• CD-DA (compact disk-digital audio, held primarily in the Music Listening Lab). The CD-DA is a digitally recorded alternative to conventional phonograph records and magnetic tapes.
• CD-ROM (compact disk-read only memory, generally held in reference collections). CD-ROM is the compact disk format for computer-processible data. Writable and rewritable optical disks and video disks are not specifically covered in these guidelines.

Both types of CDs are read only optical disks. Optical storage technology uses lasers to record information by selectively altering the light reflectance characteristics on one side of the disk. The alterations that record information on optical disks include light-scattering holes, bumps, or bubbles. Optically recorded information is read by a laser and pickup mechanism which senses variations in reflected light.

Compact disks consist of a thin reflective metal layer and protective coating which rest on a plastic substrate. The substrate material is usually polycarbonate. The reflective layer is usually composed of aluminum, though sometimes other metals are used. The protective overcoat is a layer of acrylic or lacquer. Damage to, or deterioration of, any one of these three components of the disk can render it unplayable, in which case it will need to be replaced.

The potential for damage can be minimized by following the handling procedures and precautions outlined below.

1. High temperatures and high relative humidity should be avoided. CDs should never be stored in direct sunlight or otherwise exposed to intense heat sources. They should not be placed on top of their drives where they are exposed to both heat and dust.
2. The performance of CDs is adversely affected by dust and debris. Processing, storage, and playback areas should be regularly cleaned and free of potentially harmful contaminants.
3. CDs should be handled by their outer edges and inner hole only. Skin oils, fingerprints and scratches on the disk must be avoided.
4. CDs should be removed from their cases carefully. A good technique is to place the container on a flat surface, push on the hub with your thumb or forefinger while lifting the CD gently with your other fingers. Reverse the order to replace the CD.
5. Dust can be removed from CDs with a soft, lint-free cloth by wiping them in a circular motion from the center to the outer edges. Dust should always be removed from disks prior to use. Several manufacturers offer cleaning kits. They should always be used in strict conformity with their manufacturer's instructions and should be used only when there is evidence of contamination. Conventional household cleaners should not be used; they contain solvents which can damage a disk's protective overcoat.
6. Food, spilled liquids and smoke particles can contaminate or otherwise damage CDs. Eating, drinking and smoking should be prohibited in all areas where CDs are stored and used. Boiling water or otherwise creating water vapors must likewise be avoided.
7. Physical distortion should be avoided. Do not place books or other heavy objects on the disks. Care must be taken to avoid bending disks when removing them or inserting them into their containers. They should not be stacked horizontally on desks or tabletops. To prevent warpage they should be shelved in a vertical, upright position.
8. Barcodes or other adhesive labels should never be affixed to either side of the disk. Such labels may contain solvents which can damage a disk's protective coating and reflective metal layer. For the same reason, markers should not be used to label such disks, even on the printed side.
9. CDs should be stored in their "jewel cases," other protective enclosures provided by the producer, or custom enclosures provided by the Preservation Department. Replacement containers are available from the Preservation Department.
10. The performance of compact disks is dependent on and affected by the equipment used for playback. All equipment must be in proper operating condition. Machine components should be cleaned regularly in the manner specified by the manufacturer. Disks should be removed from their drives immediately after use, unless they are stored in a tower. Some computer peripherals, such as printers, generate debris. Disks and drives should be located as far away from such devices as is practical.

Note: The primary source of this information is William Saffady's "Stability, Care and Handling of Microforms, Magnetic Media and Optical Disks", Library Technology Reports, January- February, 1991.

Prepared by Jim Wellvang who invites questions or comments via email.
July 6, 1994