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Re: Deposits on glass

    * From: "Greger Lindstrand" <greglind at algonet_se>

...........Deposits on glass
I got white deposits on the coverglass on top of the aquarium. I've tried
to remove it with vinegar essence which didn't work.
So I tried concentrated phosphoric acid (Acidum Phosphoricum Concentratum),
really powerful stuff which didn't work either.
I'm puzzled. I was under the impression that those deposits on the
underside of the cover glass was calcium and would be easily removed.
Well, here I stand without the slightest hunch on what to do next. I could
go get new glass, but that would soon have the same deposits, so that would
not be of much use.
It is quite useless for me to install new reflectors and tubes if I can't
get rid of this light obstruction.
Advice is needed and I turn to the list members for help.
What is it?
How do I remove it?........................

I know these well, and I have turned to this list for help in the past,
getting a lot of responses.  It took me a while to convince people that
these deposits are not calcium.  What they are is silica deposits that come
from water condensing on the bottom of the glass when the light is off and
then drying up when the light is on and the glass is heated.  Each time the
water condenses on the glass it dissolves some of the silica, and each time
it dries up, it redeposits it.  After a year or so of this process
happening every day, you get very clouded glass.  You can probably see
round spots on the glass that correspond to the locations of the drops that
form there every night.

Several people on the list suggested use of cerium oxide powder, which is
used in polishing glass.  I got some, but the powder seems to be too fine
to make much impression with just elbow grease and a scrubbing pad.  About
the best results I have had is to use coarse steel wool and cleanser.  The
steel wool actually does rub off the little glass "stalagtites" that form.
I havn't yet tried the steel wool with the cerium oxide powder, but will
next time.  Maybe that will turn out to be better than the steel wool plus
cleanser.  I have some glass plates that are definitely less cloudy after a
thorough scrubbing with the steel wool and cleanser.  It is the first time
I have been able to make any impression on the deposit, so, I am

Paul Krombholz, in cool central Mississippi, with partly cloudy skies.