Heat Soaking Glass

On very rare occasions toughened glass will break in service for no apparent reason. This can be caused by a variety of reasons including damage, impact or inclusions within the glass. One particular type of inclusion, nickel sulphide, has attracted publicity and it is important that specifiers understand its characteristics and the risk of breakage in service.

Nickel sulphide (NiS) is formed naturally within the glass making process, but is of concern only in glass that is subsequently toughened. The inclusions are too small to be visually detected by the glass manufacturers and when heated in the toughening process undergo a phase change that reduces their size. Cooling of the glass is far too rapid to allow reversal and this takes place over a period of time. If an inclusion is sufficiently large and is located within the centre of the glass it can cause spontaneous breakage.

It is vital to stress that the risk is very small. Glass manufacturers estimate the incidence at less than 1 in 13,000kg