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Nickel sulphide inclusion
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A Nickel sulphide inclusion, (also abbreviated to NiS), occurs during the process of manufacturing float glass (normal window glass).
In a batch of glass, contaminants that contain nickel might be present, (e.g. stainless steel.) These can combine with sulphur to form Nickel Sulphide inclusions. Furnaces produce hundreds of tons of glass every day, so it is difficult to completely eliminate any contaminants. This causes a problem later in the manufacturing process.
The process of tempering float glass can cause a NiS to change from its normal state (known as a low-temperature structure) to a different high-temperature, crystalline structure. When the glass is cooled quickly (as part of the process) the NiS particle is unable to change back to its original form. Over a certain period of time NiS will slowly convert to the original low temperature structure. This means the NiS increases in size, and the mechanical stresses caused by this cause the tempered pane to shatter, for no apparent reason (hence spontaneous glass breakage).