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.
Facade consultant Dominic Yu Wai-kin, commissioned by Sun Hung Kai to investigate the breakages, said clearing the impurity was almost impossible: "We call it glass cancer. There is still residual risk even if we follow the government procedures."
But international experts said the problem could be controlled in the panels if they were tested by being submerged in a heat bath for 24 hours compared to the minimum requirement of two hours specified by the Buildings Department.
"The test accelerates the heating process so if the glass survives the test, it should be okay to use on buildings," Bury said.