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Spontaneous Breakage of Toughened Glass

Panes of toughened (tempered) glass can shatter spontaneously years after installation.

The phenomenon is caused by an impurity in the glass known as a nickel sulphide (NiS) inclusion and is, fortunately, a rare event.

How does this occur?

Toughened (tempered) glass is created by rapid cooling to set up a tension on the skin or surface of the glass, which gives it its impact resistance.

But this can cause a problem if NiS contamination is present. A NiS stone inclusion shrinks more than the surrounding glass during the quench cooling process. This sudden quench catches the inclusion in its smaller, hot, Alpha phase, not giving it time to make its usual transition to the slightly larger, cold, Beta phase.

With further growth in size, as the phase change is completed, the surrounding glass is subjected to an increasing tensile stress. There are many different compositions of NiS inclusion. The exact composition of a NiS inclusion is a factor in determining when the full expansion and possible breakage occur. It can take many months or even years or breakage may never happen at all.


In areas where spontaneous breakage of toughened (tempered) glass would pose a safety risk, our range of safety window films can be installed to provide an invisible layer of protection. The safety film will hold the glass fragments together and, if required, can be combined with an edge retention system to keep the glass within the frame until the point at which it can be safely removed in one piece.

Spontaneous glass breakage UK coverage

Our glass breakage protection solutions are available in most parts of central England including Nottingham, Newark, Mansfield, Chesterfield, Derby, Burton, Uttoxeter, Loughborough, Leicester, Market Harborough, Hinckley, Nuneaton, Coventry, Grantham, Stamford, Melton Mowbray, Rutland, Oakham, Peterborough, Kettering, Cambridge, Northampton, Wellingborough, Daventry, Newport Pagnell, Bedford, Milton Keynes, Towcester, Stafford, Stoke, Rugby and Brackley.

The main counties we cover are Nottinghamshire, Derbyshire, Leicestershire, Warwickshire, Northamptonshire, Cambridgeshire, Buckinghamshire, Bedfordshire and Hertfordshire.