Date: Mon, 15 Apr 2002 08:08:44 -0400
From: Steve Smith <sgs at aginc.net>
To: WSFA members <WSFAlist at keithlynch.net>
Subject: [WSFA] CDs [was: Netiquette]
Reply-To: WSFA members <WSFAlist at keithlynch.net>

"Keith F. Lynch" wrote:
>
> 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.

The failure mechanism I heard about, 'way back when, was oxygen
diffusing through the plastic of the disk and oxidizing the aluminum
coating.  I'm not aware of this actually happening to anybody, however.
I can't find any real data on the net right offhand; most sites give a
lifetime of regular CDs of 30 - 100 years with no real data.

>From an archive standpoint, it would seem that it would be possible to
remove the aluminum/aluminum oxide coating and recoat the disk with new
aluminum.  The actual plastic disk should last essentially forever.  THe
disks probably will become brittle as they age, but so does papyrus.

> > (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.

Surprised me too.  From a Kodak paper on their own media:
(http://www.cd-info.com/CDIC/Technology/CD-R/Media/Kodak.html)

<quote>
That model predicts (at the 95% confidence level) that 95% of properly
recorded discs stored at the recommended dark storage condition (25°C,
40% RH) will have a lifetime of greater than 217 years.
</quote>

Other companies have different technologies.

--
Steve Smith                                           sgs at aginc.net
Agincourt Computing                            http://www.aginc.net
"Truth is stranger than fiction because fiction has to make sense."