How Long Will CD-Rs Last?
Rewritable CD-ROMs (CD-Rs) avoid many of tape's problems. For one thing, they're spectacularly cheap, costing as little as 15 cents to 25 cents each in consumer quantities, and less if you buy in very large quantities. They're relatively immune to magnetic problems, fare well at normal room temperatures, humidity, and dust levels, and require no special handling. Once recorded, they can be played back on almost any PC because, unlike tape drives, almost all PCs have CD drives.

CD-R backup software, using on-the-fly data compression, can easily fit 1.2 to 1.5 Gbytes on a standard CD-R, and sometimes effectively much more if the software is intelligent enough not to copy things like the Windows swap file. Writable DVDs will offer relief for these size limits in the not-too-distant future, but for now either "disk spanning" techniques (that allow a backup to occupy more than one CD, seamlessly) or a priori data management (to segregate valuable data files from easy-to-reproduce system files) make CD-Rs extremely attractive as a backup medium.

But what's their life expectancy? To answer that, we need to delve into the guts of a typical CD-R.

A CD-R has several parts. The bulkiest item is a clear plastic disk, usually made of some kind of polycarbonate and etched on one side with a spiral "pregroove" that later helps position and guide the laser. (It's the fine grooves that act as a diffraction grating, causing a CD-R to reflect light in a rainbow spectrum.)

The clear polycarbonate is quite tough. Barring temperatures high enough to melt plastic, extremely rough handling, or sandpaper-like levels of environmental abrasion, the core of a CD-R will last a very long time.

CD-Rs are reflective, due to a layer of metal foil glued to one side of the polycarbonate. In more expensive CD-Rs, the mirror-like foil is covered by a protective paper or plastic label. In budget-quality CDs, the foil itself is the upper surface of the CD. The foil is quite inert, and will usually last almost indefinitely, as long as it's not subjected to extreme mechanical damage (scratching with a sharp implement, for example). The metallic foil can come in different colors, with silver--actually, aluminum--and gold the most common.

The working part of a CD-R is a layer of dye sandwiched between the upper surface of the polycarbonate and the bottom of the reflective foil: The CD-R drive's laser beam is focused through the clear body of the CD onto the dye layer: When the beam is on, it actually melts a small hole or pit into the thin layer of dye on top of the polycarbonate substrate. The laser cycles on and off very rapidly (determined by the ones and zeroes of digital data), and the whole laser assembly tracks along the polycarbonate pregrooves as the CD-R spins. The resulting spiral track of melted and unmelted dye areas represents the ones and zeros of the encoded data, which then may be read back by any CD using a lower-power "read only" laser that does not heat or melt the dye layer.

Once recorded, CD-Rs may seem inert and long-lived. Compared to tapes and floppies, they are indeed Methuselahs. But CD-Rs still do have a finite life, mostly based on the type of dye used in the recording layer. Although CD-R technology is still new enough so that no one really knows with absolute precision the "natural" shelf life of CD-Rs, some dyes are more stable than others, and it's possible to generate broad estimates based on accelerated aging tests and educated guesswork.