Nickel Sulphide Inclusions in Glass
What is it?
Tempered glass is a type of glass which has been specially heat treated to improve it’s strength. Some tempered glass can withstand a blow from a hammer without breaking! But sometimes, tempered glass can break suddenly, and without any apparent reason. This rapid, surprise failure may be due to a tiny imperfection that originates from the manufacturing process: a nickel sulphide inclusion.
Impurities in the raw materials used to make the glass, can come together to form a new material, distinct from the glass, known as an inclusion. Inclusions can come in many forms and shapes, some of which may cause failure, some of which may not. What makes nickel sulphide inclusions so problematic is that over time, it will slowly grow.
When the glass is a molten liquid, the nickel sulphide transitions from a form that is stable at low temperatures, to a form that is stable at high temperatures. As part of the tempering process, the liquid glass is rapidly cooled, effectively trapping the nickel sulphide inclusion within the solid glass, stuck in its high temperature form. Slowly, very very slowly, a phase change will occur in the nickel sulphide inclusion, as it reverts back to its low temperature form. As part of this process, the inclusion grows in size, applying more and more internal stresses on the glass. Eventually, it becomes too much, and the glass shatters.
How do we detect it?
In a recent case, I was contacted by a facade inspection specialist, who had been tasked with evaluating a building in Dublin, Ireland, before being purchased. One particular window pane had broken, displaying an interesting pattern of fracture, which appeared to point to a central point. This particular window pane was quite large, and would expensive to replace. If multiple similar failures were to occur, there could be significant additional expenses in purchasing this building. This central portion of fractured glass was extracted, wrapped in tape to secure it, and delivered to the laboratory to examine for a nickel sulphide inclusion.
Finding a nickel sulphide can be a little bit like finding a needle in a haystack. Most are smaller than 200 microns - somewhere between the thickness of a needle and a human hair. Thankfully, the fracture patterns of the glass fragment points to where the search should start, at the point of convergence of the fracture lines. The glass fragment closest to the centre is extracted first, and examined visually under low-magnification microscopy. In this case, a golden-yellow inclusion was easily found embedded in one of the faces of the glass fragment. Sometimes an inclusion is buried within the glass fragment, and the glass fragment must be ground down to reveal the inclusion for the final step.
The final step is examining this inclusion is the elemental analysis, performed using Energy Dispersive X-ray Spectroscopy (EDS), alongside Scanning Electron Microscopy (SEM). SEM photography shows the texture of the surface of the inclusion in much greater detail. EDS was used in two modes, one to map the spatial distribution of the elements detected by assigning a colour to each, and the other to measure the elemental content at 4 specific points on the inclusion. Both modes proved beyond a doubt, this is a nickel sulphide inclusion.
Based on my analysis, findings, and technical report, the facade inspector was able to advise his client that a nickel sulphide inclusion had caused the failure of a large and expensive pane of glass, and that given the nature of nickel sulphide inclusions, similar failures could occur in the future. His client was able to use this knowledge to negotiate a lower price in purchasing the building.
Do you need to test for a nickel sulphide inclusion glass failure in Ireland? Are you a facade specialist, quantity surveyor, real estate agent, or even a homeowner who is concerned that a nickel sulphide inclusion has caused a glass fracture? Get in contact today and together we can solve the mystery.
