Compact discs are everywhere You find them in the home and the workplace and in the library collection. This unique medium brings music, text, pictures, and data into the lives of our users. The central CD preservation issue is twofold: How long do CDs last and how should they be cared for?

Collections of audio CDs, CD-ROM multimedia titles, data discs, and photo CDs represent significant investments of money and effort. Users of writable CDs are often especially concerned for their data and pictures.

The compact disc is a laminate of four different materials. The bottom of the disc is made of polycarbonate onto which the pits containing the digitized sound information are stamped. A thin layer of aluminum is then applied, covering the pits. A thin lacquer coating (which becomes the top of this disc) is then applied to cover the aluminum layer and finally the ink for the labeling is applied. As with any laminated product, there is a valid concern as to how the aging characteristics of each material will interact with, and affect, adjacent layers.

Compared with other digital storage media (e.g., magnetic tape), CDs have much longer life expectancies. Accelerated aging tests, which speed up the redactions of decay, can determine the rate at which slow chemical changes can make discs unreadable. These aging tests show that when writable media is stored in the dark at moderate storage conditions, its inherent life expectancy is around 200 years. By contrast, magnetic tape storage, according to the Commission on Preservation and Access, will last only a few decades.

Tapes and discs are "analog" recordings. Analog refers to the transformation of sound into "parallel," or analogous, grooves or participle alignments. Compact discs, on the other hand, are "digital" recordings. Rather than being a continuous physical image of changes in electrical voltage, digital recordings are based on a series of discrete electrical voltage measurements. In other words, the data stored on CDs is encoded in digital form. First, the data must be read by player devices and then it is converted into music, text, pictures, and so on by software programs. Because the real goal is access to the disc's contents, not merely the preservation of the disc, the survival of playback hardware devices and software formats are key. Migration of digital data from one storage medium or software standard to another is essential.

CD-ROM CARE TIPS

Maintain CD-ROMs in Proper Storage:
» Store recordings at a maintained temperature of between no more than 15° - 20° C. Fluctuation in temperature should not vary more than 2° C in a 24-hour period.

» Maintain a relative humidity of 25-45%. Fluctuation of relative humidity should not vary more than 5% in a 24-hour period.

» Maintain proper ventilation and air circulation of stacks at all times to avoid any microclimates.

» Keep sound recordings in dark storage when not being used. Fit light fixtures with fluorescent tubes, which do not produce ultraviolet radiation in excess of 75 mw/lm (microwatts per lumen).

» Provide good containers or enclosures for CDs. Cheap plastic sleeves are not suitable for long term storage. Upon exposure to the extremes of temperature and humidity, the disc and the sleeve may adhere to each other. The sheer forces generated upon forced removal of a disc in such circumstances can lead to delamination. The acrylic "jewel cases" provided by many manufacturers and distributors are good protection against scratches, dust, light, and rapid humidity changes. If the manufacturer provides a spacer card or other material as part of the jewel case package, it should be retained. Protect the individually cased CDs further by placing them in a closed box, drawer, or cabinet. This gives additional protection from light, dust, and climate fluctuations.

Maintain a CD Safe Area: It is always a good idea to handle the CDs themselves only when they are being used. Never allow them to remain out of their enclosures for long periods of time. Always handle discs by their edges. Do not eat, drink, or smoke around CDs.

In preservation, dirt can be classified into two categories: 1) foreign matter deposits, which are not part of the original object, such as grease from fingerprints, soots, stains, adhesives, etc. and 2) alterations of original object material through chemical corrosion products and palmitic acid, from acetate discs, or a gummy substance on tapes.

Since there is no physical contact at playback, there is virtually no chance of physical damage occurring during playback due to dust deposits. Nevertheless, dust will impede long-term preservation. At present, the precise, long-term degradation mechanism for the CD is still unknown. However, if dust is improperly removed, permanent physical damage will occur owing to the scratching of the protective layer.

To minimize foreign matter deposits:

» DO NOT touch the surface of a recording. Use white lint-free cotton gloves and handle by the edges.

» Do not expose recordings, unnecessarily, to air. Return items to their containers when not in use and never leave storage containers open.

» Do not place recordings near sources of paper or cardboard dust.

» Keep the surrounding areas clean. Do not consume food or beverages in the area in which recordings are handled.

» Keep storage facilities as dust-free as possible.

» Equip your air conditioning system with dust filtering equipment.

» Keep labeling to a minimum,and limit the placement of labels, especially pressure sensitive labels to the container. User-applied labels of any kind may unbalance the CD and make it difficult for the player to read. Also, labels may peel in humid conditions. Once a label is on the CD, however, it is especially important not to try to remove it. The act of peeling off a label creates a lever action that concentrates stress in a small area. Such a stress can cause delamination, especially in a writable CD. If it is necessary to write on the top side of a CD, a soft felt tip marker is preferable to other writing instruments, but with some solvent-base markers there may be a danger of the solvents migrating into the protective lacquer. So, if a CD must be marked, it should be done in a way that is approved by the disc manufacturer.

