 The reflective layer is typically aluminium, which reacts easily with several commonly encountered chemicals such as oxygen, sulfur, and certain ions carried by condensed water.In ordinary use, a surface layer of aluminium oxide is formed very 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.