Physical degradation of CD's

Many years ago, when I was a contributing editor to various mainstream tech publications, I did a large feature on disc rot. Spoke at length with engineers, large manufacturing & fab facilities, the whole nine yards. At this point, I don’t remember where it was published, maybe PC Magazine.

The bottom line is that "CD rot" today is pretty friggin’ unlikely. There were reports that the first generation of CDs were subject to this problem, and it was eventually traced back to a single disc-fab plant, I think in Europe, that was a source for discs worldwide. The issue was an improper seal that allowed air to contact the aluminum layer, causing black oxidation. This, of course, changed the reflectivity of the pits & lands, making parts of the disc unreadable. Most people I spoke with believed that this happened only when the disc was flexed or otherwise abused to the point of breaking the original seal. My conclusion was that this was an exceedingly rare occurrence, very much overplayed in the press.

Note that I’m talking about replicated discs. Home-recorded recordable CDs, for some reason, are much more susceptible to this problem. But again, it seems as though the issue was mostly with earlier generations of recordable media and, even in worst case, it would take years to manifest. Much more common than silver-disc "rot", but still pretty rare. My personal opinion (and it's just an opinion, not something I can verify) is that the greater number of problems with recordable CDs has more to do with the fact that they use potentially degradable photoreactive inks.  But at this point, who knows?

This shouldn’t be confused with "DVD rot," which was technically similar, but much more common. DVDs are composed of multiple layers of plastic glued together, and are thicker. In terms of physical structure, they’re more susceptible to damage by flexing. This problem was exacerbated by early DVD jewel cases, which made it more likely that unthinking users would flex the heck out of a disc while trying to pry it out of the case.

Again, this was a problem related to early manufacturing issues, that were later worked out. And although more common than "CD rot", which borders on urban legend, it was still pretty rare.

I have examples of "rotted" discs still sitting on a shelf somewhere. If I get the chance, I’ll take some pictures & post them here so you can see what "rot" looks like.

There are a lot of reasons why CDs fail to play correctly.

A case of "CD rot" -- i.e., oxidation of the aluminum layer -- is very, very rare in all but the earliest replicated discs. It can be identified with the naked eye as black layers of oxidation that slowly spread across the metal layer.  The change in reflectivity is what makes the oxidized areas unreadable.

The cause of this is leakage of air onto the surface of that thin layer, usually through physical damage (like extreme flexing) or because of manufacturing QA failures.

If you don’t see black aluminum oxide patches on the reflective surface of a misbehaving disc, there’s a different reason why the CD is not playing correctly.

Much of this, btw, doesn’t apply to recordable CDs -- which use far more sensitive dyes -- and to early DVDs, which are more susceptable to oxidation because of their multi-layered physical construction. It also obviously does not apply to gold CDs, since oxidized gold has the same color and reflectivity as elemental gold.

I can't speak authoritatively to the point, but I'd expect that, barring physical damage, oxidation "rot" is unlikely on any disc manufactured in the last decade or two.