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, 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 which can cause corrosion. Since disc rot is often caused by the corrosion of aluminium, 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 chemical corrosion problems. Because they are less expensive, the industry has adopted aluminium reflecting layers as the standard for factory-pressed optical discs.
Blu-ray discs usually use a silver alloy layer instead of aluminium. In 2020, several
Warner Bros-distributed discs from 2005 to 2009 were affected with disc rot, due to a factory issue with the glue used to label the DVDs. The issue was unaddressed until 2025 when Warner Bros. offered replacements for defective discs, and similar copies of previous releases which were already
discontinued. ==Signs of disc rot==