Re: Glass for Tank Top -- cracking up over water

["S. Hieber" <shieber at yahoo_com>]

Neal De Pape said, in part:

> The only statement that Scott made that I would call
> into question is this one:
> 
> > You can get ordinary glass.  It will hold up even 
> > with the lights lying right on it...
> 
> It is true that the heat from even PC bulbs will not
> melt, distort or break normal glass.  However, what
> might break it is PC bulbs right on the glass and then
> a large splash of relatively much cooler water . . .  

> Rachel's setup will apparently have the lights
> suspended a few inches above the glass, so I would
> think that she wouldn't have any issues. . .


> But in my setup, with the bulbs only a 1/8" above the
> glass, normal glass didn't hold up.  But heat
> resistant glass, such as Pyrex, has for almost a year
> now and many splashes.  It was nearly 4x the price
> though, $70 for my 54 corner instead of only $18.

Neal is right, but there's a little more too it.  After
all, I questioned it too after a piece glass on one of my
tanks broke and I figured it was the hot lamps that caused
it.  But I happened to find a glass shop that was very
hesitant to sell me borosilicate glass.  The guy running
the shop kept saying, "How hot do the bulbs get?  So why do
want special glass?"

I gave in and haven't had a problem, although my lights
right thereafter weren't right on the glass.  They were two
75watt VHOs about a 1/4" away from the glass.  But it is
definitley true that the more localized the heat, or the
greater the temperature differential between two places on
a piece of glass, the more stress in the glass and so the
bigger the risk of flaw and fracture.  Since glass doesn't
form a crystal structure when it cools from a liquid (or so
I'm told, I've never peeked ;-)  ) the slightest flaw
continues all the way across the piece.  That's why it
cracks.  Any stress can set off a flaw, including
temperature variations.  Hold a portion of a piece of glass
over a flame and you're likely to get a crack -- not when
you heat it but when you cool it back down.

At nearly 4 times the price for borosilicate glass, you
probably still won't break even compared to risking four
good splashes resulting in cracks.  So buy 3 pieces of
plain glass instead of one piece of borosilicate?  Naw, I'd
buy one piece of plain glass and just put a small piece of
something under the bulb to hold it slightly off the glass
- and that only if I expected some pretty strong splashing.
 We're only talking about 120-150 degrees F at the hot spot
so it doesn't take much to greatly reduce the risks.  The
bulb will probably last a bit longer too, but of this I am
less certain. 

Scott H.

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