Re: Glass for Tank Top

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

Rachel Sandage asked:

> . . .I'm tinking about building a new
> hood for my 72 
> bowfront, and I will need new glass for it. Is there any
> particular type 
> which you'd recommend? It will be sitting a few inches
> under 5 55W PCF 
> lights. Are there some kinds which withstand the heat
> better? How about 
> transparency? Does anybody make glass with
> anti-reflective coatings?

The answers to all your questions are yes.  How many days
in an average week does that happen?

I asked these very questions myself.  In fact I find that
Rachel's questions on APD, in general, tend to be ones that
I have asked, or ones that I *should have asked*.  Thanks,
Rachel.

About glass tops.  Short answer: Unless money is no object,
and I mean unless you really don't care at all, then plain
ordinary plate glass is what you want.  (or is that plane
ordinary plate glass?)  

Long answer:  Plain plate glass is also called "soda glass"
-- not because you can make tumblers out of it but because
soda is added to the silica to lower the melting point --
It's over 800 degrees otherwise!  Soda is one of the things
that makes the glass greenish and reduces its
transmissivity.

You can get glass with a higher than average transmissivity
-- it's not as green when you look at the edge and it lets
more light through without absorbing or reflecting as much
back.  Such glass tends to be very expensive, the raw
materials need to be more highly refined (e.g., iron
content must be reduced) and some expensive compounds need
to be added to the glass when it's made.

You can get heat resistant glass.  Well actually glass that
withstands changes in temps and and differences in temp
(hot here cold there) since most glass doesn't melt until
it reaches hudnreds of degrees.  But special heat tolerant
glass (borosilicate or pyrex, etc.) tends to be *very*
expensive and has a strong tint if the glass has much
thickness.  It can be had without very little tint but
that's even more expensive.  The most heat resitant glass
is pure or nearly pure silica glass -- it melts at
something over 850 degrees F.  The space shuttle outer
window panes are made of this.  They can afford it!

Feeling fed up with glass and all the tint, heat sensitive,
and cost issues?   Well, acrylic has none of the heat
sensitive or tint problems .  No glass has as high a
transmissitiy as good acrylic but acrylic absorbs water --
not a lot but just enough so that it will warp if both
sides are evenly exposed to water/mositure -- like when
placed over water (polycarbonate too).  You can reduce or
prevent warping by using a really thick piece  (3/4"),
apply ribbing, or having other reinforcement, like cemented
box sides.  This is all rather cumbersome and, anyway, more
expensive that some of the fancy glass.

Special coatings won't be kind to your pocketbook and will
be hard to find for such a small order.  Anyway, most plate
glass coatings are done to keep out ranges of sunlight. 
Antireflective coatings that don't shaprly decrease
transmissivity are more often used for optical lenses and
would probably cost more than you aquarium. ("Hi. I'd like
to order two lenses, without a corrective prescription but
I want them coated and I want them *THIS* big!")

You can get ordinary glass.  It will hold up even with the
lights lying right on it, has pretty good transmissivity,
and is very inexpensive.  The effect on the amount of light
transmitted will be less than the variances from one bulb
manufacturer to another, or probably, the ambient temps.

If you go to a glass dealer and say you want glass for the
top of your aquarium and he or she tells you you need
special glass, find another dealer.

It's hard to give exact measurements with a bow front.  You
might consider making a template out of cardboard and
taking that to the glass shop.

You don't need special cardboard ;-).

Scott H.

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