Source: Canadian Conservation Institute
The longevity of recordable compact discs (CD-Rs) and recordable digital versatile/video discs (DVD±Rs1) is uncertain, leading to a widespread lack of trust by libraries and archives. Research studies, anecdotal information, and manufacturers’ literature suggest that the lifetime of CDs and DVDs can range from a couple of years to more than 200 years. This Note explores several of the factors that affect whether a disc fails within a short period or continues to perform well for many years.
Poorly manufactured discs (i.e. discs that do not meet standard specifications for proper function) will probably fail sooner than good-quality ones due to rapid chemical degradation or physical damage. This was a significant problem when discs were introduced (CD-Rs first appeared in the marketplace in 1991, DVD-Rs in 1997, and DVD+Rs in 2002) and for at least 2–3 years afterwards. Poor manufacturing is still a problem to some extent, either because of cost-cutting to meet competitive pricing or speedy production runs to meet high demand.
Determining if a disc is poorly manufactured is an impossible task without thorough testing, which likely would include accelerated aging. This is not feasible for most archives, libraries, and museums. In the absence of testing, discs with a recognized brand name can generally be assumed to be of good quality. Even though some large manufacturers label discs produced elsewhere with their own brand name, most have no desire to be associated with an inferior product. Information on where the disc was actually produced may be available by consulting the manufacturer and/or examining disc coding.
The base layer of a CD-R is always composed of polycarbonate. However, the dye and metal reflective layers can be composed of various materials, each with its own inherent stability. The quality of the top protective layer is also important.
The CD-R specification was designed around cyanine dye (shades of blue) and, therefore, most of the early discs used it. Azo dye (deeper blue) was introduced in 1996. However, neither of these dyes matches the stability of light-green phthalocyanine, which is very stable to light, high temperature, and high relative humidity (RH). Phthalocyanine was available in the early days of CD-R manufacture, but it was not widely used until around 2002.
The type of dye in a CD-R can sometimes be determined by transmitting light through the disc and viewing the color (see Table 1). However, the presence of a thick, dark label may make identification impossible with transmitted light. In these cases, reflective light can be used although the color of the metal reflective layer may alter the appearance of the dye color. Note also that some discs have pigmented bases, such as black or one of a variety of fluorescent colors; these colors do not indicate a different dye or metal layer than indicated in Table 1. The type of dye in a disc can also be determined by consulting the manufacturer or their literature.
A CD-player reads a disc by directing a laser light through the base and dye layers to the metal layer, which reflects it back to the player’s signal detector. If the reflective layer is altered in any way, it will not perform as expected and the disc cannot be read. The metal reflective layer in CD-Rs has generally been gold, silver, or silver alloy. Gold is very stable, so discs with a gold reflective layer are not at risk for “laser rot” (a term used to describe the corrosion of the metal layer) and hence have excellent longevity. However, silver and silver alloys are susceptible to corrosion, so CD-Rs with these kinds of reflective layers are more likely to fail, especially if they are exposed to pollutants.
The top protective layer should have good chemical resistance and be rugged enough to protect the sensitive metal layer from handling damage. If this layer is of poor quality or has not been applied evenly and completely on the disc, then early disc failure is likely. Many manufacturers specifically mention that their discs contain rugged topcoats.
A recordable DVD provides much more capacity than a recordable CD and, therefore, is often the desired format for storing information.
Recordable DVDs are composed of two individual discs, half the thickness of a CD-R, manufactured separately and then glued together. DVD±Rs can also have two information layers.
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