Nickel Sulphide Inclusion: A Guide to Glazing’s Trojan Horse

Nickel sulphide inclusion can make or break your glass design project. What is it? Can it be prevented? This comprehensive guide answers those questions and more.


05 Mar 2023


Simon McAuliffe

Cracked glass

Stand in the centre of London and you will be surrounded by magnificent monuments of structural glass. To illustrate that point, you would have to throw a stone pretty accurately for it to miss a glass building.


But what if we told you that on the calmest of days, even without the stone or any human interaction at all, elements of glass architecture could shatter into millions of tiny pieces before your eyes? 


No, it’s not because of ghosts. 


Instead, the phenomenon is caused by nickel sulphide inclusion, three words that can regularly keep an architect up at night.


If you have woken in a cold sweat thinking not of the benefits of natural light, but instead of the glass staircase, frameless glass wall or retractable glass roof that you recently installed for a client and whether it could break at any moment, we invite you to read on.


In this guide, we hope to answer some important questions relating to nickel sulphide inclusion, provide some insights that can help you to minimise the risk of your design falling apart, and perhaps shatter some misconceptions along the way. 


Let’s begin.

What is nickel sulphide inclusion?

Nickel sulphide inclusion is the unintentional incorporation of nickel sulphide, an impurity, into the glass-making process. According to the Glass Association of America, 50 different types of inclusions have been identified. Almost all of them are harmless. Nickel sulphide, however, can cause the glass to spontaneously shatter, weeks, months or even years after installation.

How does nickel sulphide inclusion lead to breakages?

Breakages occur because of the change in the size of nickel sulphide following the thermal process by which glass is toughened. 

To toughen glass, panes are loaded into a large oven with a temperature between 600 and 700 degrees celsius. The glass becomes soft, at which point it is blasted with cold air, also known as quenching. Because glass is a poor conductor of thermal energy, when the surface layers of the glass are rapidly cooled to put them into a state of compression, the inner region cools much more gradually, solidifying, contracting and squeezing the outer surfaces to increase compression further.

Compression is what makes the glass strong. The result is, therefore, a pane of glass that has a very strong outer layer, supported by a very tensile inner region.

But the nickel sulphide is located within the inner region, the tensile region. At low temperatures, nickel sulphide has a greater volume than at high temperatures. Because the toughening process does not allow the nickel sulphide to return to its larger cold-temperature state, it does so afterwards, usually over several years. When glass is soft, it can accommodate the increase in the volume of nickel sulphide. But when it is hard, the nickel sulphide can cause fractures, which lead to sudden shattering.

Think of nickel sulphide as glazing’s Trojan horse. Troy had strong outer defences but was vulnerable within. It is within the glass’s defences that nickel sulphide inclusion does its damage.

How to identify a breakage caused by nickel sulphide inclusion

Implosion caused by nickel sulphide inclusion will centre around a singular point flanked by two pentagonal or octagonal-shaped fragments. If the destruction wasn’t so frustrating, its pattern would be very pretty, and is often compared with a butterfly or a figure-of-eight. The nickel sulphide stone will be tiny, measuring between 0.06 and 0.5 millimetres and often only visible under an electron microscope. Identification of nickel sulphide inclusion breakages is often made trickier by the fact that the pane of glass could be in a thousand pieces on the floor.

Shattered piece of glass

Does nickel sulphide inclusion affect all types of glass?

All glass has the potential to contain nickel sulphide. The difference between the glass that could spontaneously shatter or not is how it – and the nickel sulphide within – is allowed to cool. 

Annealed glass, for example, is created at temperatures of 1,480 degrees celsius but undergoes a controlled cooling process that is long enough for the nickel sulphide to return to its greater-volume state when the soft glass can accommodate the change. Thermal strengthening, however, solidifies the glass before the nickel sulphide’s change can take place. The expansion of nickel sulphide’s volume when glass is set can cause it to fracture and shatter.

In short, breakages caused by nickel sulphide inclusion affect all pieces of thermally toughened glass (also called tempered glass). Heat-strengthened glass (approximately two times tougher than annealed glass) rarely suffers from spontaneous failure. But toughened glass (four to five times tougher) is far more likely to experience a breakage because it is cooled much faster.


