Glass needs stringent testing to prevent it falling from buildings

Glass panels that have fallen from the walls of two luxury estates have been diagnosed with "glass cancer", which is caused by an impurity common in tempered glass and which can be prevented only by stringent quality control.
Expansion of the impurity, nickel sulphide, is the most likely cause of the breakages at Larvotto on Ap Lei Chau and The Arch in West Kowloon, according to the consultant of developer Sun Hung Kai Properties and independent assessments from glass experts.
Nickel sulphide is a mineral that can be present in all types of architectural glass but only affects tempered glass, a type that is heat-strengthened so that it is four times the strength of normal glass.
"During the tempering process, if there is nickel sulphide in the glass, the shape of the molecules will change and become smaller by about 4 per cent. But over a period of time, the molecules will expand back to its original volume, causing the glass to break," said Clifford Bury, managing director of JAS (Inspection & Testing).
The expansion is often caused by temperature variations such as those between night and day.
Breakages in planes of glass can occur a year after production, peak in the second and third years, and taper off by the sixth year.