| Technology |
Glass Tempering by Numbers
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Turning an 'Art' into a 'Science' with the help of some simple Rules and Numbers. By Jonathan Barr
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 The process of thermally toughening a piece of normal soda-lime glass is very simple: - just uniformly heat the glass to a point above its softening temperature, keep the glass optically flat, (or shape it if required), and then uniformly cool the glass so that the temperatures of the top and bottom surfaces are equal, and lower than the temperature at the centre plane of the glass, as it cools to below the softening temperature and becomes solid again. That is the 'Sum Total' of what a good tempering plant does.
The process relies on the fact that, as the glass then cools down to ambient temperature, the centre plane of the glass will contract more than the surfaces because it was hotter than the surfaces when the body of the glass became solid. The contraction of the centre plane pulls the surfaces into compression and the glass is therefore very strong.
So let's start to put some numbers in to help define the process.
Scientifically we know that the softening temperature of soda-lime glass is around 550°C. Below this temperature we consider the glass to act essentially like a solid, and above this temperature we consider the glass to be in a plastic condition. And, for every 9°C the glass is heated above its softening temperature, the viscosity (stiffness) of the glass halves, and so it very quickly becomes much easier to distort.
We also know that to achieve the final stress level in the glass, to pass the tempering standards, we need between 100°C and 170°C (depending on the standard) between the surfaces and the centre as the glass cools down through the softening point and becomes solid. (Glass with a very low coefficient of expansion e.g. Borosilicate glass (Pyrex) cannot be easily toughened using a thermal process)
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Photos: GOW.com
Last review: September, 2006 |
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