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Re: car windshields



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.
There are two demo's I have seen on tempered glass. One involves a fairly
massive glass bottle that can be used to drive nails, but dropping a small
piece of carbide into it will cause it to shatter. The other is Rupert's
drops. I think that the name. They are drops of glass made by dropping
molten glass into water. You can hit them with a hammer and they don't
break, but breaking off the tail will cause them to shatter.

I hope this helps.

Gary

Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU