Tempered glass explanation

I was going to give my $.02, but PC acted up, so I never did. I found a
ST*J article that explains it better anyway:
 
SUBJECT
Recently I had a really spooky experience. I had
just removed the glass from a video game and
turned to set it down when all of a sudden it
exploded right in my hands as though it had
been shot by a gun! It completely disintegrated
into a pile of marble sized pieces on the floor, still
crackling, leaving me dumbfounded.
When I related the incident to co-workers I found
that some others had similar experiences involving
thick (1/4 inch) sections of glass.
I’ve ruled out poltergeists sun spots, and bad
karma, and there were no Memorex tapes anywhere
in the vicinity. Does anyone know what
causes this?
EXPLANATION
What you had was a piece of tempered glass. It
has excellent strength and durability. And that
is exactly what it is designed for: Strength
without the bulk.
TEMPER TEMPER
When the original plate of glass was rolled it had
many surface defects and is very brittle at room
temperature. (I’m sure you’ve broken an ordinary
glass window before, the cracks were formed
along these defects.) It is these surface defects
that provide the stress lines that cause the glass
to break so easily. To avoid the surface defects
and to increase overall strength a tempering
process is used.
THE PROCESS
After the rolling process, the sheet of glass was
heated again to almost the melting point. That
smooths out all of the surface defects, then the
hot glass is exposed to a cold blast of air. That
instantly solidifies the surface and yet keeps the
interior hot. As the interior slowly cools it contracts
and pulls the surface into a compressed
state. That makes it one seriously stiff piece of
glass!
Believe it or not, your average pinball glass has
over 15,000 psi (pounds per square inch) of
surface tension on it, and pressures of up to
35,000 psi aren’t uncommon.
IN REAL LIFE
Now, say you have one of these 15,000 psi pieces
of glass that’s been hit and scratched in several
places (arcade wear). What you were carrying
around was a 15,000 psi time bomb that had all
kinds of stresses built up and focused on and
around these little scratches. Add a little thermal
stress from a pair of warm hands and a little
mechanical flexing from the movement and
BOOOOOMMMMMMM! Little bitty chunks of
stress relief, that you will find in the strangest
places for years to come, are flying.
FOR YOU FACT NUTS
Surface stresses of 1,000,000 psi have been
attained with heat and chemical tempering.
How would you like to have some of that stuff go
off in your hands!
 
+++++Ask me about Retrocade 30-in-1 MultiPac kits for Midway Pac Man and Ms
Pac Man boards and same kit already attached to a new JAMMA PCB. Also we now
have purchased the rights to WG6100 (Tempest, etc) new deflection boards and
HVU boards. Also HVU boards for Amplifone (Star Wars). These are version 2
with no extra wiring or mods necessary. Ask about Star Wars overlays and new
cabinets for Midway cocktail, upright, mini, Dragon's Lair, Williams, Golden
Tee Fore, and Ultra-MAME++++

---------------------------------------------------------------------------
** Unsubscribe, subscribe, or view the archives at http://www.vectorlist.org
** Please direct other questions, comments, or problems to chris@westnet.com
Received on Sun Mar 19 15:57:16 2006