Re: Tempered glass explanation

Pulling out an old playfield glass and having it shatter is actually not uncommon. Startling the first time it happens but it happens a fair amount, especially if you just pulled that machine off of a commercial location.

Chad
  ----- Original Message -----
  From: Electronicamuse@aol.com
  To: vectorlist@vectorlist.org
  Sent: Sunday, March 19, 2006 3:57 PM
  Subject: VECTOR: 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!

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Received on Mon Mar 20 07:45:12 2006