<VV> tempered glass-handle with care

[RoboMan91324 at aol.com]

Harry, et al,
 
It is impossible for me to say with certainty but I would like  to give a 
possible explanation of what happened with your exploding glass.   However, I 
would first like to give an oversimplified discussion of the nature  of 
tempered glass and safety glass as it would help all to understand the  
strengths and weaknesses of these types of glass.
 
Back in the good old days, it became obvious that normal sheet  glass was 
dangerous to cars' occupants in an accident.  Pushing your head  or other 
body part through glass that turned into knife edged shards would often  cause 
death even when the impact of the accident did not.  This was  especially 
the case before the use of seat and shoulder belts.  To counter  this, at some 
point, (1920s?) windshield glass was required to be laminated with  plastic 
between two layers of glass.  The glass would still shatter fairly  easily 
but it was more difficult for a body part to penetrate through and flying  
glass knives were kept to a minimum.  When I bought my home, the state  
required that I either replace the glass in sliding doors with tempered glass or  
have a licensed company cover the glass with a film of plastic thus giving 
me  laminated glass.  To my knowledge, windshields are still made this  way.
 
The use of tempered windows on the sides of cars and back  light 
applications was a significant safety improvement as well because it was  much more 
resistant to breaking and when it did break, it shattered into pebbles  
instead of shards with knife edges.  So, what are the characteristics of  tempered 
glass?  Again, this will be simplified.
 
Hot glass flows and allows the manufacturer to form it into a  wide variety 
of shapes.  Almost all present day automotive glass has some  degree of 
curve to it.  If the manufacturer allowed the glass to cool  slowly overall, it 
would not be tempered.  What they do, is quench the  outside surfaces of 
the glass so that they cool and harden relatively quickly  while the center 
remains hot, soft and fluid for a short period of  time.  When the outside 
surfaces cool, they shrink and harden relatively  quickly.  Because the center 
is still fluid, it conforms to the outside  shrinkage.  When the center 
eventually cools, it also shrinks but the  outside surfaces are already 
hardened.  The end result is that the outside  surfaces are pulled into compression 
as the inside contracts.  This is  important.
 
Glass is a brittle substance and all brittle substances are  stronger in 
compression than they are in contraction.  In effect, you can  easily support 
a significant weight from below with a concrete post but you  would never 
hang a similar weight with a concrete member.  Because the  outside surfaces 
of tempered glass are in a constant state of compression it is  very 
resistant to impact, relatively speaking.  Yes, the center of the  glass is in 
contraction but it is protected from impact by the outside  layers.  The glass is 
in a constant state of significant stress due to  the compression and 
contraction at the same time.  Significant energy is  stored in these stress 
forces.  This is why tempered glass seemingly  explodes into pebbles once its 
stress limits are exceeded.  The stresses  are immediately relieved in the 
process and release the pent up energy almost  instantaneously.  To demonstrate 
this, take a piece of tempered glass that  you no longer want and give it a 
moderate tap on the surface with a  hammer.  Then give it a tap on the edge 
with the same force.  The  glass will shatter much more easily with a tap 
on the edge.  Even though  the piece if glass has minimal compression on the 
edges, it is still weaker than  the broad surfaces and the stress patterns 
are more complex.   WARNING:  If you try this, the glass pebbles will go 
flying everywhere at  high initial velocity and they have lots of small sharp 
edges.  You can  suffer minor cuts to your skin and if these cuts are to your 
eyes, it could be  disastrous.  The glass can fly for yards in any 
direction.  Be very  careful.
 
Now, what MIGHT have happened with your piece of Allante  glass.  Glass is 
an amorphous solid substance.  This is a fancy word  for something that is 
solid but also flows.  While glass is brittle at room  temperatures, it still 
flows/bends/deforms at an extremely slow rate.  What  could have happened 
is that the metal mounting bar the glass was mounted in  could have slightly 
deformed (bent) the glass when the bar was mounted into the  car.  This 
would have created a stress in the glass but not enough stress  to make it break 
initially.  Over time, the minor deformation of the glass  may have 
relieved itself due to the amorphous nature of glass.  This relief  would have been 
a slow process but I do not know how old the car is.  When  you removed the 
window assembly from the car, the bar returned to its natural  state and 
created a stress in the glass in the opposite direction.  The  glass may have 
been more susceptible to stress in one direction than in the  other.  This 
may have been due to a minor crack on one side that became  more compressed 
when mounted in the car but then subsequently contracted when  the assembly 
was removed from the car and the stresses were relieved.   BOOM!  Of course, 
this is a theory which is not supported by  evidence.
 
Keep in mind that any scratch or even a minor crack in  tempered glass can 
seriously weaken it.  In a windshield, you can get a  traveling crack put in 
tempered glass you could get a pebble  explosion.
 
By the way, if you store windshields for an extended time, you  should do 
it on the edges.  If you must store the windshields horizontally,  you should 
support the glass at multiple points such as 1/3 in from each  side.  
Because of the amorphous nature of glass, the windshield will begin  to sag over 
the years and this is worse if the glass is supported only on the  center 
point.  This is more important for windshields that are mounted in  gaskets 
like our FCs because the gaskets will force the glass to conform to the  shape 
of the FC's mounting opening.  If the glass is mounted in  tar/silicone, 
this is less of an issue because the tar/silicone conforms to the  glass and 
does not create excessive forces.
 
How do you avoid a similar episode in the future?   Unfortunately, there is 
little you can do other than handling the glass and  window assemblies with 
TLC.  If the above described scenario was the cause  of the explosion, it 
is doubtful you could observe the potential problem before  the glass hit the 
fan, so to speak.
 
Happy sweeping,
 
Doc
 
1960 Corvette, 1961 Rampside, 1962 Rampside, 1964 Spyder  coupe, 1965 
Greenbrier, 1966 Canadian Corsa turbo coupe, 1967 Nova SS, 1968  Camaro ragtop

~~~~~~~~~~~~~~~~~~~~
 
In a message dated 3/10/2012 4:49:47 P.M. Pacific Daylight Time,  
virtualvairs-request at corvair.org writes:

Message:  2
Date: Sat, 10 Mar 2012 14:55:08 -0500
From: "Harry Yarnell"  <hyarnell1 at earthlink.net>
Subject: <VV> tempered glass-handle  with care
To: "'VV'" <virtualvairs at corvair.org>
Message-ID:  <001801ccfef7$b2715210$1753f630$@net>
Content-Type: text/plain;   charset="us-ascii"

I have just experienced a frightening event;  the explosion of a tempered
glass door window.

I was working on the  power window mechanism of an Allante. The door glass 
was out and resting on  the arms of a leather chair (don't ask), in a 
vertical position. It was  solidly placed as not to shift or fall. I continued 
working on the motor and  drive in the door when 10 minutes later, this sound 
of a shotgun goes off,  followed by this falling sleet of glass 
pebbles...WTF! 

Glass pebbles  are EVERYWHERE. All that's left of the window is the lower 
mounting  bar.

I have no idea why this happened, but obviously there was stress  somewhere.

Oh, it was Italian   glass...