To: WSFAlist at keithlynch.net
Date: Sun, 14 Apr 2002 01:08:03 -0400
Subject: [WSFA] CDs, CD-Rs, and CD-RWs
From: ronkean at juno.com
Reply-To: WSFA members <WSFAlist at keithlynch.net>

On Sat, 13 Apr 2002 23:45:20 -0400 (EDT) "Keith F. Lynch"
<kfl at keithlynch.net> writes:

> Steve Smith <sgs at aginc.net> wrote:
> > and I've heard that the "expected" lifetime of a normal (aluminum
> > coated) CD is only about 10 years.
>

> Definitely not.  Like Ted, I have lots of 18 year old CDs, and
> they're all still good.  Even though I keep my apartment much warmer
than
> most, which is likely to accelerate any deterioration.  Except for
> CDs with a blatant manufacturing defect (e.g. "bronzing"), I expect
> them to last for centuries, since they're basically just bent metal.
>

I have also heard that the expected life of a CD is ten years, but I
think what is meant by that is that a CD may be expected to last at least
10 years in normal use, with normal storage and handling precautions,
while admitting the possibility that they might last very much longer
under those same conditions.  In other words, the ten year life
prediction is a conservative guideline for archival planning purposes,
much like saying floppy disks have an expected life of a year or two.  In
reality, floppy disks seem to be lasting 10 or 20 years (so far), if
seldom used, long enough that they have usually become obsolete long
before they lose data.

> > (CD-R recordable CDs have a much longer expected life -- 100-150
> > years.)
>
> Are you sure you haven't got those reversed?  CD-Rs use a chemical
> process, which might fade like old photographs.
> --

CD-RWs, which are recordable, erasable and re-writable, are apparently
able to be erased and re-written by strong IR laser radiation.
Therefore, I would expect those disks to deteriorate (albeit slowly) when
exposed to ordinary artificial or natural light, which includes IR, and
perhaps to slowly degrade even when stored at room temperature in the
dark, by the same token.  Storing them at very low temperatures would
invite stressing the plastic, or delaminating the sensitive coating.  So
I would think CD-RWs would be a poor choice for long term archiving.

But I don't see why the expected life of CDs should be any different than
that of CD-Rs, because both types seem to consist of thin aluminum foil
laminated on a plastic disk.  As an experiment, I bent a CD-R between my
heel and the floor, intending to break it and delaminate the foil.  The
disk bent nearly to a 'U' shape before breaking into several pieces,
including some thin slivers.  After the disk broke, some of the foil
delaminated, and most of the rest could be peeled away.  To all
appearances, the CD-R was basically just a plastic disk with laminated
aluminum foil so thin it is translucent, with an additional thin layer of
plastic on top of the aluminum to accept the ink markings.

I have heard a report that CDs have been attacked by fungus in tropical
environments, with the fungus getting between the aluminum foil and the
plastic disk.

Ron Kean

.

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