Let's take a look at how CD media are made and at what that means for their durability. There are darned few hard data available on many of these matters and even fewer reliable numbers, so apply a substantial fudge factor - and give me better numbers if you can find them. Please remember in what follows that the light used to read a CD is infrared and not visible to the human eye. What you see is not what the reader sees, so do not be misled. I am also neglecting here the whole business of the grooves pressed into all discs and containing both positioning information and code for the type of medium.
To begin with, we should examine a pressed disc. Essentially, it begins with a fairly thick layer of tough, clear plastic. The top surface of the plastic has been inscribed with pits and is covered with a very thin layer of metal. On top of that is a thin layer of that plastic, then the graphics which are silk-screened on the very top. Going back to that inscribed top of the bottom piece of plastic, it is usually created by pressing a metal stamper made from a glass master into the surface. Where the result is flat (parallel to the face of the disc), the reflection from the metallized layer is strong; where there is a slant to the plastic, the illumination is scattered and there is no significant return signal. Thus, the maximum brightness is high and the minimum is quite low.
An erasable disc (CD-RW) is similar in some ways to the pressed disc. The top and bottom layers are the same, but the plastic is smooth. The big difference is that instead of a metallized layer which simply reflects the light uniformly, the erasable uses an alloy which changes state when illuminated strongly. In the crystalline state, the alloy reflects light quite well. But it can also be switched to an amorphous state in which its reflectance is substantially lower. The frequency of illumination for reliable reading of an erasable is different from that for a pressed disc or for a CD-R, so special hardware is needed in a reader to allow it to get the signal back. It is also significant that the change of state is not completely reversible. After a number of cycles, the alloy becomes stubborn at a spot and insists on being either crystalline or amorphous, ignoring all the urging that the writing laser may offer. As a result, the disc will gradually develop errors over repeated erasures; eventually, they will be too numerous for error correction to deal with and the disc will be a coaster.
A write-once disc (CD-R) uses yet another arrangement. The layers are similar: thick plastic, smooth surface, metallized reflecting layer, thin plastic over the top. The essential difference is that the thick plastic layer has a dye in it which changes color when illuminated by the writing laser. Before illumination, it may be relatively opaque to the reading frequency; after illumination, it will lighten. The dye is deliberately unstable (otherwise, it would not change transparency) in two different senses. One is that it is able to be written - to respond to the writing laser by changing color - only for about five years; the other is that over time and depending on handling, the dye can fade. When working right, the dye absorbs some of the light when it would ideally be clear and allows some through when we want it to be opaque, so it does not offer the contrast of a pressed disc.
The top plastic layer on a pressed disc is pretty sturdy - not as durable as the thicker bottom layer, but pretty good especially when it has a nice coat of silk-screen ink on it. An erasable is usually pretty good in this respect as well; not as resistant to mistreatment as a pressed disc, but a lot better than a typical CD-R. A write-once blank may have a durable top surface added or not. A durable surface, like one advertised for 'long life' or used to support ink-jet printing, is still much more vulnerable to scratches than is the top of a pressed disc, but the least expensive blanks, with no additional protection or surface printing, are the most fragile of all.
In general, if you want to write onto an uncoated disc, you should use a pen made for the purpose. TDK has one available for about $3. Many uncoated discs will take writing from the felt-tip (not the metal-sheathed ultra-fine) Sharpie, but there is a slight risk that the solvent in the Sharpie's ink can etch some plastics which may be used. Other pens may well be safe, but why not hedge your bets and either find out from your medium's manufacturer or stick with something made for the purpose? As for pencils and ballpoints: you might as well use a dentist's drill or a sandblaster.
Another option is to apply an adhesive-coated paper label such as are available from Avery, Neato, Stomper and others. That's a fine solution, but there are some risks. Obviously, the label needs to be pretty well centered to avoid problems in high-speed readers. Also obviously, you don't want loose adhesive to foul up your reader or its optics. However, the biggest problem is the adhesive used. It must not let go. If it does, the label will peel away in part, snag in the drive and potentially peel some of the adjacent plastic layer. And that is disaster. If you scratch the thick layer moderately, there will probably be no effect at all. But an uncoated disc has a remarkably thin upper layer and even a tiny scratch or hole will disrupt the metal layer below; a disturbed reflecting layer means no reflections, which means no data - dead disc.
Okay, now you're prepared to handle the disc with due respect for the ultrathin lacquer layer on top. How long will the disc last if you don't sandblast it? If it's a pressed disc, the answer is likely to be 'forever'. That is, there is no decay mechanism known for the CD's sandwich of plastic and metal if it is made well. We may learn better some day, but for now when we find a CD that has gone bad, we suspect that it was made badly, that something led to oxygen reaching the metallizing layer or some extraordinary event - like a fission bomb - turned the plastic opaque.
Write-once media are another story and one which is less understood. Remember, that dye which holds the information is unstable. Time alone will cause the dye to fade or to grow opaque. Either way, it will mask the data. From accelerated tests, manfuacturers will claim life expectancy of 100 or 200 years, but they don't know. On top of that, the real life depends on how you store the disc. Intense light - sunlight or most other forms - will change the dye. After all, the only difference between the sun and the writing laser is that the laser concentrates its light much more finely. How much exposure does how much damage is ... unspecified. Even worse is uncertainty about storage temperature. Life tests are accelerated by holding the disc at a higher temperature than normal; no one seems to be willing to speculate on what happens to that 100-200 year estimate if the disc is not held at 20 C (or whatever).
The erasable disc's decay is probably something like that of the write-once. There have been reports that intense illumination (in one case, with an ultraviolet PROM eraser) will erase an RW and that seems quite likely. My guess is that cosmic rays will also lock the alloy in one state, but I have found no data from manufacturers or others on the matter. But the real killer is that 1,000-cycle estimated life. Well, I certainly don't expect to write and erase the blank a thousand times - come on, be sensible! Unfortunately, you may be doing more erasing than you think. In particular, if you are writing fixed-length packets, the area which holds the directory (think of it as a FAT, though it isn't) is written and rewritten each time you modify a file. It does not take a lot of scrubbing of that part of the disc to reach a thousand cycles to become unreliable. By now, you probably will not be surprised to know that we don't know how to assess that vulnerability, to predict how much use of a disc written in fixed-length packets will kill it.
De Beers claims that 'A diamond is forever.' But diamond is an unstable form of carbon under normal conditions and degrades from the time it is mined. A CD-R or CD-RW treated well may last effectively 'forever', too - as long as we will care about it (though not as long as a diamond). Mishandled, 'forever' for a CD-R may be measured in minutes.
E-mail me at cdrecording@mrichter.com
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