Re: car windshields

[Doug Craigen <dcc@ESCAPE.CA>]

Gary Karshner wrote:
> Fred,
>         What your student is describing is tempered glass. The glass is cooled
> rapidly at the end of its processing which produces internal stresses in it
> and makes it very hard. If you scratch the surface it relieves the strain
> catastrophically with the glass breaking into little nuggets, as you thought.
> This kind of glass is used in the rear windows of cars, but I thought it
> wasn't used in windshields because it is harder then your skull bone. For
> windshields they use a double layered glass with a plastic adhesive in
> between so the glass can't shatter. The strain glasses nuggets are not
> sharp, where as shatterproof glasses shards can cause cuts.

The rationale that I heard was that if the front windshield was tempered
then the driver could suddenly find themself without a windshield - or
in the laminate case with a windshield that they cannot see through.  So
rather than a stone flung up from a passing vehicle causing a crack in
the windshield, it causes a major accident.  Another possible factor is
the difficulty in tempering curved glass.  Whatever the reason, it is
clear enough looking at North American cars that have been in accidents
that the front windshields are not tempered here.

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Doug Craigen
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