That may have been true in the old days, but isn't that case any longer.

Sputtered coatings are just too difficult to handle in the window-manufacturing process because
there is no true bond between the coating and the glass, so the coating can be scratched or rubbed
off too easily. Basically, an electron beam is aimed at a disc of the material to be applied to the
glass. As the atoms and molecules are sufficiently energized, the "sputter" off of the surface
(vaporize) of the source, and deposit like soot on the surface of the room temperature glass.

Essentially all of the Low-E glass made now is the pyrolytically-deposited "hard coat." As it is
applied when the glass is still quite hot, early in the forming process, the organic chemical
precursors react with the glass to form a chemically bound (Si - O - Sn), ceramic oxide coating that
is almost impossible to remove (we've had to do it in the lab, and it's a pain...).

The so-called "oil slick appearance" was due to varying thickness in the coating, leading to the
formation of interference colors - again, no longer a problem due to advances in the coating
application equipment.

The application surface is selected by what the climate you will be placing the window in. For the
most part, they are used in places where you are trying to trap heat in the home. Basically, the
coatings are transparent to short wavelength infrared (the rays that heat you and objects up), but
reflect long-wave IR, the heat generated. So, sunshine comes in, warms the floor and furniture, and
as the heat attempts to be reradiated out through the window, the interpane gap, often filled with
argon (a poor thermal conductor), stops some of the rays and the oxide layer on the second surface
(as counting from the outside) does the rest.

If the climate is very warm, the coating is on the third surface so that the
low-thermally-conductive gap is before the film, reducing the heat entering the home. That is not a
common application, as the savings don't seem to warrant the added cost. Folks with second-surface
coatings do get some benefit in the heat of the summer, but less so, as there is no gap before the
coating...

Incidentally, every glass container you've had your hands on in the last 35 - 40 years also has a
(much thinner) tin oxide coating on it (that's where we started the low-e development idea), but for
protection of the glass during forming and subsequent handling, not anything energy related.

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Books, Artwork, and Lots of Free Info!

.. . . . . . . . . . .
"TRAINMAN9" wrote in message
...
Actually, the amount and specific wavelengths of light blocked by Low-E
coatings depends on the
chemistry of the coating, it's thickness, and to a lesser degree, which
surface it coated.

Most of the modern spudder coat Low E is on the number two surface and is made
up of two layers of silver with several layers of oxides to make it possible to
see through the glass.

Hard coat Low E glass used tin based coatings that were fused when the float
glass was made. These coatings are typically on the number three surface. They
do not perform as well as the spudder coated products, have an oil slick
appearance and in most cases do not meet the new Energy Star requirements for
shading co-efficient or solar heat gain. They typically only block about 55% of
the UV.