» Keep equipment clean, well-adjusted, and in good working condition.

» Remove a CD from its case by pressing your thumb and third finger on edges near the top and bottom of the case and pressing on the plastic clasp in the center with the other hand.

Washing the CD: CDs become dusty or dirty and occasionally require a careful cleaning. Dirt, dust, and fingerprints are more significant when they are on the bottom (the laser-incident) side of the disc. Light dust or dirt may be safely brushed off with a nonabrasive tissue. Always be gentle and wipe from the center hub toward the outside edge of the disc. The motion should be in a radial direction (like the spokes of a wagon wheel) not in a circumferential direction (around a circle).

Since dust is usually held in place by electrostatic attraction, dry wiping on its own does not work effectively. The added friction created by the duster will cause the dust to impact on the charged surface.

Distilled water is used for cleaning records and CDs for many reasons: its precise chemical makeup is known; it will not leave any residue; it is safe to use; and it is inexpensive. Water disperses static charges and contracts the increase in conductivity from the pickup of salt deposits from fingerprints. However, water alone cannot dissolve grease. Thus surfacants are used as additives to enable water to be a grease solvent. Surfacants break grease surface bonds and allow water to penetrate grease solids, causing swelling and then random dispersion.

An air gun should be used to blow off any light surface dust. If fingerprints or other stains must be removed, .5 part of Tergitol 15-S-3 and .5 parts of Tergitol 15-S-9 per 100 parts of distilled water can be utilized safety. Carefully blot the area of the disc needing washing with a soft cloth (preferably a soft cotton that has been washed several times) imbued with a concentration of Tergitol and distilled water. Rinse well using a second cloth soaked in distilled water. Blot dry using a soft cotton cloth and use the air gun to blow off any lint that is left.

Avoiding CD Handling Stresses: The worst handling stresses for a CD are caused by severe flexing or application of a sharp point to the top surface. These actions deform the substrate, wiping out pits and causing areas of the disc to become unreadable. A sharp stylus, a ballpoint pen for example, can cause compression of the polycarbonate substrate and the metallic reflecting layer in the area under the pen point. This can happen to both CD-ROMs and writable CDs, but the danger is more acute for writable CDs. The adhesion between the dye and the other layers in a writable CD is weaker than the interlayer adhesion in a CD-ROM where no dye is needed.

Compared to the printed word, compact disc technology is relatively new. The knowledge about its capabilities and limitations is small. There are many "care and handling" articles for maintaining a collection of compact discs, whether they are audio, ROM, interactive or recordable, but very few directly address the issue of CD longevity. One reason, of course, is that the technology is young, and not much time has passed since it was introduced. Another reason is not that the compact discs themselves are short- lived, but that the hardware needed to read them is ever evolving. Support for today's technology may not be available tomorrow. What use is a disc that can last 500 years, even 100 or 50 years, if there is no machine that provides access to the information on the disc? An early problem of compact discs, disc rot, is now thoroughly understood and documented in the literature. Manufacturers changed their practices and disc rot is no longer a problem with newly manufactured discs. De facto standards exist for most types of compact discs, the exception being compact-disc recordable (CD-R), which is under development and will be published soon.

The principal fact of life for all digital storage media is the rapid obsolescence of hardware and software. Users of the CD technology should be reassured by the long physical life of CD discs, but they must not lose sight of the need to maintain a viable path for migration of data to new hardware and software platforms. Digital storage media impose a strict discipline that human-readable records do not; their rapid evolution creates a continual progression of technology that cannot be safely ignored indefinitely. However, increased knowledge of the physical nature of the media and of appropriate archival approaches will allow users of CD technology to have the best of both worlds -- the functionality of digital media and a long life for stored data and pictures.

I have provided below a bibliography on the care issues for CD materials:

BIBLIOGRAPHY

Adelstein, Peter Z. "The Stability of Optical Disks." The Commission on Preservation and Access Newsletter, no. 58 (July 1993): 3-4.

Altord. Roger C. "CD-ROM Inside and Out: Here's How CD-ROM Drives Work, How CD-ROM Discs Store Data, and What the Standards Mean." Byte 8, no. 3 (March 1993).

Bansa, Helmut. "The New Media: Means for Better Preservation or Special Preservation Problems?" Restorator 12 (1991): 219-232.

Behera, B. and H. Singh. "New Approach to Optical Storage Modeling, Performance Analysis, and Evaluation." Optical Engineering 30, no. 9 (Sept. 1991): 1347-54.

Beiser, Karl. "CD-ROM Report-What a Difference a Year Makes: CD-ROM Developments." Online 7, no. 3 (May 1993):109-11 I.

Blom, G.M. and D.Y. Lou. "Archival Life of Telitirium-based Materials for Optical Recording." Journal of the Electrochemical Society 13, no. 1 (1984): 146-151.

Botiley, Raymond J. "The Life and Death of CD-ROM." CD-ROM Librarian (Jan. 1992): 10-17.

