More than you ever wanted to know about glass.
I test windows for hurricane performance at my laboratory. There are many
different kinds of glass. Annealed glass is just regular glass that breaks
into large pieces. Heat strengthened glass is heated and cooled to make it
stronger than annealed but still breaks into large pieces. Tempered glass
is heated and cooled rapidly. This makes it very strong. It can be up to
four times stronger than annealed glass of the same thickness. When it
breaks it breaks into lots of small pieces. Laminated glass is two or more
pieces of glass with a plastic interlayer between. It can be annealed, heat
strengthened, or tempered. Typically the interlayer is poly vinyl butylene
or polycarbonate. All windows in cars are tempered as well as windows in
your house that are in the kitchen, bathroom, hallways, and doors or next to
doors. Car windshields are laminated tempered glass. It used to be that
the glass was laminated annealed. This gave rise to the dreaded glass
necktie which was a condition where the head would go through the laminate
in an accident and the large annealed shards did a number on the neck. Now
they use tempered glass and less brittle PVB interlayer which allows the
plastic to stretch without the head rupturing it. The small tempered pieces
still make cuts but don't cause the horrific glass necktie.
One of the tests we run in the lab is to swing a 100 pound ball into a piece
of tempered glass and then sort through to find the 10 largest pieces. We
weigh the pieces. If they are too big then the manufacturer is not allowed
to etch the certification mark on the corner of the glass. Another test is
for hurricane flying debris protection. We shoot a 2x4 at the window.
Laminated glass with a 0.090" thick innerlayer will typically keep the 2x4
from penetrating. We then blow 9000 wind gusts at 150 mph at the window.
Sometimes the sharp edges of the large annealed glass shards will saw a hole
through the innerlayer while the window heaves in and out as the wind gusts
blow. If this happens the manufacturer sometimes opts to test again with
tempered laminated glass. This beaks into smaller pieces when we impact
with the 2x4 and has less chance of forming a rupture during the wind
cycles.
Tempered glass cannot be cut to size after it is tempered. Typically a
window manufacturer will order crates of annealed glass that are 10' x 10'.
They then cut it to the right size for the window. Tempered glass has to be
supplied in the pre-cut sizes and are therefore much more expensive.
Another problem with tempered is that it gets wavy during the tempering
process.
If anyone is still awake or interested I can post up information about
sealed insulating glass, fogging, condensation, the myth about argon fill,
energy efficiency, solar heat gain, or just about anything else window
related.
Andy
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