Our teams have just completed glazing replacement work to the iconic I-Quarter, in Sheffield. Glass had been reported as having broken through nickel suphide impurities.
Spontaneous breakage of glass with impurities may occur as a consequence of differing rates of expansion/contraction under heat and cooling to the glass itself. Nickle Sulphide impurities occupies a slightly greater volume and so generates a local stress concentration.
Nickel sulphide breakage of glass exhibits a characteristic fracture pattern. Assuming that the fractured pane remains in place, at the origin or epicentre of the fracture there should be located two fragments which are larger than the rest and which form a ‘figure of 8’ or ‘butterfly’. In the centre would be located the inclusion which is a small, round, shiny, yellow-black particle which may be as small as 0.05mm in diameter.
As part of the results of the normal float glass process, one large reputable manufacturer has reported that only 1 critical inclusion occurs in every 13 tonnes of glass that they produce, and that 90% of the inclusions will cause premature breakage during their heat soaking process. This means that only one nickel sulphide inclusion will ‘get through’ for each 8,470 m2 of 6 mm heat soaked toughened glass.