces rather than sharp shards.

Video: This large sheet of tempered glass was broken on purpose to demonstrate the appearance of spontaneous glass breakage due to a nickel sulfide inclusion. Spontaneous glass breakage might happen even when nobody is near the glass. Even though tempered safety glass breaks in small pieces, the pieces can remain interlocked with one another and come down in dangerous clumps.

The Role of Nickel Sulfide Inclusions

Despite the robust nature of tempered glass, a microscopic anomaly can compromise its integrity. Nickel sulfide inclusions are tiny metallic spheres formed from impurities during the glass manufacturing process. These inclusions, often barely visible to the naked eye, pose a significant threat because they are trying to grow inside the glass, but the glass keeps them locked and prevents the growth.

Suppose a NiS inclusion is located within the tension layer (the central 60%) of the glass. In that case, its growth can exert sufficient localized pressure to disrupt the delicate balance of tension and compression between layers of the glass. This localized stress can eventually overpower the glass’s inherent strength, leading to sudden and spontaneous shattering.

While the breakage is often attributed solely to the inclusion’s growth, sometimes a seemingly minor external force, like opening a shower door or cleaning the glass, can be the final trigger, tipping the scales and initiating the catastrophic failure.

Identifying a Nickel Sulfide Break

Determining the cause of a glass breakage can be challenging, especially if the glass has completely fallen out. However, if the broken glass cubes remain in the opening, a telltale sign of an NiS inclusion break is the “butterfly pattern”.

This distinctive pattern consists of a pair of polygons bordering each other at the focal point of the break. If this pattern is seen, an NiS inclusion is likely the culprit. A closer examination with moderate magnification at the border between the polygons may reveal the tiny inclusion itself.

The Origin of Nickel Sulfide Inclusions

Nickel sulfide inclusions are an unfortunate byproduct of the glass manufacturing process. They form from nickel and sulfur, both undesired impurities present in the raw materials or equipment used in float glass plants.

Nickel, a component of stainless steel, can be found in furnace components like nozzles and mixers. Sulfur can originate from combustion fuels used to melt the glass or in fining agents that reduce bubbling in the glass. In the molten glass mixture, these elements can combine to form the problematic NiS inclusions.

Conversions:

• 1 mm = 1000 μm = 0.0394 in

• 1 in = 25.4 mm = 25400 μm

Video: Here are some views of glass sinks, glass stove doors and glass doors breaking. The two clips at the end of the video shows glass doors experiencing actual spontaneous breaks captured by a security camera. The building staff cleaned up the broken glass, so there was no evidence to examine to determine of the spontaneous glass breakage was a result of a nickel sulfide inclusion.

Frequency and Timing of NiS Breakage

While spontaneous glass breakage due to NiS inclusions can feel like a pervasive issue, the overall incidence is relatively low. Estimates vary widely depending on the source and the quality control measures implemented by glass manufacturers.

Some plants employ scanning equipment or specialized lances to remove impurities, while others adjust their ingredient mix to reduce the likelihood of NiS formation. This explains the significant variation in reported breakage rates, which can range from one break per 4 tonnes to one per 450 tonnes of tempered glass.

The timing of these breakages is often linked to temperature cycling. Glass in windows in the Northern hemisphere, for instance, tends to break first on the East and West sides of a building, followed by the South, then finally the North. Initial breaks can occur within the first year or two of installation, but it’s not uncommon for spontaneous shattering to continue for 10 years or even longer in severely affected buildings.

The growth of the NiS inclusion is tied to a phase change, similar to how water expands when it freezes into ice. At 379 °C (714 °F), the NiS inclusion undergoes a phase change from a denser alpha state to a less dense beta state. During the rapid cooling of the tempering process, the inclusion is “frozen” in its smaller alpha state.

Over time, as it experiences temperature fluctuations, it “tries” to expand to its beta state, exerting immense localized pressure (estimated at 125,000 psi) on the surrounding glass, creating a microscopic flaw that eventually leads to catastrophic failure.

What to Do After a Spontaneous Glass Break

If you experience a spontaneous glass break, documenting the incident thoroughly is crucial, especially if you suspect a NiS inclusion:

• Take immediate photos: Capture wide shots of the entire piece of glass from both sides if possible. If the focal point of the break is still intact, zoom in for detailed pictures.

• Record details: Note the date, time, and outside temperature when the break occurred.

• Stabilize the glass: If the glass remains in the opening, use carpet protection film (available at hardware stores) or duct tape to secure it.

• Preserve evidence: To keep the origin of the break for analysis, place clear tape or carpet protection film on a 3″ x 3″ area around the focal point. Use strong duct tape to create a border around this area, then carefully chip away the surrounding glass with a screwdriver and hammer to free the sample.

• Send for analysis: Package the sample securely for professional study. A microscopic examination can confirm the presence of a NiS inclusion and provide a detailed report.

Even if it’s the first spontaneous break you’ve encountered, starting a record of these incidents is advisable. If it becomes a recurring problem, detailed documentation will be invaluable for seeking redress or implementing preventive measures.

Contact us if your glass has nickel sulfide inclusions or if it breaks spontaneously.

Nickel Sulfide Inclusion – A World Inside Glass