HOW CD ROT HAPPENS
CD Rot occurs in some older disks that were manufactured in the technologies early days.
The aluminum layer that reflects the light of the player’s laser is separated from the CD label by a thin layer of lacquer. If the manufacturer applied the lacquer improperly, air can penetrate to oxidize the aluminum, eating it up much like iron rusts in air.
The problem being that when the discs were cut the aluminum layer was too close to the edge of the disc and not sealed properly from the environment, thus exposing it to oxidation. The indexing information of a CD is on the inside the disc, i.e. nearest the center, and that the discs are read from the center out. This explains why in discs that succumb to CD Rot, the last tracks on the discs are first affected, i.e. because they are on the outside edge of the disc and hence the first to be subjected to oxidation.
A second cause was that some labelling inks used in the silk-screening process were chemically active even after UV curing. This interfered with the reflective layer, again, causing readback problems.
So how can you tell whether one of your disks is infected with CD Rot? Firstly, the silver color on the “label” side of the CD will have started to change to a color variously described as bronze, copper, golden-brown or rusty-orange color. The discoloration doesn’t necessarily show up on the “playing” side of the CDs. This symptom happens on 100% of “rotting” discs, the worse the “rot” - the more pronounced the discoloration will be.
On playing the disc, there will be an inordinate amount of “static-like” background noise. The level of noise that can be heard rises and falls with the volume of the music on the disc. The louder the actual music, the more apparent the background noise will be. This symptom is not apparent at the outset, but eventually creeps in and gets worse and worse over time. This also seems to show up earliest on tracks towards the end of a disc rather than at the beginning.
Unofficial estimates put the number of affected discs at between one and 10 per cent, however the good news is that both the issues of ink and aluminium layers were solved for pressed CDs several years ago, and the CD manufacturers made changes to their manufacturing process and material selection. As a result, CD Rot should not be a common problem in the future.