T3D Re: CD ROM Lifespan

[lunazzi <lunazzi@xxxxxxxxxxxxxx>]

tech-3d@xxxxxxxxxxxxxxxxx wrote on CD ROM Lifespan:

Just because the subject is interesting, let me try to put and get some more
information.

I have a natural tendency to compare CD's with holograms, because in both cases
you have the same degree of high resolution embossing and a metallic coating.

In the earlier times when CD technique  was searched, Phillips company tested a
technique of registering CD's in a holographic way, such that a local scratch
could be recovered by the information on the whole plate.
The digital technique  was finally elected, and somebody told me that for
audio, the registering exists many times in delayed sequence in the disk, such
that if the system lose their sequence, can soon recuperate it in a side band.

Well, but my main question is:  Which is the material employed for the coating?

For the gold colored disks, is it gold?
For the silvered colored disks, is it silver?

Although silver is not expensive now a days, as it was in the past, mainly
because the photographic industry does not demands it any longer (do you
confirm this information I get on Mexico?), silver oxidizes more than aluminum.

For holograms, aluminum is employed.  Its oxidation does not come appearent,
does not destroy the surface.  We use first surface mirrors in holography made
of aluminum, for many years, while silvered mirrors would last just days if
uncovered.

I was surprised to see holograms losing its reflectity, becoming transparents
in irregular paths, closely resembling that of fungus. These two holograms I
saw in two opportunities, maybe were exposed to humidity, I never saw that in
well stored holograms.
Maybe you think fungus would not attack inorganic materials as aluminum, the
fact is that they (at least the brazilian ones) attack glass very actively,
digging paths on it.

Would degradation of CDs be caused by fungus?  Where could we find technical
information on this important subject?

José

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