But each metal is different. Aluminum is used as the metal reflective layer in CD-ROM discs because of its ease of application and low cost. Dyes used as CD-R recording layers are very corrosive, and rapidly destroy aluminum. Gold is an inert noble metal that is not corroded by organic dyes, and therefore was used exclusively in CD-R discs for many years. Because gold is expensive, even very thin gold layers become costly as CD-R prices rapidly decline.
Silver is less expensive than gold, and has equal or better optical properties.
At present, the thickness of the silver layer is not a significant cost factor, although this may change in the future. Thick silver may actually be superior to thin gold because of its constant reflectivity. Small variations in thickness of a thick coating have minimal effect on the reflectance, while the same variations significantly modify the optical properties of a thin coating. All metal layers have thickness variations, but reflectance from thick layers is not affected because the penetration depth of light is only a fraction of the thickness. Thin layers also have thickness variations, but these are now comparable to penetration depth and significantly modify reflectivity.