I trust that it is not news to anyone that CDs of any flavor can be damaged. Scratches, chips and dirt on the bottom layer lead to misreading and in the worst cases failure. Total failure usually comes from the top side, though, where anything which gets through to the metal layer is likely to turn the CD into a small and leaky coaster. Whatever is put on the top of the disc has a chance of seeping through the protective layer (which may be only a thin wash of acrylic lacquer) to damage the metal and demolish the data.
Such disaster aside, pressed discs are pretty nearly invulnerable to degradation. Handled with reasonable care, they will outlast the users and probably the technology unless they were defective in manufacture. Erasables ought to be similar - but they're not. For whatever reasons (and I've found nothing but anecdotal documentation), they seem to have a tendency to grow forgetful over time. Without systematic assessment and tests against specifications, the only safe course is to reserve erasable media for testing and for short-term use. From here on, then, we'll talk about write-once media, a.k.a., CD-R.
A CD-R consists of several layers with different 'responsibilities'. We begin at the bottom with clear polycarbonate to provide spacing and refraction for the illumination. (Note that "clear" applies to the wavelength - color - of interest and not necessarily to the appearance to the human eye. Pigment can be in that layer to provide a colorful disc which is still essentially clear to the lasers used. In all of what follows, optical properties are those for the dye not the eye.) Next comes a layer with a photosensitive dye. Above that is a metal reflector and on top of that is an acrylic lacquer topped by any paint, printing or label which may be used. Of all of those, the one of interest for life of the disc is the layer with the dye.
There are two operation points of interest for the dye. At one intensity and wavelength, the transmissivity - transparency - of the dye can be read. At another point, it can be changed. That means that a write laser can hit a spot of dye at one wavelength and intensity to make it dark to a wavelength and intensity used by a read laser. The process is not reversible in the usual sense; you can't undo writing, just as you cannot unexpose film once light has struck it. In fact, there is a useful analogy with old photographic film, the sort which was sensitive only to blue light. Once it was exposed, it could be processed under another color - usually red - without damage. That's still true for monochrome photographic paper which is processed in a darkroom illuminated in colors to which it is insensitive. So you may think of the dye in a CD-R as sensitive only to one color but visible (readable) in another.
Staying with the analogy, we know from experience that over a long enough period, a photo will fade. Properly processed, stored and handled, that takes decades or centuries. The same is believed to be true for CD-R. Once written, the information will remain for decades when handled properly. However, mistreatment will damage any such medium. In the case of CD-R, that means exposure to intense light or to an environment which leaks corrosive material to the metal. It is not easy to see the damage done by ordinary light on some media, but if you have an old CD-R with an azo (blue) dye, you can try a simple test. Lay the disc in bright sunlight, blue (or green) side up. Block a portion of the disc with something opaque. Give it some hours of exposure, bring it into ordinary light and notice that the unblocked area has been bleached to a lighter shade. While the blueness itself does not indicate sensitivity, its fading means that the dye has degraded and the former ones and zeroes are no longer readily distinguished from one another.
In normal operation (not intense light), the time for a CD-R to degrade is supposed to be a century or more. That assessment comes from "accelerated life test" where unfavorable conditions for a short time are extrapolated to normal conditions for a longer time. No doubt, there is validity to those tests and manufacturers have no better way to determine how long information will last, but the multiplier from days of test to years of use is far from proved. Still, as long as we expect the disc to remember what was written for decades, that's probably good enough.
Photographic film has a second sort of lifetime as indicated by its expiration date. Over time and as a function of storage, an unexposed area will acquire some exposure, if only due to cosmic rays. The silver halide also changes sensitivity to exposure so the speed rating will vary. In all, after a while it becomes less able to record the exposure. The same sort of thing happens to the dye in a CD-R. After a few years on the shelf, the dye breaks down, gets 'tired', and is less able to record a clear signal when exposed to the write laser. That can be corrected to some extent by calibration at the start of writing, but gray is still gray and age will reduce the quality of a recording. (See the page on "It's not all Ones and Zeroes".)
These effects of aging are not simple go/no-go distinctions. For each type of aging, the difference is an increase in error rate, not complete inability to record anything or to playback anything. Audio becomes dull as error concealment kicks in; data reading becomes slower. Eventually, good information can no longer be recovered - a CRC fails or a track will not advance. Nothing can be done to stop or to reverse the ravages of time. Preventive measures are needed if your information is to last on CD-R.
Step one is to buy enough good media to use up in reasonable time. With the demand on media today, what you buy is likely to be no more than a few months old. My rule of thumb is to use blanks within three years of purchase, though usually I run out much sooner than that. I have been able to measure degradation in a very good line of media (Mitsui silver) over four years and reluctantly trashed my last hundred blanks when they exceeded my threshhold for quality of recording. I have some slow media (Kodak 4x) that are even older and still write acceptably; when they give up, I'll have no way to record for an old laptop and will probably have to retire it.
Step two is to copy an archived disc before it fails. This does not apply to ordinary home use, where the lifetime of a recorded disc will appear infinite to those with normal human life expectancy. However, if you have a vital record on CD-R, it is wise to make at least two copies on different media and to check them every few years. When one begins to show correctable errors, use the clean copy to make some fresh ones. That's overkill for most purposes, but you may have some cases where it's worth the effort just to be sure that that ambertype of your great-grandparents survives to be seen by your great-grandchildren.
E-mail me at cdrecording@mrichter.com
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