Heat Soaked Glass FAQ’s
Heat soaked glass reduces the risk of toughened glass spontaneously breaking due to nickle sulphide inclusion (NiS).
What causes toughened glass to break?
There are many reasons toughened glass can break. Often the cause is obvious e.g. accidental impact or vandalism. On other occasions the cause isn’t as clear. This is referred to as spontaneous.
The cause can often be identified by the break pattern of the glass. Some causes of spontaneous glass breakages include:
Face Damage
The glass sustained an impact which did not exceed the elastic strength of the glass. A break with this type of damage can become apparent at a later date. Frequently after the glass has expanded and contracted causing heat stress, from hot days and cold nights.
Edge Damage
The edge of the glass has become damaged either before, during or as a result of the installation process. If the glass comes into contact with the fixings and fittings, with no allowance for the glass to expand and contract, an edge break can occur.
Design
The wind / structural load, clearances, design flaws, excessive heat stress etc, all need to be taken into account when the glass is spec’d. Incorrectly spec’d glass can cause breakages.
Nickle Sulphide Inclusions
Nickel Sulphide Inclusion (NiS) occurs during the process of manufacturing toughened glass. As a result of nickel contaminants present in the glass melt, reacting with sulphur.
Visually you are not be able to see these particles, but they could still be present within the glass. The particles remain benign until they are aggravated.
NiS has two states; a high temperature state and a low temperature state. When the glass is cooled quickly during the toughening process, the NiS is unable to change back to its low temperature state. Over time (the exact time it takes is unpredictable) the NiS changes back to its original state, increasing in size. Causing the glass to seemingly shatter for no apparent reason.
Raw materials, manufacturing methods and processing techniques, have all been adapted over time to try and eliminate NiS. However it is not possible to eliminate it completely. Meaning all glass comes with the very rare risk of spontaneous breakage due to NiS.
It is this cause of spontaneous breakage, that heat soaking can help reduce the chances of occurring.
What is heat soaked glass?
Heat soaked glass is toughened glass which has been heated to a very high temperature (290°C), and left for several hours. Speeding up the aging process and encouraging the glass to break during the test if NiS is present, rather than when in service.
How does heat soaking reduce the chances of toughened glass breaking due to NiS?
The heat soaking process is designed to return the NiS to it’s natural larger state. This will cause the glass to break during the testing process rather than once the glass is in use. If the glass doesn’t break during testing, there is a high probability that NiS is not present, however it is not possible to eliminate the risk completely.
The Glass and Glazing Federation estimate the risk is reduced from 1m2 in every 10,000m2 of glass, to 1m2 in every 1 million m2 of glass.
When is heat soaked glass recommended?
Heat soaked glass is recommended where a breakage could cause serious injury. Or for pieces of glass which would be very difficult to change at a later date, e.g. where a crane would be required, or when the glass is on the first floor or above.
Specifying heat soaked glass minimises the risk of spontaneous glass breakages and therefore having to replace the glass in the future, the future cost to do so and the disruption caused.
Is heat soaked glass recommended for annealed non-toughened glass?
As annealed non-toughened glass doesn’t go through the rapid heating/cooling process, spontaneous breakages of this nature does not occur.
Does heat soaked glass occur an additional cost or effect the lead time of the glass?
Due to the additional process the glass has to go through, there are cost and lead time implications. However these are minimal, for the peace of mind this process can offer.
Is spontaneous breakages of heat soaked glass covered by the warranty?
As glass companies are unable to eliminate NiS completely, however rare, our glass suppliers are unable to offer warranty against spontaneous breakages of this nature or spontaneous breaks of any nature. Therefore Exeter Trade Aluminium are unable to pass on any warranty to our customers.
