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Re: Aquatic Plants Digest V4 #195


>>Wright Huntley wrote:

>.....Back to the redeposited glass... When the glass surface is rough, cerium
>oxide or rouge are not effective agents for fixing it. You need to start
>with a grinding compound (silicon carbide or diamond), and a tool that
>doesn't trap it (cast iron or glass). Grind the surface flat and smooth,
>first. Gradually wash off and reduce the size of the grinding compound
>particles to get a smoother and smoother surface. Go down to at least as
>small as 5 microns for the final grind particle size. Then switch to a tool
>that *does* trap the cerium oxide or jewelers rouge (hard wax or pellon pad)
>for the polish stage. Glass is too cheap for this much fuss (apologies to
>starving students, of course).
>

Well, I am a starving professor, and can't afford to do that, even though
it sounds like the only way to do it if I had to polish off the deposited
glass.  I think I will try a coating with spray on acrylic.  Hopefully,
there won't be anything toxic in the acrylic.

Bob Dixon wrote:
>...........but how come I can get
>the stuff off the underside of my cover?  How come the rest of us aren't
>experiencing this "redeposited glass" thing?  What's so different about
>Mississippi water that you are seeing this and I'm not?
>
>The carbonate buildup in a pot that is used to boil water is not soft, it is
>rock hard.  Maybe your lights are baking the stuff on?  I'd still try the
>CLR, and if it comes off, it is carbonates.
>
It is not a difference in the water.  It is probably a difference on how
much air circulates under the cover.  I have covers that cover the entire
tank with no air circulation.  The lights sit on the cover and heat up the
top layer of the water.  When the lights go off, the cover cools, and there
is extensive condensation on the cover from the warm water, below.  When
the lights go on in the morning, this condensed water dries up.  Repeating
this cycle daily builds up in several months a noticable silica deposit.  I
would think that you may have more air circulating under your covers, so
that you don't get very much condensation on them.


The same sort of fogging of glass due to dissolving and redepositing can
occur in the double windows that the newer houses are equipped with if
these windows develop a leak and get some water inside.  As the temperature
cycles during each day, there is condensation and then evaporation, and
after a year or less, the glass is badly fogged by the etching and
redepositing.  I have seen windows in this condition when I was
house-hunting, and the house we finally purchased had two fogged windows
that we required the seller to replace.

The deposit really is silicon dioxide, and silicon dioxide is slightly
soluble in water.  It can be removed from silicates in the presence of
carbon dioxide or bicarbonates.  My limnology text says that natural waters
can contain "milligrams to centigrams" of silicic acid.

I have also had calcium carbonate deposits on my tank covers, and they are
easy to remove.  Compared to the silica deposits, they are not hard.

Paul Krombholz, in Central Mississippi, where we are getting
drought-busting rains, starting yesterday.  At least 2 inches, so far.  The
frogs love it!