I guess its the strength of the glass? I always had a weird feeling that Pyrex felt weaker.

 

pyrex is extremeley strong, it has physical properties unlike normal glass.
in fact, the secret to making pyrex has been just that for quite a long time and still is, property of Corning USA and licensed to one other manufacturer in Germany I think.

Pyrex has much more tensile strength than normal glass. Tempered glasses can have varying tensile strengths and hardnesses. I know that pyrex has Boron in it (boro-silicate), which is maybe a heavy metal? I'm no chemist. Perhaps its the boron that makes it unsuitable for fish and plants, but its still suitable for cooking so maybe there's a more complex reaction.

from wikipedia:
"Boron is an essential plant nutrient, although soil concentrations of > 1.0 ppm can cause marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8 ppm can cause these same symptoms to appear in plants particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of boron in the soil, will show at least some symptoms of boron toxicity when boron in the soil is greater than 1.8 ppm. When Boron in the soil exceeds 2.0 ppm, few plants will perform well. Plants sensitive to boron in the soil may not survive. When boron levels in plant tissue exceed 200 ppm symptoms of boron toxicity are likely to appear. As an ultratrace mineral, boron is necessary for the optimal health of animals, though its physiological role in animals is poorly understood."

however, there is a kind of glass which is nowadays referred to as quartz, which is horribly misleading marketing. It maybe be made from melted down Quartz, but once quartz melts it's just sand or glass, good ol' silicon dioxide. However, as the name truly implies, quartz can easily be nigh on totally pure SiO2, and when glass is made from pure SiO2 it melts at an extremeley high temperature, is perfectly clear (blow your mind), and DENSE. DENSE AND HARD, HARD AND STRONG.
this kind of glass is so expensive to produce, however, it is rarely used for nonscientific purposes. It'd make a beautiful, heavy, expensive tank though.

 

So it must be cost prohibitive. Pyrex has been around a long time. I imagine the manufacturing patent has expired, but not the copyright of the name.

 

In the course of exploring options for a new aquarium, I asked about availability of Starphire glass from several glass vendors. More than one said things along the lines of, "We can't get Starphire, but we can get borosilicate, which is cheaper and even clearer."

I finally settled on an acrylic tank, but I did end up buying some Starphire for use as lens guards for my lighting. It does have a slight blue tint at the edges, but it's not nearly as blue as standard glass. However, I have quite a bit of Pyrex cookware, and I observed no tint whatsoever at the edges of any of it.

That's what lead me to wonder why there are so many aquarium builders who favor Starphire as an "ultra clear" option instead of borosilicate.

 

In the course of exploring options for a new aquarium, I asked about availability of Starphire glass from several glass vendors. More than one said things along the lines of, "We can't get Starphire, but we can get borosilicate, which is cheaper and even clearer."

My initial reaction is to assume borosilicate in large sheets is not uniform enough (like old windows) to provide a uniform viewing window. It should be good enough for the sides and back though.

 

My lily pipes are made from chemistry-grade, borosilicate glass. The stuff is tough as nails.

Interesting discussion...

 

Now if it is tougher/stronger glass, would that mean it would bow less and be a better choice for a custom rimless tank?

 

I would think the Pyrex is better than Star whatever, I've seen several starfire glass tanks, blah.................

You cannot tell.

But if that does make a difference, the pyrex sounds like the better choice from the vendor. I'd go with that if I were you and wanted something beyond normal glass.

I cannot think of reason why the starfire cost more than the pyrex though...........well, gouging and marketing perhaps...........

Regards,
Tom Barr

 

There's a product called Borofloat made by Schott. 1/4 inch thick is about $20 a sq ft.