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.
- I think its probably a good idea to just wire the fans on the same timer as the lights - PCs come on first with 2 fans - MH comes on with 2 more - then off in reverse.
