Spontaneous Crack of Fully-Tempered Glass: Causes, Risks, and Prevention

Spontaneous Crack of Fully-Tempered Glass: Causes, Risks, and Prevention

by Engr. Md. Abdul Wazed, B.Sc. Engineer (Civil)

Summary

Spontaneous cracking of tempered glass is a phenomenon mainly associated with the inclusion of Nickel Sulphide (NiS) during the manufacturing process of annealed glass, along with other factors such as damaged glass edges or frame contact with the glass. The phenomenon was first acknowledged in 1940, with the first documented case reported in 1961. Since then, extensive research by companies, institutions, and scientists has been carried out to identify, prevent, and resolve the issue. However, NiS has not yet been eliminated completely.

NiS exhibits two different crystalline structures depending on temperature: the alpha phase (high temperature) and the beta phase (low temperature). During the rapid cooling process used to convert annealed glass into tempered glass, there is insufficient time for NiS to complete its transition from alpha to beta. As a result, the inclusions become trapped in the glass in their high-temperature alpha phase. Over a long period of service, NiS slowly transforms into the beta phase, increasing in volume by 2–4%. This expansion creates highly localized tensile stresses at the glass–NiS interface, which ultimately leads to fracture.

Because tempered glass is considered safety glass, the International Building Code (IBC) and other standards define specific areas where safety glazing is required, such as doors, windows, railings, point-fixed glazing, and other panes susceptible to human impact. However, if tempered glass is used more widely, one must be prepared for the possibility of NiS-induced failures and the need to replace broken panels.

Keywords: Glass, fracture, fully-tempered, heat-strengthened, spontaneous, Nickel Sulphide, inclusions, alpha phase, beta phase

1.1 What Is Fully-Tempered Glass?

Float glass, also known as annealed glass, is a super-smooth, distortion-free glass commonly used as a base material for producing various types of processed glass, such as laminated glass, heat-strengthened glass, fully-tempered glass, and insulated glass units. When broken, annealed glass shatters into large fragments with sharp edges, making it unsuitable for use as safety glass.

Among the many types of processed glass, ASTM C 1048 defines two categories of heat-treated glass:

• Kind HS – Heat-Strengthened Glass

  Heat-strengthened glass shall be flat glass, either transparent or patterned, that meets the applicable requirements of ASTM C 1036. It is further processed to conform to the specifications for heat-strengthened glass.

  
  

• Kind FT – Fully-Tempered Glass

  Fully-tempered glass shall be flat glass, either transparent or patterned, that meets the applicable requirements of ASTM C 1036. It is further processed to conform to the specifications for fully-tempered glass.

  
  

Although both types share similarities in their manufacturing process, there are key differences to note. In both cases, glass sheets are heated to approximately 650°C (1200°F) and then cooled rapidly to increase strength. The primary distinction lies in the cooling rate:

• Fully-tempered glass undergoes a much faster cooling process, resulting in higher surface compression and significantly greater strength.

• Heat-strengthened glass is cooled more slowly, leading to lower surface compression and moderate strength.

The heating duration for both types is typically 40–50 seconds per millimeter of glass thickness. However, the cooling time for tempered glass is very short—only a few seconds—whereas heat-strengthened glass requires a longer cooling period.

(diagram: Process of Tempering):

(diagram: Stress Distribution in Tempered glass):

Table 1.1 Comparative properties of Fully-tempered and Heat-strengthened glass ⁽¹⁾

*Polyvinyl butyral

The difference between heat-strengthened and tempered glass is the speed at which they are cooled, which results in different surface compressions and therefore different overall glass strengths and properties, which are compared as under:

1.2 Types of Glass Cracks

Different types of glass—annealed, tempered, and laminated—exhibit distinct breaking patterns.

• Annealed glass, when broken, typically fractures into large, jagged shards rather than shattering into small pieces.

• The crack pattern in annealed glass often reveals the point of impact and the direction of force. For example, impact-related cracks usually radiate outward from the point of contact, resembling a spider web.

• Fractured annealed glass pieces have sharp edges, which may cause cuts or injuries.

1.2.1 Possible Causes of Tempered Glass Breakage

Tempered glass can break due to several factors, broadly categorized as follows:

• Impact: A strong blow can shatter the glass, resulting in its characteristic diced or granular fracture pattern.

• Edge Damage: Chips or flaws along the glass edge can initiate cracks that propagate and lead to breakage.

• Inclusions: Impurities such as Nickel Sulphide (NiS) can create internal stress points, causing spontaneous breakage.

