At 01:25 PM 4/17/99 -0400, you wrote:
>James Rogers wrote:
>> The biggest problem is deterioration of the media. The storage life of CD
>> media isn't even close to 50,000 years. Normal aluminum film CDs are only
>> good for about 100 years and recordable CD media is around 20 (although it
>> varies widely depending on the type). The longest lasting ones, gold film
>> CD masters, will last less than 1,000 years.
>What are the criteria for decay of the media?
Decay of CD media is usually due to chemical reactions between the different layers of the media or with the environment. Ionizing radiation is a primary culprit, both directly and indirectly.
The recordable CDs have organic/metallo-organic recording surfaces that are by their nature more susceptible to ionizing radiation, heat, and oxidation than the metal media. However, the rapidity of decay varies widely depending on which of the many types of recording surfaces is used, ranging from 10 to 75 years of useful life with most being around 20-25 years.
Normal production CDs have an aluminum recording surface. The aluminum slowly oxidizes, either by exposure to atmospheric oxygen (it leaks in along the exposed edges), or from very slow oxidation reactions with the surrounding media.
Gold film masters have a gold film recording surface that is essentially impervious to oxidation. Unfortunately, the polymer media layers surrounding it (polycarbonates et al) will degrade to the point of ruining the media over a few hundred years, mostly as a function of exposure to ionizing radiation.
It would be possible to design a read-only media that could last 100,000 years without degradation, but there is not enough of a market to justify the cost over current solutions. Most people are satisfied with media that will probably outlive them.
My knowledge of this comes from when I worked in the recording industry. This was and is a big deal, since a huge amount of money is invested in the information stored on mastering media, and a good master can maintain a commercial value of some type for a century or more.