Hammer cracking glass panel

Other considerations for nickel sulphide inclusion

Can you predict nickel sulphide inclusion?


You cannot predict nickel sulphide inclusion. Sulphur, the first of the two components, is introduced by adding sodium sulphate, a fining agent that encourages large bubbles to form from smaller ones during the initial heating period. It is a necessary inclusion.

Nickel, however, is neither easily traceable nor necessary. Its origins likely enter the process as part of the raw materials or the handling of them. When experts identified that fuel oil contains traces of nickel, a switch to natural gas burners reduced the occurrence of nickel. But it has still not been eliminated.

Can you prevent nickel sulphide inclusion?


At the time of writing this, there is no way to 100 percent eliminate the possibility of nickel sulphide inclusion in the glass-making process. There is, however, a way to significantly reduce it, a process called heat soak testing.


What is heat soaking?


Heat soak testing is a stress test that replicates the conditions required for heat-toughened glass to break as a result of nickel sulphide inclusion. The glass is reheated to 290 degrees celsius for two hours (as regulated by Standard PREN 14179-1). At this temperature the glass does not go soft, but the nickel sulphide – if present – is allowed to return to its low-temperature state. As the glass is not malleable, fractures can occur, leading to shattering. According to some estimations, heat soaking can remove 95 percent of the problem windows without significantly reducing the temper of the glass.


Is nickel sulphide inclusion covered by the warranty?


Because it is currently impossible to eliminate any chance that glass has a nickel sulphide inclusion, and the spontaneous shattering usually happens after several years, no manufacturers offer a warranty that covers nickel sulphide inclusion. Manufacturers refer to it as a phenomenon, rather than a defect.

Swimming pool enclosure with sliding glass doors open to reveal garden outside

Future solutions to nickel sulphide inclusion

Heat soak testing glass is currently the best process for eliminating nickel sulphide inclusions from heat-toughened glass. But it can significantly extend lead times and add to your project cost. Furthermore, when a pane breaks dramatically during the heat soaking process, it has the potential to damage its neighbours. 

Future technologies may provide ways of detecting nickel sulphide inclusions in the glass without its destruction.

Methods of detection that are currently being developed include ultrasound and laser imaging. Both methods require analysis of individual panes by skilled technicians. Laser imaging also requires both sides of the glass to be accessible and transparent – something not possible for glass spandrel panels.

The only sure means of eliminating the risk of failure due to Nickel Sulphide is through the avoidance of heat-toughened glass altogether.  The use of heat-strengthened glass is increasing and often now specified as a viable option.  Heat-strengthened glass cannot be considered a ‘safety glass’ as it breaks into large shards. 

To be used safely, two or more panes of heat-strengthened glass need to be laminated together to prevent the individual panes breaking apart. This, along with having to use thicker panes, makes heat-strengthen a considerably more expensive alternative.  The final decision can only be made after weighing up the additional cost against the costs of any future failure.

London City Hall - Award-winning building challenged by nickel sulphide inclusion

In 2002, Foster and Partners completed London City Hall, an iconic building located on the south bank of the Thames that was described by The Guardian as “a tight ball of architectural muscle”. While standing in its atrium and looking up, your gaze is met by concentric walkways bathed in light that floods through the inter-connected triangular panes of glass.

But it too experienced teething problems related to nickel sulphide inclusion two years after completion. Despite its modern excellence, making it one of our favourite buildings in all of London, the lesson is that no project is immune to impacts of the phenomenon.

Many architects and contractors, Foster and Partners included, will no longer accept toughened glass in their schemes even if it has been heat soaked. Cantifix almost exclusively use heat strengthened and laminated glass, because uncertainty is the biggest restriction to innovation.

Nickel sulphide inclusion can be devastating to your design project. While it only affects glass that has been thermally toughened, its destruction is unpredictable and can be very dangerous. 

If you want to know how nickel sulphide inclusion could impact your next project and how you can reduce the risk it poses, get in touch with us today.