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At ToughGlaze, we are finding the use of toughened glass in buildings is becoming increasingly of demand. With the routine of construction industries making use of large glazing screens, glass roofs and curtain walls to benefit there projects, nickel sulphide inclusions has become an issue.
Glass is produced from bulk natural materials which are then processed to an extremely high standard, naturally the occurring products inevitably contain a level of low level of impurities. Nickel Sulphide is one of the causes in which is the failure of toughened glass. The presence of Nickel Sulphide in a batch of glass is variable and it is quite likely that large quantities of glass may contain comparatively low levels of Nickel Sulphide and in reverse this is true.
An understanding of the problem by the manufacturers has caused them to take steps to eliminate this impurity.
When Nickel Sulphide impurities are present they are in the form of small crystals, differing crystalline forms each stable within various temperature ranges. They are not generally a problem in annealed glass but become an issue in toughened or heat strengthened glass. When rapidly cooled the glass is hardened around a shrunken version of the crystal. This process is so fast that it does not allow the crystal to revert to its more stable form.
The modified Nickel Sulphide crystal must revert to its low temperature form but as it is trapped in the glass it can only do this slowly, the crystal expands and it is at this process which places intolerable stresses on the hardened glass and provides failure in results. This will only happen if the crystal is large enough to place sufficient stress on the glass.
The actual rate of occurrence of Nickel Sulphide is low but the user of the glass must consider the effect and threat to Health & Safety of such an inclusion in all the glass specified.
ToughGlaze offer a heat soaking process that complies with BS EN 14179. In this process the glass is heated and maintained at a temperature then slowly cooled in a heat soaking oven which encourages the conversion of the Nickel Sulphide inclusion to its more stable form.
Please note that even heat soaking to the most rigorous category will not guarantee the conversion of all Nickel Sulphide so there is still an inherent possibility of failure however the failure rate is greatly reduced.
Based upon the European rate of Nickel Sulphide inclusion, there will be one critical inclusion in every 4 tons of glass. |
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