Craig H.K. "Nondestructive Testing and Life Determination of Disc Material." British Journal of Non-Destructive Testing 34, no. 8 (Aug. 1992): 395-9.

Day, Rebecca. "Where's the Rot? A Special Report on CD Longevity." Stereo Review 54, no. 4 (Apr. 1989): 23-24.

Fox, Barry. "CD Makers Perform in Unison to Stop the Rot." New Scientist 134, no. 1815.

Fox, Barry. "Technology: Tests Prove that CDs Can Self-Destruct." New Scientist 119, no. 1620 (7 July 1988): 37.

"Gold Discs Stop the Rot. (New Compact Disc Coatings Last Longer Than Aluminum)." New Scientist 118, no. 1616 (9 June 1988): 51.

Harvey, O.A. "CD-ROM Drives: How Good is the Third Generation?" Byte 16 (Summer 1991): 168-70+.

"Laser Rot." The Perfect Vision 1, no. 1 (Winter 1986-1987): 35-45.

Marshall, Mary E. "Compact/Disc's 'Indestructibility': Myth and Maybe." OCLC Micro 7, no. 1 (Feb. 1991): 20-23.

Martin, Mike. "Compact Disc Media Evaluation-What We Know about Disc Quality." CD-ROM Professional, no. 2 (Mar. 1993): 74-77.

Meyer, Frederick P. "Out with the Old, in with the New-Why CD-ROM May Have a New Standard." CD-ROM Professional 5, no. 5 (Sept. 1992): 150-152.

Meyer, Frederick P. "Recordable Capability Compact Discs Rolling in the '90s." CD-ROM Professional, no. 3 (May 1992): 127-129.

Mohilienrich, Janice, ed. Preservation of Electronic Formats and Electronic Formats for Preservation. Fort Atkinson, Wisconsin: Highsmith Press, 1993.

Murray, William P. "CD-ROM Archivability." NAFL Bits: Newsletter of the National Media Lab 2, no. 2 (May 1992): 4-7.

National Association of Government Archivists & Records Administrators. Digital Imaging and Optical Media Storage Systems: Guidelines for State and Government Agencies. Albany, NY, 1991.

Nikles, D. E. and C. P. Forbes. "Accelerated Aging Studies for Polycarbonate Optical Disk Substrates." Proceeding of the Society of Photo-optical Instrumentation Engineers-International Society of Optical Engineers 1499, (1991): 29-38.

Nugent, William R. "Research in Extending the Longevity of Information on Digital Optical Disks and Videodiscs." Paper Summaries, Electronic Imaging '86. Boston: IGC, 1986.

"Once and Future Standards." Byte 18, no. 2 (Feb. 1993): 130.

Oudard, Denis. "Testing Compact Discs for Longevity." Paper Presented at the 14th Annual Conference on Interactive Systems for Training, Education, and Job Performance Improvement. Arlington, VA, Aug. 1992.

Podio, F.L. Test Methods for Optical Disk Media Characteristics (for 356mm ruggedized magneto-optic media). Report NIST SP 500-191. Washington, D.C.: National Institute for Standards & Technology, Sept. 1991.

Rosenthal, Steve. "Passing Sunday Drivers." MacWEEK no.6, no. 41 (Oct. 12, 1992): 48.

Saffady, William. "Stability, Care, and Handling of Microforms, Magnetic Media, and Optical Disks." Library Technology Reports 27, no, I (Jan./Feb. 1991): 5-116.

Salzer, Jerome. "Fault-Tolerance in Very Large Archival Systems (Position Paper)." Operating Systems Review 25, no. 1 (Jan. 1991): 81-82.

Tamuru, N. and N. Amano. "Data Reliability Evaluation of Flexible Optical Disk System Using Accelerated Test of Dust density." Applied Optics and Optical Engineering 31, no. 35 (Dec. 10, 1992): 7464-70.

Videodisc and Optical Disk Technologies and Their Applications in Libraries: A Report to the Council on Library Resources. Washington, D.C.: The Council on Library Resources.

Woodcock, Roderick and Marc Wielage. "Laser Rot." Video (April 1987):49-52.

Wong, J.S., LJ. Fortunati, B. Hong, A.P Kitiska, L.L. Nebenzahi, M.F. Mercado, Y.J. Rodriguez, and Y.S. Yen. "Testing of Commercial Optical Media." Conference Record. Institute of Electrical and Electronic Engineers Instrumentation and Measurement -Technology Conference (Catalog No.91CH2940-5) (1991). New York: IEEE, 1991, 44-47.

Yamaguchi, H., Y. Tsukmoto, F. Watanabe, et al. "Extremely Durable CD-ROM with a Novel Structure." Proceeding of the Society of Photo-optical Instrumentation Engineers- International Society of Optical Engineers 1499 (1991): 29-38.

Standard References:

"'Red Book". CEI IEC 908 (Audio)
"Yellow Book" ISO lO141989 (CD-ROM)
"Orange Book" (CD-R)
(Available from: ANSI, 1436 Broadway, NY, NY 10018)