Ask A Scientist© Chemistry Archive

name         Bud B.
status       other
age          50s

Question -   Glass can contain a defect that will cause it to
spontaneously explode after a period of years, with no obvious
cause.  This is often seen in auto glass when for no obvious reason
a rear window or windshield will explode as if it has been shot out.
It seems like th epresence of nickel sulfate or some suce contamination
is the cause.  What is the cause and the failure mechanism?
Here is the origin of my question. Our pastor was simply driving down the
street when
the rear window of his car simply exploded. No impact of any kind, it
simply exploded. I know there is an explanation but cannot find it.
I first heard that this was a relatively common condition when I was at a
ballistics test lab in Aberdeen, Maryland.  A man pulled in and thought his
windshield had just been shot out as he drove along the road. He was in a
real panic. There were two glass company technicians there and they said
knowingly between themselves what I think I remember as "nickle sulfate" and
nodded in agreement. The motorist was eventually assured that he had not
been shot at and after calling the police he drove away. I never got the
explanation from the glass guys and have wondered about it since.
I have read your responses on glass as a liquid, (I remember
vividly when someone gave me the "Glass is a river" speech) but see nothing
that relates to this question.
Things do not just happen as a rule, something causes them. I was just
curious but will assume that you have not the faintest idea.
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More likely, the glass broke from "simple" harmonic oscillation.  Every
object has a natural vibrational frequency.  If that frequency is supplied
to the object from an external source, the object will vibrate
sympathetically.  If the external source supplies enough energy such that
the elastic modulus of the object is exceeded, the object will break.  You
have probably seen opera singers break goblets by singing a tone.  No
chemical impurities in that glass!  No defects either, structural or
chemical!! (How do they do it?  They vary their pitch until the glass starts
to vibrate, then hold that pitch until the vibration exceeds the elastic
modulus of the crystal.)  Where did the vibration come from?  Could be any
one of a zillion possible sources...

Tim Spry
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I do not know the role of nickel sulfate, but windshields "explode" like
you describe in hot weather that induces stresses in laminated windshields.
And once a crack is initiated it rapidly propogates -- explosively. This can
also occur in some cases if the windshield is struck by a flying pebble, and
frequently happens when the auto is involved in an accident.

Vince Calder
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Hi, Bud
Well, the answer I have for your question is based upon tensions
that somehow may occur and cause the glass to explode. For instance,
an exagerated heating and no place to the required expansion in the steel
frame where the glass is fixed. This seems to me more likely to happen
than temperature differences between internal and external surfaces,
which in turn could cause a thermal shock wave that could destroy
the glass. I couldnīt find anything related to chemical impurities

Regards
Alcir Grohmann
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