Basically, all the CDs are constituted on a plastic surface, that can vary a lot from each manufacturer. This does not matter much to us, and it is necessary to know just if the kinds of plastic are harder or less hard, some more easily broken, others with characteristics that make it look opaque with time and etc. Fortunately, the great majority of manufacturers chooses good quality material, even because it does not make the production that much expensive. Even so, there are episodes of CDs left in the car that, with the excessive heat, give off corrosive gases coming from this plastic that damaged them, and the worst: damaged the CD Player's reading head.
The CDs that guarantee longer durability are usually made of better quality plastic and less susceptible to weather changes. Less quality plastic, (usually found in cheaper media) are naturally more vulnerable to thermal dilatation. So, the simple act of putting the CD in a drive that heats a lot can reduce its lifetime in months and can make it not possible to be read in other units.
On top of this plastic, on only one side, it is put a layer that is sensitive to determined wavelengths. It is the recording layer itself.
On the blue face CD, this layer is made from a special material called cyanine. This material is a metallic compound with special electromagnetic properties which composition is not revealed by the manufacturers. Because it is commercial technology, we would probably have access to its components only if we paid for it. Cyanine has a predominantly bluish color (cyan).
On the gold, silver and green face CDs, this layer is formed by a variation of the compound cyanine, called phythalocyanine that can be more or less bluish, according to the concentration of the material basic compounds. Some specialists stipulated that CDs with phythalocyanine should be necessarily cheaper, because the cyanine concentration is lower and those CDs would be designed to the low cost market, but this is not true. What happens is that some of the original cyanine's compounds have been substituted and others have been added to this new cyanine, for non revealed reasons, but it is connected to the durability and we will soon talk about it. The "low cost CDs" reputation created a culture that even nowadays can be easily detected, that states that the blue colored CDs are the best without any technical or scientific base.
Over this layer, it is applied the metallic reflective material. This layer can be made up of two different alloys: a silver alloy or a 24k gold alloy. Naturally, gold has more durability and less susceptibility to damages than silver.
The final color that results from the CD's recording face is determined by the reflective layer and its concentrations.
The blue media is made of cyanine in high concentration with a silver-based reflective layer. The gold media and silver media are made of a phythalocyanine layer over a layer of gold. So, that old thought that silver CDs are made of silver come to naught. Some models take a specific proportion of aluminum in this gold alloy, that helps generating the resulting final color. Practically every silver media is made with gold alloy layers and the industry has not contradicted this statement yet, which means that there is not silver media made with exclusively silver reflective layer on the market yet.
Specifically in the case of the green CD it is necessary to know that it can be both made of phythalocyanine and of cyanine. Always with the reflective layer in gold alloy.
