 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.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.