[APD] Glass myths

[Wright Huntley <whuntley at verizon_net>]

There has been a great deal of what sounds like misinformation about 
glasses here, lately.

First, can someone cite any reference that tempered glass is stronger 
than ordinary sheet glass? It was stated as fact several times, without 
proof.

IMHO, it may be slightly stronger in some applications, but it usually 
isn't a great deal stronger. Its advantage is the way it breaks without 
leaving large slicing pieces. It has a deliberately built-in stress 
pattern that causes the little cubes to form when it shatters. That 
makes it far weaker than other glass when scratched, BTW. Really dumb to 
use in any aquarium where gravel can accidentally scratch and shatter it 
during cleaning. Good idea for terrariums, maybe (except for cobras?). I 
don't even like it in the bottom glass, but shower-door material is too 
common and available in those thicker sizes, so they do use it in larger 
tanks. The bottom is less likely to get scratched during cleaning, anyway.

It isn't a bit lighter than the same thickness of sheet glass of the 
same chemical formulation. It normally is still just single-thickness 
soda-lime glass with a built-in stress pattern. [Of course the glass can 
be chemically formulated to be stronger, but IME it usually isn't.That 
has nothing to do with the tempering.] Look at it between crossed 
polarizers if you want to see the pattern. You can often see it in car 
side and back windows when you are just wearing Polaroid sunglasses, if 
the sunlight is at a low enough angle to be polarized a bit.

Most aquariums are made of sheet or float glass. Plate glass disappeared 
many many years ago as it was way too expensive and labor intensive. 
They quit making it here in the US, before most of you were born. Float 
glass was getting just too good to distinguish them, but was far 
cheaper. There is no such thing as plate glass in the American consumer 
market. It is imported for some fancy mirrors, coffee tables, etc. but 
not for windows or aquariums. There is not, and hasn't been, since about 
the '50s, any major American maker of plate glass. Plate glass is 
regular window glass that has a final step where it is ground flat and 
parallel, and then polished like fine optics. Expen$ive!

Water-white glass is not just "low iron" glass. Basically, it is the 
same as the brand-named "Pyrex" in that it has higher purity and 
higher-temperature fluxing agents. It is also often formulated to have a 
lower thermal expansion coefficient (more soda, less lime?), so it 
doesn't break from quick heat changes. It has to be handled at higher 
temperatures so is more expensive for most applications. It usually is 
stress relieved (annealed), so tends to be very strong, too. Still 
glass, it is made by casting (float) or drawing (sheet), like other 
glasses. You can spot it instantly as the edges do not look green.

Tempered glass is very different from laminated safety glass. The latter 
is used in windshields mostly to keep you in the car in a collision, and 
(perhaps) avoid the blinding if tempered and it didn't all fall out when 
hit by a pebble. It also doesn't cause polarized "rainbows." It is two 
thin sheets of glass bonded to a plastic center layer. You cannot, as a 
practical matter, usually cut either safety or tempered glass at home. 
Most auto glass is tempered, after the outline and curvature are 
established, as a final step. Safety glass is usually annealed to 
relieve all stresses, make it stronger, and to avoid the polarization 
"rainbows."

Wright

-- 
Wright Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net - 760 872-3995

"...watching Mayor Nagin and Governor Blanco handle the New Orleans/ Louisiana debacle is like watching Moe and Curly trying to work a Rubik’s cube."
Doug Giles

                 
http://www.self-gov.org/wspq.html

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