From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Saturday, February 28, 1998 5:08 PM
To: Tesla List
Subject: Re: glass cutting
> Subject: glass cutting
> If anyone out there is planning on making plate glass caps, please be
> warned, our neighbor gave us a couple of fine sheets of Hercules Safety
> glass. The sheets were 24" long and 8" wide. Harold thought to saw them
> into 8" square pieces to make a glass cap. I do a bit of lapidary work,
> so we have a diamond edge abrasive saw that is used to cut rocks, glass
> and the like. Harold marked the glass, donned a rubber apron, a hat and
> safety goggles, and turned on the saw. He had not cut an eighth of an
> inch into that glass when it literally exploded.
What you had was tempered safety glass (no doubt, it had the little marking
in the corner, often called the "bug"). It is made with high internal
stresses by a heat/cool cycle (after cutting) so that any little defect
causes it to turn into small glass "gravel" as you noticed. It is called
safety glass because the little shards aren't as dangerous as the huge
daggers, swords, and guillotines created by standard plate glass. For
instance, that sliding glass door to your patio is (should) be tempered
glass, so that when your 3 yr old runs into with the trike, they don't get
cleaved. The "gravel" does have sharp corners, but nothing like the
slivers and really sharp edges from regular plate glass.
Most automobile side and rear windows are tempered glass. When repairing
the window regulator on my old Honda Civic, I tapped the window with the
screwdriver, just wrong, and wound up with a door full of glass gravel. The
windshield is typically a "laminate", i.e. a sandwich of
glass/plastic/glass, which tends to stay together when hit by an object.
You can imagine why this might be handy.
We use a lot of tempered glass in the special effects industry with a
variety of means to break it. For instance, a small pyrotechnic charge in a
cylinder driving a hardened steel point into the corner of the window will
cause it all the shatter. A steel ball bearing in an air blowgun also works
nicely. (The window the stunt guy jumps through is typically not glass, but
sugar carefully crystallized into a sheet: "candy glass", which does form
realistic looking shards). To tell the difference, look at the shards on
the ground: tempered looks like gravel, untempered gives huge pieces that
are all jagged.
The upshot is, you can't really machine tempered glass.
The gravel will continue to self destruct for a day or two, as the internal
stresses are relieved. It eventually turns into, of all things, sand (i.e.
that from which it was originally made).
We also wonder what might have happened,
> should he have had access to the size he needed, constructed the cap and
> put the high voltage to it. Big explosion?
Presumably, the cap would be in a container, so it would contain the shards
when it self destructed.