By Ben Li
Many students probably received CDs, DVDs, or some kind of electronic device that uses optical discs of various flavours during the recent holiday season. Brisk sales of consumer and computer optical disc players and recorders–driven by commodification of computing devices capable of producing large quantities of data–means we are increasingly reliant on the contents of ephemeral optical discs.
A guide recently co-published by the United States National Institute of Standards and Technology and the Council on Library and Information Resources offers insight into protecting data on optical discs for both institutional and home users. Fred R. Byers’ Care and Handling of CDs and DVDs–A Guide for Librarians and Archivists explains both practical techniques for maintaining optical discs as well as technical explanations for those who are curious about the inner workings of optical discs.
Some suggestions are obvious, such as keeping discs in their protective storage containers, while other recommendations may be surprising. For example, many CD/DVD storage units orient discs so they are flat to the ground, a practice NIST warns against.
"The disc should be stored in its case and placed vertically, like a book, on a shelf," writes Byers. "Long-term horizontal storage, particularly in a heated environment, can cause the disc to become permanently bowed. While the data may still be intact, the disc may not operate properly in the drive or permit the laser to follow the track."
While Byers warns improperly stored discs may bow in five to 10 years, NIST predicts all current optical discs will remain readable for much longer than the technology will exist to read them, a fate comparable to the Betamax and punch card formats.
"Among the manufacturers that have done testing, there is consensus that, under recommended storage conditions, CD-R, DVD-R, and DVD+R discs should have a life expectancy of 100 to 200 years or more," writes Byers. "CD-R, DVD-R, DVD+R, and DVD-RAM discs should have a life expectancy of 25 years or more."
Pressed, non-recordable discs are estimated to remain readable for 20 to 100 years, consistent with news reports of early or low-quality CDs with aluminum surfaces that have rusted or been colonized by fungus due to exposure to the environment. As the aluminum surface reflects the laser light required for reading of optical discs, a tarnished surface makes data unreadable.
Useful tips
According to the guide, the main factors contributing to unreadable discs in regular use are surface contamination, heat and physical damage–conditions not uncommon to student backpacks, cars and living areas. Though Byers’ strict recommendations for optical disc storage are probably excessive for random mix CDs, discs containing important data such as research, images, schoolwork and computer backups would benefit from careful treatment.
Optical discs will absorb water and contaminants if left in humid conditions or submerged. NIST recommends keeping discs in their protective containers in cool and dry environments, even during regular use. Adhesive labels may also bring moisture and contaminants in contact with discs, as well as mechanically unbalance the discs themselves when spun by the disc reader. Unbalanced discs may skip or damage lower-quality readers.
Dirty discs may yield particles that can be spun off by disc readers, and should be cleaned of surface contamination by rinsing with water or water-based detergents, according to Byers. Only if the dirt cannot be removed with water should a dry cloth be used to wipe from the centre outwards, perpendicular to the direction of the tracks on the disc. Abrasive products such as lens paper should not be used to clean discs as they may scratch the disc surface.
Although extensive testing has not been done, NIST experiments show that excessive heat from sunlight exposure, car interiors and low-quality disc readers can damage rewritable (+/-RW) discs by degrading the phase-changing film. Direct sunlight will not damage rewritable discs, but will damage recordable (+/-R) discs with light and heat that mimic the writing process, which uses heat to cause changes to organic dyes. The plastic used for all discs may be damaged by heat generated by sunlight.
Other considerations
Other physical damage occurs when either the plastic or metal recording surface are scratched accidentally or through labeling. When either process causes damage severe enough to overwhelm the discs’ built-in error correction, content may become unreadable.
NIST recommends hand-labeling with soft-tip markers be restricted to the clear inner hub and mirror band, where no data is stored. CD-safe markers should be used, although other markers not containing strong organic solvents will also work. Labeling on the data surface, or with markers based on organic solvents may accelerate the degradation of optical discs.
Though Byers does not recommendations specific brands of recordable discs, the manufacture of recordable discs used to store data is as important as the care and handling of the discs. Those with a branded or printable label surface are thought to be more durable than generic unlabeled discs, as that label provides an additional layer of physical protection for the recording surface.
As for the format of media to use, Byers warns: "In theory, -R discs should have a limited number of read times (several thousand) because of the cumulative effect on the data layer from the laser light. As with -ROM discs, the polycarbonate [clear plastic] may also eventually be affected, but there is no recorded evidence of ill effects of laser light, so such effects are deemed negligible."
The guide can be purchased in print or downloaded for free from www.itl.nist.gov/div895/carefordisc/