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Although we back up our information by burning it to disc to keep it safe from hard drive crashes, it should be kept in mind that CDs and DVDs are fragile media themselves. They degrade in a number of ways over time, and are susceptible to damage from mishandling and misuse. If you have backed up your important files to CD, these should be kept safe and in good condition so that your information can be retrieved when needed in the future.

Construction and Materials of Discs

CDs and DVDs are made of a number of different materials. Both have a lacquer layer on the top surface, on which labels may be printed or text handwritten. Underneath that is a thin layer of metal, usually aluminum, which holds the information that is “written” on the disk. This layer is the most important to keep safe. On Read-only discs, the kind you purchase commercially with music or movies on them, the information is stamped physically into this layer. On burnable discs, most commonly CD-Rs or DVD-Rs, there is below this metal layer a layer of dye, which when “burned” by the laser of your disc drive, is removed in a pattern of dots that then reveal the metal, and it is this pattern that the laser in the disc player identifies and translates into information. On the bottom of all discs is a thick layer of polycarbonate plastic for protection. It sounds complicated, but what you need to know is that the dye and metal layers are where the information is stored, and these are what need to be protected.

Weak Points in the Disc Format

Although manufacturers claim that discs will last for up to 200 years, the Canadian Conservation Institute advises that discs will remain playable for only between 5 and 100 years, the range varying depending on the disc quality, storage environment, and usage.

Scratches on the bottom are, of course, one of the main dangers that frequently used discs face, however this is not the most vulnerable area. The plastic coating here is very thick, and only the deepest scratches will affect playback. The top surface however is protected only by a very thin lacquer, and this is where the metal layer – which holds the actual information – is most easily reached and damaged. Any scratch that disrupts the metal layer will render the disc unusable.

Moisture can also affect this metal layer, causing the aluminum to rust and corrode, also rendering the disc unreadable. Discs should be kept away from liquid, as this can penetrate the protective coatings and cause corrosion; a high relative humidity in the environment, such as is found in tropic regions, can cause corrosion as well.

If exposed to direct light, the dye in the bottom of the disc can fade, interfering with the laser’s reading of the disc’s information. Discs should be kept in a dark place, or in a case that is opaque both on the top and bottom.

Heat can cause the disc to warp, making playback impossible, and once warped there is very little hope of retrieving the information.

Quality:

If you are concerned about the stability of your media, you should take care to purchase discs made of high quality materials. Kodak makes a burnable CD with a gold metallic layer instead of aluminum, a product called Kodak Gold, and this is recommended for any discs with long-term use. Gold is an inert metal and will not corrode as aluminum does.

The dye is the second most important and fragile feature, and so a disc with stable dye should be selected if at all possible. Little research has been done on the stability of the dyes used, and often manufacturers won’t reveal the type of dye in their products. As far as is known, the most stable dye available in discs is phthalocyanine; phthalocyanine discs are blue on the underside, and the ones that are green are made of a cheaper, less stable dye.

Storage:

- Discs should be stored in their plastic cases, for added protection.

- Cases used should be the variety with a hub in the centre so disc is not resting on any part of case.

- Disc cases should be stored vertically.

- You should remove any paper liner from the disc case – this is probably acidic, and can leach acid into the disc environment. Put a label on the outside of the disc case for identification instead.

- The average room temperature should be kept at approximately 20C, and the relative humidity at 50%. If you must deviate from this standard, the most important thing is to keep the environment constant, so do not allow abrupt plunges or increases in temperature and RH. 

- Discs should be protected from bright light, as this can fade the dye in the disc.

- When needed, discs should be handled with care, only by the edges – a lot of fingerprints or dirt will interfere with the laser reading.

- You should do a yearly inspection of your discs, including playing them all the way through to ensure that all the information is still present; this is known as “exercising” the discs, and is a process recommended for magnetic tape media as well.

- If you notice the information beginning to degrade, migrate the information immediately by transferring to a new disc. 

Maintenance and Repair:

- If you need to clean your discs, start with the most gentle application and work your way up. First, use compressed air to blow off any dust or dirt particles with minimal handling to the disc.

- A soft cloth can be used to polish the disc bottoms – rub from centre outwards, like spokes of a wheel. Never rub circular in a circular direction, with the disc shape, because a scratch in this direction will interfere with reading the most.

- For stubborn dirt, you can use a soft cloth with a little bit of water or ethanol or isopropanol.  NEVER use any other or stronger solvent, as you run the risk of softening the plastic. Acetone, for example, will melt the polycarbonate plastic layer on the bottom.

- NEVER immerse your discs in water for cleaning, as the water could get through the seal and corrode the metal.

- For deep scratches on the underside, as a last ditch attempt try a commercially available polishing kit, but this must never be undertaken unless the disc is absolutely unplayable.

- If you have a seriously warped disc, which is entirely beyond repair in any other way, there is a small chance you can revive it by heating and reflattening it. This is a really last-ditch attempt and must never be undertaken unless there is no other hope. Bake the disc in the oven on a low low temperature, for several hours, or until it is slightly soft, then remove and press it between 2 sheets of glass. For more information on this, please read technical bulletin 27 by Joe Irrachi on the conservation of CDs and DVDs, available at the website of the Canadian Conservation Institute.