Andrew Fildes wrote:
> Decent ones are good for over 100 in a case on a shelf. Strangely, it's the
> coating on the back that's important - once that scratches or breaks down and
> light can pass right through, they're cactus.
>
Minor, technical points. The bit about the label side is true only for
CDs. The outside, label layer is just paint. Scratching it alone
doesn't, in and of itself, hurt the data. The next layer down is the
reflective metal. If it's gold, breaks in the lacquer seal don't matter
much. With any other metal, the air and dampness let in oxidize the
layer, destroying its reflectivity.
Pens with aggressive solvents, like sharpies, and labels with the wrong
adhesives are also a danger to the lacquer layer. I suspect that such
factors are partly the reason for the very different experiences people
have had with CD longevity.
DVDs are a different design, with two full plastic layers. The weak
point for leakage is where they are glued together around the rim.
That's why DVD cases have center posts designed to release without
bending when pushed. Bending that won't hurt a CD may break the edge
seal of a DVD.
In both cases, it's not just the gold reflective layer that gives the
longevity. The second major component in computer writable CDs and DVDs
is the dye that changes color when the laser hits it. The only dye so
far known that doesn't fade/deteriorate relatively rapidly is patented
by Mitsui. I think all name brand gold disks use the Mutsui dyes, but
there's nothing to stop someone from making gold disks with inferior
dyes to make a quick buck.
Moose
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