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Re: [APD] tempered glass (william ruyle)
The glass v. acrylic "DIY" pops up every so often
on the list (and for me personally too ;-), as does
the "temper v. annealed (non-tempered)". And,
there's some great info on the web with new products
(like the relatively recent low-iron glass, laminated
Below is a my post from Oct 1995 (10 years already?!)
that might be somewhat helpful -- it's still pretty
much accurate, although we have new products we might
consider. (This post regards the tempered glass
An addendum is that you *can* drill tempered glass,
but you have increased chance of breakage (see below).
And, micro-fissures from the drilling process never
go away and *always* get bigger (but slowly, and based
on movement/vibration). But, I've done it and you can
still run your tanks for at least a decade (?or
Sorry, it's a long post: It was originally (aptly)
entitled, "Tempered Glass: More Than You Want To Know"
> This is a response to several recent
> aquarium-drilling posts.
> 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 processes:
> 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). [UPDATE: Steve Miller corrected
> this to describe the phase transition of steel
> tempering through iron carbide particles, see:
> 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 tank.
> 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
> 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. [UPDATE: Windshield glass and
> tempered glass for some windows/doors are laminated
> with plastic or other coatings to hold the broken
> pieces together. This doesn't impact the "strength"
> (that's the temper), but rather holds all the broken
> pieces together for other safety reasons.]
> 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 quality.
> 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
> 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
> 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
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