Nickel Sulphide Inclusion in Glass: An Analysis

nickle sulphide in glass

Nickel sulphide inclusion in glass is a long-standing concern in the glass manufacturing industry, posing significant risks to the safety and reliability of tempered glass products. These microscopic inclusions formed during the manufacturing process have the potential to cause spontaneous breakage, posing hazards in architectural, automotive, and other applications. Understanding the causes, characteristics, and detection methods of nickel sulphide inclusions is critical for maintaining the integrity of glass structures and protecting public safety. This article investigates the origins, visual identification, detection methods, and mitigation strategies for nickel sulphide inclusions in glass, shedding light on an important aspect of glass quality control and risk management.

What is Nickel Sulphide Inclusion?

The term “nickel sulphide inclusion” refers to the occurrence of nickel sulphide (NiS) particles in a substance, especially in crystalline materials like glass. These inclusions may appear while the material is being manufactured. Nickel sulphide inclusions are thought to be a common cause of spontaneous glass breakage.

During the glass-making process, nickel sulphide impurities may precipitate and form tiny crystalline particles inside the glass when it is heated and cooled. At high temperatures, these particles might be stable, but at lower temperatures, they could go through a phase change and expand. Significant pressure from this expansion may be applied to the surrounding glass, causing spontaneous breakage, which frequently happens months or even years after installation.

Causes of Nickel Sulphide Inclusions in Glass

Sulphur and nickel can have significant but distinct roles in the production of glass. Nickel is often regarded as an undesirable impurity in glass production due to its tendency to form nickel sulphide (NiS) inclusions, which can cause spontaneous glass breakage. When nickel traces found in raw materials precipitate as NiS crystals during cooling, inclusions like these arise. Nickel contamination is still a problem even with controls in place, especially for batch materials and recycled glass. 

Sulphur, however, can be used as a fining agent when making glass. Sulphur helps to eliminate bubbles and other flaws from the glass melt by reacting with other ingredients, improving the final product’s clarity and quality. Nonetheless, sulphur levels need to be closely monitored to avoid excessive gas formation since it can lead to foaming and defects in glass. 

  • Nickel Contaminants: The main cause of nickel sulphide inclusions is the existence of nickel impurities in the raw materials used in the glass-making process.
  • Production Process: The formation of nickel sulphide inclusions may be aided by specific production procedures or environmental factors. For instance, precipitation of nickel sulphide can occur more frequently when glass is made at rapid cooling rates.
  • The composition of the glass can influence the formation of nickel sulphide inclusions. Precipitation of nickel sulphide crystals may occur more frequently in some types of glass formulations. 
  • The quality and purity of raw materials used in glass production have a significant impact on the occurrence of nickel sulphide inclusions. Nickel can enter glass through contaminants or impurities in the raw materials, causing inclusions to form.
  • Temperature Fluctuations: Nickel sulphide inclusions may form as a result of temperature changes that occur during the glass’s manufacturing process or during its service life. The nickel sulphide crystals may undergo phase changes as a result of temperature cycles between high and low, which could cause them to expand and shatter the glass.

How To Identify Nickel Sulphide Inclusions?

Glass containing nickel sulphide inclusions can usually be identified by visual inspection as well as by using specific instruments and methods. This is how they are usually recognised:

Visual characteristics:

  • Appearance: Nickel sulphide inclusions typically appear as small, dark-coloured specks or particles embedded in the glass.
  • Location: Glass inclusions can be found inside the glass’s body or close to its surface.
  • Size: The size of inclusions can range from barely visible to several millimetres in diameter.
  • Shape: Nickel sulphide inclusions can be irregular, spherical, or elongated, depending on the conditions under which they formed.
  • Distribution: The inclusions in the glass may be dispersed uniformly or grouped together in specific regions. 

Tools and Techniques for Detection:

  • Visual Inspection: Skilled workers look for inclusions by visually inspecting the glass. This can be accomplished with the use of magnification instruments like magnifying glasses or microscopes, or it can be done with just the naked eye.
  • Polarised Light Microscopy (PLM) uses polarised light to examine thin sections of glass samples under a microscope. This method can help identify inclusions by illuminating their internal structure and composition.
  • Ultrasonic Testing: High-frequency sound waves are sent through the glass during ultrasonic testing in order to find any internal inclusions or flaws. The presence of inclusions may be indicated by modifications in the sound wave pattern.
  • X-ray Diffraction (XRD): XRD is a useful technique for examining the crystalline structure of glass inclusions. It is possible to identify the phase and composition of the inclusions by analysing the diffraction pattern of X-rays that are passing through the glass sample.

What Is Heat Soaking In Glass?

Heat soaking is a process used in the production of tempered glass. Tempered glass panels undergo a controlled heat-soaking process wherein they are exposed to high temperatures for a prolonged duration, usually a few hours. The goal of heat soaking is to lessen the possibility of tempered glass spontaneously breaking because of inclusions of nickel sulphide, which can occasionally occur.

Heat soaking is significant because it can detect and reduce the possible hazards related to nickel sulphide inclusions. Even though tempered glass is extremely strong and impact-resistant, nickel sulphide inclusions may cause it to shatter spontaneously.


Nickel sulphide inclusions present a significant challenge in the production and quality assurance of tempered glass products. These latent defects could lead to spontaneous breaking, which could be dangerous in a number of applications. While methods like heat soaking can help reduce the risk of breakage, complete prevention remains unclear. As the demand for tempered glass increases, ongoing research and development efforts are required to improve detection methods and reduce the occurrence of nickel sulphide inclusions, thereby ensuring the safety and reliability of glass products in the built environment.


  1. Is Nickel Sulphide Inclusion a latent defect?

Yes, nickel sulphide inclusion is considered a latent defect in glass.

  1. What is the Colour of nickel sulphide?

The colour of nickel sulphide is typically dark or black.

  1. What is the pattern of nickel sulphide glass breakage?

Nickel sulphide inclusions can cause glass to break in a characteristic “spider web” or radial pattern..

  1. Can nickel sulphide inclusions be prevented or treated?

Nickel sulphide inclusions can be prevented through processes like heat soaking during glass manufacturing, but complete prevention is challenging

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