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This was in the post for drilling a tank that toufic posted. Thought it was interesting:

Tempered Glass: More Than You Want To Know

by charleyb/gr.hp.com (Charley Bay (Contract))

Date: Mon, 30 Oct 1995

Newsgroup: sci.aquaria,rec.aquaria,alt.aquaria

This is a response to several recent aquarium-drilling


After incrementally seeing my own ignorance in glass topics,

I spent some time with a good friend of mine with an extensive

history in the glass and optics industry.  He has several patents

and has run several very large glass manufacturing companies. 

If we have more questions about this stuff, I'll just invite

Alan Beatty over for another beer and we'll get the answers

right away.

Tempering glass and tempering steel are roughly the same


o  Typically, the material is fed horizontally into large

  furnaces that heat it to a glowing red-hot.  Glass is

  malleable at this point.

o  Quickly remove the material from the furnace and cool

  both surfaces down QUICKLY using "quenchers" (which blast

  very cold air on both sides).

The result:  The surface of the glass (both sides) is under

very high comression because they were cooled so quickly.

For fully tempered glass, this should be 15,000 PSI.

The internal portion of the glass cools more slowly, and is

under severe tension, being pulled to both surfaces.  The

density of tempered glass and annealed (non-tempered) glass

is exactly the same:  tempered is merely under compression

and tension, while annealed glass is not.

Note that this compression/tension between the surface and

the interior of the substance is the desired result of

tempering (same for glass or steel):  The opposition of

force results in internal stress that makes the material

far stronger (but more brittle).

Once the glass is tempered, you can no longer work with it.

Becuase it is under severe compression (the surfaces) and

tension (the internals), it will shatter when one of the

surfaces gets a hit greater than that the surface is tempered

to handle (15,000 PSI on fully tempered glass).  Thus, you

should be able to hit your aquarium pretty hard with a hammer

or baseball, but a small hit from a needle or an icepick may

shatter the whole thing (you need to exceed 15,000 PSI at

only one point).  This is why a cute fuzzy little bristle

worm (marine) can shatter the side of your 29 gallon tempered


NOTE:  Annealed (non-tempered) glass is typically under

no (or very low) internal pressure, closer to 400 PSI.

Thus, it won't shatter (it's not under such high tension

and compression), but it also will crack or break more

easily (it has a lower threshold:  400 PSI).

If you want to break your tempered tank, you must exceed

this 15,000 PSI limit at some point on the glass surface.

Thus, standing in a tank supported in only two corners is

fine, as long as you have 14,999 PSI (or less) stress on

those two corners (theoretically).  (Ditto with the annealed

glass and the 400 PSI threshold).

RECALL:  Tempered glass:  15,000 PSI must be exceeded. 

  Annealed glass:    400 PSI must be exceeded.

Tempered is thus better than 10 times stronger.

Automobile windshields are tempered around 10,000 PSI, and are

thus not considered "fully" tempered.  When they break, you see

pieces maybe 1/4".  When fully tempered glass is broken, the

pieces are very small, like 1/8" to 1/16".  In fact, the temper

on automobile windshields are measured by breaking a few of

them and then counting the number and sizes of the pieces.

You can otherwise measure the temper of glass (if you don't

want to break it) by measuring a poloroid light through the

glass (tempering glass tends to polorize the glass).  This

is why tempered glass often seems to have poorer visual

quality:  the glass is slightly polarized, so looking at

our beautiful aquascapes at an angle may lower the viewing


You must shape the glass, put holes in it, etc. before the

tempering process.  For automobiles, the glass is cut to

the desired shape (including any holes), and when it comes

out of the furnace red-hot (and malleable), it is curved.

Then, the quenchers blow cold air on it and the piece is

tempered.  You can no longer cut it.  Curved glass never

tempers as well as flat glass because the quenchers cannot

cool the surfaces as evenly.

Because the glass was heated to a glowing red-hot (and was

malleable), tempered glass is NEVER as straight as annealed

(non-tempered) glass.  Tempered glass always has small

ripples, warps, or twists in it.  Thus, there is a chance

that your aquarium won't line up as well when assembling

pieces of tempered glass.  (These ripples can also

contribute to a lower viewing quality, in coordination with

the partial polarization).

However, most big tanks don't use tempered glass:  While

tempered glass may be 10 times stronger than non-tempered,

the big tanks need that extra thickness for support so

nobody bothers with the tempered expense.  It's better for

the little 10 gallon aquariums where the thinner, stronger

glass can save on space, shipping, and weight requirements.

Since the total stress is lower on these smaller tanks, it

is far easier for our sillicon adhesive caulk to compensate

for any un-evenness in the surfaces of the glass (it can

cover the cracks caused by tempered warping).

The tint in some glass is a result of melting the silica

with iron oxide, cobalt, selenium, or other elements to

help the glass resist alkaline etching.  That's partially

why the glass is so resistant to chemical reactions even

in marine systems with a very high pH.  Also, some lower

quality glass can have other photo-sensitive impurities that

may show up with time, decreasing the clarity of the glass

(recall turn-of-the-century old windows that have yellowed).

In summary, tempered glass is under severe compression at

the surface and tension internally, which allows it to shatter

when any part of its surface exceeds its temper and the

tension can "leak out".  For fully tempered glass, this is

15,000 PSI.  It doesn't take a lot of force for a needle

to exceed this pressure, but it takes far more for a hammer

or a baseball (with a larger surface area) to exceed this

pressure.  Thus, the stories of dropping a filled 200 gallon

aquarium two feet with no breakage can be absolutely true.

Annealed glass (non-tempered glass) is in a relatively

non-stressed state (no tension or compression), which works

out to about 400 PSI surface pressure it can withstand.  In

fact, many glass processing practices (cutting, drilling,

shaping) require glass to be in an annealed state (minimal

internal stress, less than 400 PSI compression).  Then, you

can temper it when you are done processing it by heating it

and quenching it. 

You can't ever remove the temper from tempered glass unless

you heat it to molten red-hot.  (Nobody does this).


--charley                              #include <stdisclaimer.h>

charleyb-at-gr.hp.com    -or-    charley-at-agrostis.nrel.colostate.edu

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I liked this the best - "This is why a cute fuzzy little bristle

worm (marine) can shatter the side of your 29 gallon tempered



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Interestingly I had some 10 mm tempered shower screens made recently. The one in my shower as a slight bow in it. It's only about 3 mm over 2 m but even though it's sitting next to another sheet it's only just noticeable. It looks odd as there is no way there would be that much flex in 2m of 10mm glass so I'm guessing it's the warp described here.

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