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Nickel Sulfide

Introduction

Nickel sulfide is one of the many minor impurities present in the raw material used for the manufacture of glass.
The amount of nickel sulfide present in the raw material can vary significantly from batch to batch and, although glass manufacturers can take measures to minimise the presence of nickel sulfide, it cannot be eliminated completely.
The presence of nickel sulfide in Annealed Glass is not a problem. The presence of nickel sulfide only becomes an issue when the glass is Toughened Glass.

When glass is toughened the heating process causes a chemical change in the nickel sulfide crystals, from their alpha-form to their beta-form. The alpha-form is denser and occupies a slightly smaller volume than the beta-form. During cooling, the molten glass reforms around the alpha-form crystal in its reduced size.
The nickel sulfide crystals gradually revert, over a long period of time, to their beta-form. If the crystal is large enough this expansion will generate sufficient stress to break the toughened glass from within. This is often referred to as "spontaneous" glass breakage.
There are a number of reasons why toughened glass may suddenly break "spontaneously" and the presence of nickel sulfide inclusions is only one of them. Breakages due to nickel sulfide have a characteristic pattern of fractures generally referred to as a 'butterfly', an example of which is shown below.
There is no statistically valid way of calculating the likelihood of toughened glass breaking due to nickel sulfide inclusions, and thus no figure to compare with the one instance in 400 metric tonnes quoted below for heat soaked glass.

The photograph below is a good example of the typical 'butterfly' pattern resulting from a nickel sulfide breakage.
It is, however, possible that mechanical damage might generate a similar breakage pattern. To be reasonably sure that the breakage is due to nickel sulfide you need to look for the expanded inclusion at the centre point of the 'butterfly'.

Heat soaking is a process during which the toughened glass is re-heated, maintained at a given temperature for a specified period, then gradually cooled.
The process is intended to accelerate the failure of any sheets of glass with 'critical' nickel sulfide inclusions, i.e. those inclusions which might cause the glass to break in normal use. Heat soaking is not a 'test', the intention is solely to induce early glass breakage and thus minimise the potential for future problems.
Properly carried out, heat soaking will considerably reduce the possibility for future failure, but it cannot eliminate it entirely.

Once the problem of nickel sulfide inclusions had been understood, and heat soaking identified as the most satisfactory method for minimising the potential for failure, glass manufacturers started developing their own heat soaking methods and standards.
These varied and, for many years, the only internationally-recognised standard was the German standard DIN 18516-4. This required the glass to be "conditioned" for eight hours in an oven at a mean temperature of 290^oC +/- 10^oC but provided no guidance as to how this was to be achieved.
BS EN 14179 was first published in 2005. Although the "holding period" of two hours would, at first glance, appear to be a lower standard than the DIN requirement for eight hours, the EN standard has requirements for control of the heat soaking process, including calibration of the oven and measurement of the surface temperature of the glass. Expert opinion believes that these make the EN standard superior to the DIN standard.

It is generally accepted that heat soaking to BS EN 14179 should reduce the statistical rate for critical nickel sulfide inclusions to one instance in 400 metric tonnes, i.e. 400,000kg, of processed glass.
The weight of glass is approximately 2.5kg/square metre/mm of thickness.
The total glass weight can be simply calculated once the glass specification has been finalised, and the total area of glass required for the project determined - not forgetting to allow for the combined thickness of both panes in double glazing, etc.
This will provide a very approximate guide to the theoretical likelihood of glass failure due to nickel sulfide inclusions. There are many other factors that determine whether a critical nickel sulfide inclusion will actually cause glass failure and a proper risk assessment should be carried out.

There is no valid basis for a direct cost-in-use comparison between heat soaked glass and non-heat soaked glass as it is impossible to estimate the number of breakages likely to occur if toughened glass is not heat soaked. (There is no generally accepted figure to compare with the one instance in 400 metric tonnes quoted above for heat soaked glass.)
In August 2010 one of the major international glass suppliers quoted a budget figure of £0.33/mm/m² as the cost of heat soaking, e.g. £3.33/m² for a 10mm thick pane.
This figure may be used to estimate the approximate cost of heat soaking the glass for an entire building, and this can be compared with the cost of replacing individual panes of glass in different locations around the building.
There is, however, evidence that many of the breakages caused by nickel sulfide inclusions (in toughened glass which has not been heat soaked) can be related to specific batches of glass, e.g. if one pane from a batch fails there is a increased possibility that others from the same batch may fail. In these cases, the cost of the glass replacement will be far greater than the cost saving gained by omitting the heat soaking.