Digital Data Preservation

This article provides some guidelines for the preservation of digital data using CDs and DVDs. This information has been gathered from a number of sources including NIST (National Institute of Standards and Technology), DVD and CD manufacturers and data archiving specialists.

The consensus of opinions, based on accelerated lifetime testing, is that a high quality CD or DVD, if properly handled and stored, should preserved data for at least 75 years. Some studies even suggest a lifetime as long as 200 years before the data becomes unreadable.

Of course whether the hardware required to read a current CD or DVD will be around in 75 or 200 years is another matter. Most system today don't have even the hardware to read 5.25" floppy disks (which were common and popular only 10 years ago).

The key points here are that long life depends on using HIGH QUALITY archival DVDs and CDs and handling and storing them properly. The less you handle them, the better, so for real archival use, make two copies. Put one away in storage under the conditions outlined below (vertical storage in a case under low temperature and low humidity conditions) and use one as your "working" copy.

Things to Do:

• Handle discs by the outer edge or the center hole.
• Use a non solvent-based felt-tip permanent marker to mark the label side of the disc.
• Keep dirt or other foreign matter from the disc.
• Store discs upright (book style) in plastic cases specified for CDs and DVDs.
• Return discs to storage cases immediately after use.
• Leave discs in their packaging (or cases) to minimize the effects of environmental changes.
• Open a recordable disc package only when you are ready to record data on that disc.
• Store in a cool, dry, dark environment in which the air is clean.
• Remove dirt, foreign material, fingerprints, smudges, and liquids by wiping with a clean cotton fabric in a straight line from the center of the disc toward the outer edge.
• Use CD/DVD cleaning detergent, isopropyl alcohol or methanol to remove stubborn dirt or material.
• Check the disc surface before recording.

A standard jewel case is adequate for storage. In jewel cases,the disc is suspended by the center hole and isolates the written area [read side] from contacting any surface, It also offers some protection against rapid temperature changes

Things NOT to Do:

• Touch the surface of the disc.
• Bend the disc.
• Use adhesive labels.
• Store discs horizontally for a long time (years).
• Open a recordable optical disc package if you are not ready to record.
• Expose discs to extreme heat or high humidity.
• Expose discs to extreme rapid temperature or humidity changes.
• Expose recordable discs to prolonged sunlight or other sources of UV light.
• Write or mark in the data area of the disc (area where the laser "reads").
• Clean in a circular direction around the disc.

"The worst handling stresses for a CD are caused by severe flexing or application of a sharp point to the top surface" - Kodak

"Cheap plastic sleeves (for example, those sometimes used to hold CDs in the backs of books or in some types of multiple CD holders) are not suitable for long-term storage" - Kodak

For CDs, Things REALLY not to Do:

• Scratch the label side of a CD.
• Use a pen, pencil, or fine tip marker to write on the disc.
• Write on the disc with markers that contain solvents.
• Try to peel off or re-position a label.
  

General recommendation for long-term storage conditions

• For archiving recordable (R) discs,discs that have a gold metal reflective layer are recommended. It's said that with current disks, which are better sealed than they used to be, the use of gold as a reflecting layer is less important than it was, however it certainly doesn't hurt!
• For general storage, a temperature between 4°C (39°F) and 20°C (68°F) is recommended with a relative humidity of  20% to 50%
• For long term storage, 18°C and 40% RH are recommended.
• For extended term archival storage even lower temperature and humidity are recommended.
• Storage in the dark, while not absolutely required, can't hurt.

Differences between CDs

Writable CDs are made using several different photosensitive dyes:

DYE	COLOR	PROPERTIES
Metal-Cyanine	Blue	Original CD-R Dye, Least stable
Metal-Azo	Very Deep Blue	Increased stability over Cyanine dyes
Phthalocyanine	Very light Green	Most stable, transparent and durable

When stored under optimum conditions, CDs using Phthalocyanine dyes and a gold metallization layer should have the longest life. Mitsui MAM-A Gold Archive 74 minute CD-Rs match these requirements. They are a little more expensive then "regular" CD-Rs, but still only about $1.50 each in quantity.

Delkin's eFilm Archival Gold CD-R's incorporate a Phthalocyanine (thalo-sy-a-neen) dye and a 24k gold reflective layer into every CD-R. Delkin claim that eFilm Archival Gold CD-R's can store images for more than 300 years. I'm not aware of any independant tests that verify this, but gold/pthalocyanine disks should have the longest lives.

It's a little more difficult to make specific DVD recommendations as DVDs haven't been quite so well characterized as CDs. Certainly some are more archival then others, though it's hard to specify a particular product right now. It's almost certainly true the cheapest, spindle bundled, multipack DVDs aren't going to have the longest life though. Verbatim claims a projected life of 100+ years for their Metal-Azo DataLifePlus series of DVDs, if stored in jewel box cases in a non corrosive atmosphere at 25C and 55% RH. However I don't know of any independent study of DVD lifetimes.

Both Memorex and Delkin make a line of archival gold DVD-R disks which are stated to have a lifetime in excess of 100 years. As far as I know there are no gold DVD+R disks available, but DVD+R is probably an intrinsically better recording scheme to start with, having a somewhat more advanced technology with better tracking and error correction as well as more reliable disk burning. Most of the time the difference between DVD+R and DVD-R is negligible for consumer applications (both work fine), but for archival storage and the ability to read old disks, I'd go with DVD+R as the better bet. Verbatim have DVR+R disks in their "DatalifePlus" series, which should be pretty archival.

     

One thing you may want to do if your data is really important is to make multiple backups and don't store them togther. While any one disk can fail or be accidentally damaged, the chances of that happening to two disks at the same time are much lower.

Some useful links and references:

CD Check - A freeware program (for personal use) which can check the integrity of data on a CD.

Stability Comparison of Recordable Optical Discs : a Study of Error Rates in Harsh Conditions
Byers, Fred; Lu, Richang; Slattery, Oliver; Zheng, Jian; Tang, Xiao