By Diane Vogt-O’Connor, Senior Archivist, Museum Management Program, National Park Service.
Compact Discs (CD) have many advantages. Researchers search speedily through large quantities of documents on CDs, while protecting the original materials from excessive handling. An inexpensive distribution tool, CDs hold large quantities of data in a small space. During playing, no contact occurs between the playback device and the sound carrier so wear is minimized. Many CDs incorporate an Error Detection And Correction (EDAC) system that allows reconstruction of digital data when small errors or losses occur, making perfect copying possible Since CDs vary little over time (until they fail), they may be duplicated without generational loss of information.
CD Life Expectancies
CDs are complex laminate structures vulnerable to damage by light, humidity, temperature mishandling, and pressure. Since CD information is stored in blocks of data with EDAC correcting codes it isn’t easy to determine when a CD is about to fail. CDs can be destroyed in a few minutes through poor handling or damaged from a few hours of being stored outside of their jewel cases.
Storage or handling that would not destroy tape or paper, such as bending, pressure, or light exposure, can destroy a CD. Don’t count on CDs to last many decades because the polycarbonate substrate used on most CDs has a shorter life than paper or film. Few companies warranty their discs for more than a decade. Don’t expect CD playback equipment for today’s CDs to be available in 20 years, so equipment (and where appropriate, software) maintenance is essential. Don’t dispose of your paper or film originals when using CDs for access copies.
Why CDs Fail
Most CDs fail because of:
- Physical stress leading to delamination, warping, and/or improper tracking
- Dirt or grit scratching media and leading to losses of information
- Yellowing of the plastic or light recording layer
- Low reflectivity due to oxidation of the aluminum layer (also known as laser rot)
- Natural aging
Standard CD Construction
Many CDs include a lacquer for durability, a reflective layer (usually aluminum; sometimes more stable gold), dyes (most frequently organic), and a substrate (often polycarbonate plastic, sometimes metal or etched glass) onto which the signals are etched by laser light. A marking agent, such as ink or an ink printed label, may be placed on the CD. Many of these materials, particularly the substrate, vary over time and by manufacturer.
Types of CDs
There are many CD technologies. The following are some of the most common types:
- Write-Once, Read-Many Times (WORM) format CDs are standard CDs commercially produced that may contain text, images, video, software, or sound.
- Compact-Disc-Digital Audio (CD-DA) is almost identical to standard WORM format CDs. CD-DAs are produced commercially for the popular music market.
- Recordable Compact Discs (CD-R) are WORM format CDs that are produced one-by-one non-commercially by a recording disc drive. CD-Rs are playable on standard CD-DA or CD-ROM players.
- Rewritable CDs can be erased and used again like magnetic media. Available in 90mm and 130mm digital optical formats, rewritable CDs require special players. Rewritable CDs use two separate technologies:
- Magnetic-Optical Rewritable CD-MO uses heat and magnetic fields to write the CD. Non-standard CD-MO players use polarized laser beams that indicate the magnetic orientation of each spot.
- Phase Change CDs use laser heat to reflectivity of the recorded section
Evaluation of CD Materials
Look for CDs with a scratch resistant lacquer for durability, a gold reflector layer. Thalocyamine dyes, and a stable glass substrate. For these discs, manufacturers quote life expectancies equivalent to paper and microfilm records; however, testing data is incomplete Don’t use CDs made with cyamine dyes because they are less light stable. CDs with aluminum reflection oxidation (caused by excessive humidity coupled with airborne contaminants)
Most Durable CDs
According to conservators, such as William Nugent (see References), the following are the most durable CDs:
- Super CD
- Kodak Writable CD
- Kodak Photo CD
- Digipress Century Disc Gold
- Digipress Century-Disc Ark
- Digipress Century-Disc Eon
Where to Find Guidance
Don’t get your preservation data from vendors. In the past vendors have often not disclosed when their products were non-archival or short-lived. Watch for media production, storage, housing, and player standards from the American National Standards Institute (ANSI) and the Association for Information and Imaging Management (AIIM) and recommendations and manuals from non-profit professional associations, such as the American Institute of Conservators (AIC) or the Society of American Archivists (SAA).
3M Software Media and CD-ROM Services
Bldg. 544-2N-01, 1185 Wolters Blvd.
Vadnais Heights, MN 55110
18 Rue Bailey
14050 CAEN Cedex, France
Fax (43) 31 47-25-02
Writable and Photo CD, Eastman Kodak
460 Buffalo Road
Rochester, NY 14652-3834
CD Computer Aided Test System
Advanced Audio Development USA, Inc.
5335 Merle Hay Road, Suite 9
Johnstown, IA 50131
Compact Disc Inspection System
Automatic Inspection Devices, Inc.
P.O. Box 6295
Toledo, OH 43614
To Preserve Your Archival CDs
|Do This…||Don’t Do This…|
|Handling and Use
||Handling and Use
Bikson, T.K. and E.J. Frinkling. “Preserving the Present: Toward Viable Electronic Records.” The Netherlands: Sdu Publishers, The Hague, 1993.
Fontaine, J.M. “The Preservation of Compact Discs, Principles of Analysis,” “Archiving the Audio-Visual Heritage” by G. Boston, ed. Northants, UK. Technical Coordinating Committee and UNESCO, 1992.
Nugent, William R. “Compact Discs and other Digital Optical Discs,” “Storage of Natural History Collections: A Preventive Conservation Approach.” York, PA.: Society for the Preservation of Natural History Collections. 1995.
Diane Vogt-O’Connor, Senior Archivist, Museum Management Program, National Park Service. Washington, DC 20013-7127
This article originally published in Conserv O Gram September 1996 Number 19/19 and Colorado Preservation Alert Winter, 1996 Volume 6/Issue 4
The Conserve O Gram series is published as a reference on collections management and curatorial issues. Mention of a product, a manufacturer, or a supplier by name in this publication does not constitute an endorsement to that product or supplier by the National Parks Service. Sources named are not all inclusive. It is suggested that readers also seek alternative product and vendor information in order to assess the full range of available supplies and equipment.
The series is distributed to all NPS institutions and interested individuals by subscription through the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402: FAX 202/512 -2250 . For further information and guidance concerning any of the topics or procedures addressed in the series contact the NPS Museum Management Program, P.O. Box 37127, Washington, DC 20013-7127; 202/343-8142