• Thermal Stress: Sudden temperature changes may cause uneven expansion or contraction, potentially leading to failure.

  
  

Additional contributing factors include:

• Poor glazing practices

• Glass-to-glass or glass-to-metal contact

• Use of excessively hard setting blocks

  
  

Improper or uneven tempering may also result in post-installation breakage, especially when stress variations across the same pane are significant. This type of failure is identifiable by a highly irregular breakage pattern, featuring large islands of unbroken glass surrounded by smaller fragmented pieces.

Although approximately 50 types of inclusions are recognized in glass manufacturing, most are harmless. Nickel Sulphide (NiS) is the notable exception, posing a risk specifically in fully-tempered glass.

In this article, we will focus exclusively on NiS-induced spontaneous breakage in tempered glass. While all float glass types—including annealed, heat-strengthened (HT), and fully-tempered (FT)—may contain NiS impurities, spontaneous fracture due to NiS occurs only in fully-tempered (FT) glass.

1.3 Spontaneous Fracture of Tempered Glass

Spontaneous fracture refers to the sudden breakage of fully tempered glass without any apparent external force or impact. When such a fracture occurs, the residual tension within the glass core rapidly propagates cracks throughout the pane, resulting in a characteristic diced fracture pattern.

In most cases, monolithic fully tempered glass disintegrates and falls out of its frame upon breakage. However, if the fragments remain in place—such as in tempered laminated glass—a distinct series of primary cracks can be observed radiating from the point of origin. This is commonly referred to as a "starburst" pattern, which reflects the release of internal stresses.

At first glance, the broken glass may appear to have been struck by a sharp object, with the point of origin clearly visible. Upon closer inspection, the origin typically reveals two fragments shaped like butterfly wings, forming what is known as the "Butterfly Pattern," "Double D Pattern," or "Cat Eye Pattern" (see Fig. 1.1). This pattern is a strong indicator of spontaneous fracture.

Notably, the origin of such breakage is usually located away from the glass edge, distinguishing it from edge-induced failures.

Even if the monolithic glass appears partially intact, the presence of a starburst crack significantly compromises its structural integrity and resistance to further stress. The most common cause of this type of spontaneous fracture is the presence of Nickel Sulphide (NiS) inclusions within the glass.

Fig. 1.1 "Starburst" and "butterfly" pattern spontaneous crack of fully tempered glass ⁽¹⁾

1.4 Inclusion of Impurities Like Nickel Sulphide (NiS)

Float glass may contain various types of inclusions—microscopic or macroscopic imperfections formed during the manufacturing process. Common types include:

• Bubbles (Seeds): Gaseous inclusions, often referred to as seeds or bubbles.

• Stones: Solid, crystalline inclusions that result from unmelted raw materials during the float process.

• Nickel Sulphide (NiS): Microscopic impurities that can form during the thermal toughening of glass and are particularly significant due to their potential to cause spontaneous breakage.

There is ongoing debate regarding the exact mechanism by which nickel enters the glass mix. However, commonly recognized sources include:

• Raw Material Contamination: Nickel may be present as a trace impurity in silica sand, soda ash, or recycled glass (cullet).

• Equipment Wear: Nickel can be introduced through wear and tear of nickel-containing components in production equipment, such as heating elements or conveyor systems.

Fig. 1.2 Microscopic NiS particle causing crack in tempered glass ⁽¹⁾

During the thermal toughening process, small nickel particles can react with sulphur to form Nickel Sulphide (NiS) inclusions. In glass manufacturing, the primary source of sulphur is sodium sulphate (Na₂SO₄)—a common and cost-effective fining agent used to remove bubbles from molten glass.

While most impurities in raw materials are benign, Nickel Sulphide (NiS) is an exception. NiS inclusions, typically around 100 µm in size, can lead to spontaneous fracture in fully tempered glass, making them a critical concern in glass quality control and safety assurance.

1.5 Cracking Process Due to Nickel Sulphide (NiS)

Nickel Sulphide (NiS) inclusions are small, spherical, opaque solids typically ranging from 0.1 to 0.5 mm in diameter (approximately 0.005" to 0.020"). NiS is a unique compound due to its ability to exist in two distinct crystalline phases:

• Alpha Phase: High-temperature form, characterized by greater density, smaller volume, and less stability.

• Beta Phase: Low-temperature form, with lower density, larger volume, and greater stability.

