When you find a butterfly shaped pattern at the origin of a broken glass pane, it might be the clue that you are in front of a spontaneous breakage.
 |
| The butterfly shape is noted at the point of origin of the break. |
The inclusions are impurities
introduced in the mass glass prior to the production of float glass. The tempering
process is based upon to heat up the glass firstly and afterwards is cooled
down to compress the surface of the glass pane, obtaining improved mechanical
properties.
If one of those inclusions within
the mass glass is Nickel Sulphide (NiS), this remains in a high temperature
form because it did not get enough time to cool down as the rest of the mass
glass. Subsequently, the NiS will return to its low-temperature after a period of
time. During the process, the NiS volume will increase its volume, causing the
sudden breakage of the glass pane. This is known as spontaneous breakage.
NiS will only cause the breakage
of the tempered glass providing they are situated in its tensile zone. As the
inclusion increases the volume, it releases all the energy embedded in the
compressed zone.
 |
| Stresses in toughened glass |
Nowadays, the production of float
glass totally free of nickel sulphide does not appear to be possible.
The spontaneous breakage follows a Gaussian function; such graph is a characteristic symmetric bell curve shape. This shows that the number of breakages is initially reduced, and then it goes up to a maximum value and finally decreases following a similar curve. This function is formulated collecting data of the number of breakages over a period of time.
 |
| The spontaneous breakage follows a Gaussian function, at the vertical axis includes the number of breakages and at the horizontal includes the time |
One of most effective way to
prevent this phenomenon is to carry out the Heat Soak Test (HST) after the tempered
fabrication process. This is a destructive method that consists of introducing
the tempered glass panes into an oven during a reduced period of time to
accelerate the expansion of the NiS inclusions. In such way, the contaminated
glass panes will fail inside the factory prior to be delivered to the Client.
 |
| Tempered glass panes within a Heat Soak Test oven |
Façade engineers should always undertake such failure investigations to determine the most likely root of the glass failure and if the NiS inclusions are the cause of the failure.
