Ever notice how your oldest CD mines a copy of Dire Straits Money for Nothing is starting to skip, no matter how well you may have taken care of it over the years? You check it over for scratches and smudges, find nothing, and still it wont play?
Welcome to the wonderful world of CD-rot, the dirty little secret that the manufacturers dont want you to know about. Back when they were introduced in the 1980s, CDs were heralded as indestructible no more worn out grooves on records, no more faded or broken magnetic tape in your cassettes. The CD was supposed to be the last format you would ever buy, with technology that was supposed to last for a hundred years if you took care of it.
It turns out that some of your CDs that can no longer play properly were simply manufactured poorly, allowing air to oxidize the thin aluminum layer that is sandwiched between a couple layers of lacquer. If you look closely on some disks you can actually see evidence of this in the form of black spots and tiny holes that let the light through.
Most manufacturers attribute CD-rot to the actions of the disks owner. If you scratch the protective layer by not putting a disk away properly by stacking them, then air will get in and oxidize the disk.
Damage can also occur if you are too aggressive when removing disks from their protective cases if you bend them you can damage the glue that holds the wafer together, once again letting air in.
Where it gets confusing is the fact that most people tend to protect the readable side of the disk rather than the label side. In fact, the lacquer is always thicker on the bottom. Most of the wear and tear occurs on the label side where the lacquer is more fragile.
Its not only CDs you need to worry about, either. The same basic flaws that allow your CDs to rot will also rot your CD-ROMs, CD-Rs, CD-RWs, DVDs, DVD-Rs, and other laser media. DVDs are slightly tougher because the layers are encased in stronger plastic, but its all relative.
Rewriteable CD and DVD disks also require special care and should not be used for long-term storage because they have a heat sensitive layer that decays faster than layers in disks that only let you write to them once.
The lack of certainty in storage has some government institutions and businesses worried about the safety of their records, as entire archives of information have been scanned and stored on disks over the past decade. They want to know exactly how long the disks will last and if they need to start making back-up copies of their back-ups.