Preserving CD-Rs, and Other Topics from 2004

Last week, a number of blogs broke the news that Hot Topic would be selling a Rugrats themed eye-shadow palette in the shape of a “VHS cassette.” In comments sections and on social media, however, many expressed horror – “That’s not a VHS cassette – it’s an audio cassette!”

Image via Hot Topic.

While this gaffe might startle those of us who remember the reigns of VHS and audio cassette, it’s an undeniable fact that media become obsolete – or at least less-favored – over time. The CD, successor to the audio cassette, is a great example.

CDs, compact discs, have been in use since 1982, with ‘writeable’ formats like the CD-R being on the market since 1990. More than an audio medium, CD-Rs were considered an excellent way to store and share computer files throughout the 1990s and early 2000s. Compared to size-limited floppy disks and not-so-portable hard drives, CD-Rs filled an important niche. So when I asked to see some Special Collections files from the early 2000s, it was no surprise to find said files stored on 700 megabyte CD-Rs. As I browsed the files (after taking ~30 seconds to remember how to open a CD drive), I wondered: How do CDs work? How stable are CD-Rs? How can we best preserve files like these?

Most mass-produced audio or software CDs are made of layers of polycarbonate with molded ‘pits’ and ‘lands’ – not entirely unlike a vinyl record. The polycarbonate is covered with aluminum or silver, then sealed with a clear lacquer on the top of the disc. CD drives read the disc by focusing a low-intensity laser on the disc and reading the textural pattern of the polycarbonate.

Image via Science ABC.

CD-Rs aren’t so different, except that instead of having the data molded into the polycarbonate, they contain a layer of heat-sensitive pigments. A CD-R drive uses a high-intensity laser to alter the molecular structure of the layer and destroy the transparency in certain areas of the layer – thus mimicking the pits and lands. When the low-intensity laser is used to read the disc, the pigmented sections provide the illusion of texture. CD-RWs (the kind you can write and erase over and over) work much the same, but the pigments are replaced with a metal alloy and the pigmentation be undone and re-done.

Image via Infocellar.com.

So, how stable is this medium for storage? On one hand, CD-Rs are not quite as susceptible to water damage as a hard drive, and they can handle magnetism and x-rays much better than hard drives or camera film. On the other hand, the chemical dyes within CD-Rs are subject to degradation which can be sped-up by environmental factors. CDs of all kinds owe their lightweight portability to their polycarbonate ‘skeleton’ which is both flexible and heat-sensitive. Given these factors, how can you best preserve digital files stored on CDs? Here are some tips:

  1. Prioritize CD-Rs and CD-RWs. Mass-produced CDs (think Encarta Encyclopedia) have their data physically molded into the structure, so there’s no risk of dyes failing you. However, they are prone to warping and scratching.
  2. Speaking of warping and scratching, always store CDs ‘book style’ – with the spine vertical. Vertical storage helps stave-off warping if the CD is exposed to high heat. Whenever possible, store CDs in plastic jewel cases. These prevent scratching as well as moisture damage.
  3. Since the exterior of CDs is plastic, many people assume that moisture isn’t a concern. However, if the lacquer of a CD is sufficiently scratched, moisture can reach the reflective aluminum or silver layer and cause damage, eventually rendering the CD unreadable.
  4. Avoid solvent cleaners like acetone and solvent-based permanent markers. Solvents can disrupt the lacquer, again raising the risk of moisture damage, or even damage the polycarbonate itself, making the CD unreadable.
  5. Don’t use adhesive labels. These labels can attract and trap moisture against the CDs surface. If they begin to detach, they can also disrupt playback in a CD drive.
  6. Keep CDs away from direct sunlight and heat. Remember that data is encoded on a CD-R using the heat energy of a laser to modify a layer of dye. Over time, sunlight and heat can impact the dye in much the same way. One study by Slattery et al. suggested that just a few weeks of direct sun exposure could damage some CD-Rs considerably. Even mass-produced CDs should be kept away from heat to prevent warping.
  7. Clean moisture off a CD with a soft, dry cloth. Wipe from the center of  the CD toward the edge – never in a circle. Scratches which are concentric to the CD’s edge can result in large patches of unreadable data.
  8. Finally – make a backup copy of the CD! Slattery et al. suggest that CD-Rs with a phthalocyanine dye and a silver and gold alloy reflective layer may offer the best long-term storage. Store the backups as described above.

Even after you’ve backed up your CDs, keep in mind that data and software also become obsolete. For instance, one CD I recently used contained files in a now-obsolete format from an antiquated version of a proprietary computer program. Thankfully, the program was created by a large company which is still in operation, and which provides information on ‘upgrading’ the files – but this isn’t always the case. Some filetypes may only be accessible via a specific program, and that program might require obsolete hardware or software. For very important files, consider making copies in a format indicated for long-term storage. The Digital Preservation Coalition’s handbook is a good place to start.

New options for data storage are routinely marketed to consumers and professionals alike, but enduring forms like the book are a rarity. Projects like The Museum of Obsolete Media demonstrate how quickly the devices which store our memories and artifacts become lost to time. Obsolescence always brings us back to an interlocking set of questions that trail collectors of all kinds: What should I keep? Why? How? Answers to these questions remain contentious even among archivists, but in the rush to preserve or the calm of letting something slip away I think we learn something interesting about ourselves and the historical moments we live in.


References:

Byers, Fred R. Care and Handling of CDs and DVDs – A Guide for Librarians and Archivists (National Institute of Standards and Technology and Council on Library and Information Resources, 2003).

Harvey, Ross, Martha R. Mahard, Preservation Management Handbook (Rowman & Littlefield Publishers, 2014), <http://www.myilibrary.com?ID=595041> ( 31 January 2018)

Slattery, O., Lu, R., Zheng, J., Byers, F., & Tang, X. (2004). Stability comparison of recordable optical discs-A study of error rates in harsh conditions. Journal of Research of the National Institute of Standards and Technology, 109(5), 517-524. Retrieved from http://ezproxy.library.arizona.edu/login?url=https://search.proquest.com/docview/214789596?accountid=8360

2 thoughts on “Preserving CD-Rs, and Other Topics from 2004

    1. Thanks! I never knew there was so much to know about CDs. I really liked learning about the pigments in CD-Rs and how it’s kind of a mix of the digital and visual.

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