Disc rot

Disc rot is the tendency of CD or DVD or other optical discs to become unreadable because of physical or chemical deterioration. The causes include oxidation of the reflective layer, physical scuffing and abrasion of disc, reactions with contaminants, ultra-violet light damage, and de-bonding of the adhesive used to adhere the layers of the disc together.
Causes.
In CDs, the reflective layer is immediately beneath a thin protective layer of lacquer, and is also exposed at the edge of the disc. The lacquer protecting the edge of an optical disc can usually be seen without magnification. It is rarely uniformly thick; thickness variations are usually visible. The reflective layer is typically aluminium, which reacts easily with several commonly encountered chemicals such as oxygen, sulfur, and certain ions carried by liquid water. In ordinary use, a surface layer of aluminium oxide is formed quickly when an aluminium surface is exposed to the atmosphere; it serves as passivation for the bulk aluminium with regard to many, but not all, contaminants. CD reflective layers are so thin that this passivation is less effective. In the case of CD-R and CD-RW media, the materials used in the reflecting layer are more complex than a simple aluminium layer, but also can present problems if contaminated. The thin 0.25-0.5mm layer of protective lacquer is equivalent.
DVDs have a different structure from CDs, using a plastic disc over the reflecting layer. This means that a scratch on either surface of a DVD is not as likely to reach the reflective layer and expose it to environmental contamination and perhaps to cause corrosion, perhaps progressive corrosion. Since disc rot is often caused by the corrosion of aluminum, this means that DVDs are more resistant to disc rot. Each type of optical disc thus has different susceptibility to contamination and corrosion of its reflecting layer; furthermore, the writable and re-writable versions of each optical disc type are somewhat different as well. Finally, discs made with gold as the reflecting layer are considerably less vulnerable to corrosion problems, though no less susceptible to physical damage to that layer. Because they are less expensive, the industry has adopted aluminium reflecting layers as the standard for factory pressed optical discs.
Blu-rays, used to distribute movies (often as merchandise) and games, usually use a silver alloy layer instead of aluminum.
Signs of disc rot.
On CDs, the rot becomes visually noticeable in two ways:
In audio CDs, the rot leads to scrambled or skipped audio or even the inability to play the disc.
Using surface error scanning to check the data integrity allows discovering loss of data integrity before uncorrectable errors occur.
Variants.
Laser rot.
Laser rot is the appearance of video and audio artifacts during the playback of LaserDiscs, and their progressive worsening over time. It is most commonly attributed to oxidation in the aluminum layers by poor quality adhesives used to bond the disc halves together. Poor adhesives separate over time, which allows oxygen in the air to corrode the thin aluminum layer into aluminum oxide, visible as transparent patches or small dots in the disc. Corrosion is possible due to the thinness of the layer; normally aluminum doesn't corrode because it is coated in a thin oxide layer that forms on contact with oxygen. Single-sided video discs did not appear to suffer from laser rot while double-sided discs did. The name "laser rot" is not a misnomer; although the disc degradation does not involve the player's laser, the "rot" refers to the laser disc itself.
Laser rot was indicated by the appearance of multi-colored speckles appearing in the video output of a laserdisc during playback. The speckles increased in volume and frequency as the disc continued to degrade. Much of the early production run of MCA DiscoVision Discs had severe laser rot. Also, in the 1990s, LaserDiscs manufactured by Sony's DADC plant in Terre Haute, Indiana, were plagued by laser rot.
HD-DVD Rot.
Many HD-DVDs, especially those produced by Warner Bros. between 2006 and 2008 developed disc rot not long after production. Disc rot was also more common on double-sided HD-DVDs than on single-sided HD-DVDs.