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Toughened (or tempered) glass is made from standard Float Glass to create an impact resistant, safety glass.
If float glass is broken it will break into very sharp, hazardous pieces of glass.
The process of toughening the glass introduces tensions into the internal and surfaces of a glass panel to increase its strength and also to ensure in the case of breakages the glass shatters into small, harmless pieces of glass.
Glass has to be cut to size and shape accordingly before the toughening process, if the glass is cut afterwards it will crack and shatter.
The cut float glass panels are put into a toughening furnace. Here the glass panels are heated to upward of 600 degrees C and then the surfaces are cooled rapidly with cold air. This produces tensile stresses on the surface of the glass with the warmer internal glass particles. As the top thickness of the glass cools it contracts and forces the corresponding glass elements to contract to introduce stresses into the glass panel and increasing strength.
Toughened glass units have a much higher resistance to impact and heat than float glass, reducing the chance of thermal shock, but their clarity, colour, light transmission and chemical makeup remain mainly unchanged.
Toughened glass sizes are largely dependant on the size of float glass cuts and the size of the toughening furnaces.
In the UK the largest toughened glass panes available are 6m by 3.2m. There is a specialist toughening plant that can toughen glass panels at 8m by 3.2m known as ‘Supersized Glass’.
To further improve the thermal stability of glazing, a secondary process called ‘heat soaking’ can be undertaken.
This involves cooking the toughened glass to very high temperatures for extended periods in the hope of identifying which panels are likely to include a Nickel Sulphide particles and reducing the chance of Nickel Sulphide Inclusion.
Read more about Heated Soaking and Nickel Sulphide Inclusion