A Dye To Die For -- Data Preservation Thoughts...

Or, Data Safety and CDs (and more), Revisited --

Several issues ago, we found that the seemingly ageless CD that an increasing number of us use to backup critical data, isn't, as we might have believed, quite as invulnerable as Superman. While the unusual problem described in that story had to do with a fungus consuming the reflective foil within a CD (rendering it unreadable), reader Carl Taylor points us to an excellent anatomy lesson on CD-Rs in the July 23 InformationWeek.com; it helps us to understand what makes a CD-R tick, which makes it easier for us to better understand these disks' strengths and weaknesses.

To summarize, a common writable CD (CD-R) is primarily a solid disk of clear plastic, usually polycarbonate, which has a spiral "pre-groove" etched within it to act as a guide for the laser. (These "pre-grooves" are the fine lines that act as a diffraction grating, producing the rainbow colors you see reflecting from a CD.)

Next comes a reflective layer, typically made of aluminum or gold foil, which is glued to the plastic disk. Then, a layer of organic dye covers the foil. Finally, a plastic or paper label goes over the foil (this is the "top," or "back" side of the CD -- the "business end" is the other, or clear side).

When you write to a CD-R, the reasonably strong "write laser" melts tiny pits in the dye in just the right places to represent your data. When you later go to retrieve your data, the lower-powered "read" laser can determine the difference between a spot in the dye that had been zapped, and adjacent spots that have not. This allows the CD reader to recreate the ones and zeros of your digital detritus.

From a longevity standpoint, the polycarbonate in a CD is pretty strong - if you don't melt or scratch it too badly, it should last for a long time. The same goes for the sealed-in-plastic foil; especially if it's the gold variety. But that leaves the dye as "something to die for," from a CD-R's perspective.

'Dem Dyes...

There are three common dyes in use today:

"Cyanine," the blue-green dye used in many less-expensive CD-Rs, has a lifespan of between 10 and 75 years, which puts it at the bottom of the dye-lifetime scale. In a word, if you use these CD-Rs, be "conservative" in how long you expect it to last.

"Phthalocyanine," on the other hand, is the longest-lasting dye, producing golden-colored CD-R disks which have, "...a reputed shelf life of something like 100 years."

Finally, with dye lifetimes falling between those two, we first find the greenish-gold "fromazan" that produces CD-Rs with a somewhat better lifetime than cyanine. We also find the dark blue "metallized AZO" CD-Rs, which exhibit a lifetime more towards the higher end of the scale.

As we can see, the type of CD-R we buy can significantly affect the longevity of our data -- but you may have noticed that few CD-R packages disclose their chemistry! So aside from the inexact art of eyeball color matching, how can we tell what type of dye a given disk contains? One answer comes to us from the Aug. 30 LangaList, which points us to a free utility from G&M called "CDR Identifier." In many cases, this little program will read the information block that is factory-written to the CD-R disk, returning an indication if the CD-R is designed for short or long term storage.

(CD-RW: Although I haven't yet seen definitive figures for the lifetime of CD-RW disks (those special disks that you can RE-write up to 1,000 times), you might be interested in how they work compared to a write-once CD-R.

Brought to our attention by the Sept. 29 Stavance Newsflash, the "working layer" on a CD-RW disk is made up of silver, indium antimony, tellurium, and other rare earths, which have the ability to change from a crystalline state (which reflects the laser as if no data were written to a spot), into an amorphous state (which reads back like a "pit" of data in a conventional CD). The laser can "write" a spot using high power, and then "erase" a previously-written spot (causing it to return to the amorphous, or "no data" state) using a medium power. Hence, on CD-RW disks, we can write -- erase -- and write again and again...

Of course, nothing is exact in the data preservation game. Any one CD, even with the most long-lived of dyes, might suffer from a manufacturing problem or from a bad dye lot, and the problem might not show up until long after your backup software completes its full "verification pass" (you DO configure your backup software to do a read-after-write "verification pass," don't you?) So be sure to keep your really critical data backed-up onto multiple media, storing them at different sites! (You can find more about CD-R and CD-RW CDs -- a LOT more -- in Andy McFadden's excellent "CD-Recordable FAQ."

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