During the tempering process, glass is rapidly cooled (quenched), and any NiS inclusions present shrink more than the surrounding glass. This shrinkage alone would not pose a problem—except for the fact that NiS undergoes a phase transformation from alpha to beta, which involves a volume expansion of approximately 2–4%.

The rapid quenching traps the NiS inclusion in its alpha-phase, preventing it from transitioning to the larger beta-phase during cooling. This is critical because:

• Below 380°C (716°F), NiS is stable in the beta-phase.

• Above 380°C, it remains in the alpha-phase.

Since glass manufacturing typically occurs at temperatures between 1000°C and 1500°C (1832°F to 2732°F), and tempering is performed at 620°C to 700°C (1148°F to 1292°F), NiS inclusions are almost always trapped in the alpha-phase during tempering.

In annealed glass, the slow cooling process in the lehr furnace allows NiS to gradually transition from alpha to beta phase without issue. However, in fully tempered glass, the rapid cooling prevents this transition, leaving the inclusion in a metastable state.

Over time—ranging from months to years—the trapped NiS inclusion attempts to revert to its beta-phase, causing a volume expansion. This expansion generates high localized tensile stress at the interface between the NiS particle and the surrounding glass. Although the stress diminishes rapidly with distance from the inclusion, it is often sufficient at the interface to initiate micro-cracking, which can eventually lead to spontaneous fracture of the glass.

1.6 Breakage Period and Frequency

Tempered glass containing Nickel Sulphide (NiS) inclusions can fracture spontaneously at any time—from days to decades after installation. However, the most frequent failures tend to occur within the first few years. While some studies suggest a decline in breakage rate over time, others report continued failures even after several years, underscoring the unpredictable nature of NiS-induced breakage.

Statistically, NiS inclusions capable of causing spontaneous fracture in fully tempered (FT) glass occur approximately once in every 4 to 12 tons of glass, on a global average. Of these, 50% of breakages typically occur within the first 800 days (approximately 26 months) after tempering.

1.7 Preventive Measures

While eliminating nickel from the glass production process may seem like the most straightforward solution, it is extremely difficult to achieve. Despite rigorous quality control, trace amounts of nickel can still enter the system through raw materials or equipment wear.

To mitigate the risk of NiS-induced breakage, the following preventive strategies are recommended:

• Use Heat-Strengthened Glass: Where technically feasible, opt for heat-strengthened glass instead of fully tempered glass.

• Prefer Laminated Heat-Strengthened Glass: This is ideal for most applications, except for point-fixed glazing (also known as spider glazing), which requires high-strength glass supported at discrete points.

• Apply Heat Soak Testing (HST): In cases where tempered glass is necessary due to structural or code requirements, HST should be performed. This process accelerates the phase change of NiS inclusions, reducing the risk of spontaneous breakage.

• Follow Building Code Guidelines: Avoid using tempered glass in areas not mandated by the International Building Code (IBC) or other relevant standards.

• Plan for Replacement: If tempered glass is used extensively, stakeholders should be prepared for potential NiS-related breakage and have a strategy for timely replacement.

1.8 Conclusion

Although the overall percentage of buildings affected by NiS-induced breakage is low, a contaminated batch can result in high incidence on a specific project. The use of fully tempered glass as the exterior lite of a gas-filled Insulated Glass Unit (IGU) in a high-rise curtain wall—especially in densely populated urban environments—raises critical safety concerns.

Given the unpredictable nature of spontaneous fracture, this issue must be addressed during the design phase of any building where tempered glass is being considered. Building owners do not expect glass to fail without warning, potentially causing injury or property damage—nor should they.

Therefore, it is essential for all members of the design and construction team to engage in transparent, early discussions about the risks associated with tempered glass. Alternative solutions, such as heat-strengthened glass, should be carefully evaluated for different zones of the building to ensure both safety and performance.

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References

1. Wang, S. Spontaneous Breakage in Fully-Tempered Glass: Why It Happens and What to Do About It. KOHLER – Senior Sales Executive.

2. Jacob, L. (2001). A Review of the Nickel Sulphide Induced Fracture in Tempered Glass. Jacob and Associates Pty Ltd, Australia.

3. Centre for Windows and Cladding Technology. (2001). Nickel Sulfide Induced Failure of Glass. University of Bath, England.

4. ASTM C1048 – Standard Specification for Heat-Treated Flat Glass – Kind HS, Kind FT, Coated and Uncoated Glass.

5. ASTM C1036 – Standard Specification for Flat Glass.