The cold storage of information media can help preserve optical discs and video tapes long-term. Electronic media such as optical discs and magnetic tapes have a short lifetime when compared to a traditional information carrier such as paper. At standard conditions of 23°C and 50% relative humidity (RH), projected lifetimes for CDs and DVDs range from 2 to 100 years and those for magnetic tapes from 10 to 30 years. In contrast, the lifetime of paper under these conditions ranges from about 50-75 years for poor-quality materials to more than 500 years for better-quality ones. Regardless of the longevity of electronic media, the technologies associated with reading or playing them likely have even shorter lifetimes. Therefore, to ensure the survival of the information stored on electronic media, it is essential to transfer it to new formats. This fact would seem to indicate that cold storage to slow chemical degradation and prolong the life of electronic media is not important because the information will not reside on the carrier for an extended period of time. But this is not the case.
Upon entering a collecting institution, CDs, DVDs, and tapes are often left on shelves or in boxes for a long time before being catalogued. Whether due to minimal resources or large volumes of material, the transfer of the information to a new format may not occur for many years. Cold storage of information media can provide collecting institutions with additional time to formulate preservation plans before media degradation leads to irretrievable losses. In addition, cold storage can help to preserve any original media that are not being discarded following transfer of the information.
Cold storage of information media can also be a valuable preservation tool when the quality of the media is unknown. Some brands are far less stable than others, and it is difficult for archives, libraries, museums, etc. to identify the unstable materials without thorough testing. Some experimental work has been performed to help clarify the stability of optical discs (Iraci 2005), but no comparable research has been performed for tapes. The variety of formulations and proprietary recipes used to manufacture tapes make it even more difficult to determine the stability of different brands and formats. Storing discs and tapes at temperatures below freezing can help to eliminate the uncertainty in stability due to variations in media quality.
Therefore, cold storage of information media does have a role to play in the preservation of CDs, DVDs, and magnetic tapes. By keeping information carriers in the best possible condition, cold storage can help to make the transfer or migration process of the information less difficult, less time consuming, and less expensive. The health of the media should not be the factor that drives the migration process, because unlike technological change, media health can be controlled.
With the benefits of cold storage being obvious, the important issues to explore are:
Does this type of cold storage of information media affect the media negatively to the point where they become unreadable or unplayable?
Does repeated removal of the media from cold storage and into room temperature conditions cause damage?
This cold storage of information media study examines the effects of continuous and fluctuating cold storage on optical discs (CDs and DVDs) and magnetic tapes (VHS tapes). VHS was chosen as the magnetic tape format due to its widespread penetration into collections.
There have been no published scientific studies examining the effect of cold temperatures on optical discs. However, because optical discs contain many thin layers, delamination is always a concern when they are subjected to extreme temperature and RH conditions or extreme fluctuations in these conditions.
There is also a lack of published studies on the effects of cold storage on magnetic tapes. However, cold temperatures could cause the tension in a wound tape pack to loosen, resulting in tape damage. Cassette shells might also be damaged under cold conditions, which would make the tapes unplayable unless transferred to a new housing. But the main concern is that of lubricant separation or lubricant exuding from the tape binder at freezing temperatures, although most of this concern is based on anecdotal evidence (Dance Heritage Coalition, Vidipax 2007, Telemetry Group Range Commanders Council 2004, Brothers 2004, Van Bogart 1995).
When magnetic tape is manufactured, lubricant is incorporated into the binder mixture prior to the binder being coated onto the tape base. The binder is a polymeric material that holds the magnetic material onto the tape base. (More modern tapes, such as metal evaporated tapes, do not have a binder layer and use surface lubrication.) In the finished tape, the lubricant resides in small pores in the binder layer. When the tape is played, the stress of the tape transport system deforms the tape and causes the lubricant to exude to the tape surface, thus lubricating the tape. Once the deformation or stress is removed, the lubricant retreats back into the tape binder (Klaus and Bhushan 1985).
This lubrication system works well under standard use conditions. However, the dynamics change under freezing temperatures. It is speculated that cold temperatures cause the lubricant to solidify and expand (Gilmour 2006), squeezing it out of the tape binder pores and causing it to crystallize on the tape surface despite no physical stress being applied to the tape. An alternative theory is that the binder contracts when exposed to cold temperatures and this causes the lubricant to squeeze out to the tape surface. Some claim that the lubricant simply retreats back into the tape binder when the tape is warmed (Brothers 2004), but others indicate that it may not (Lindner 2008). This problem is most likely to affect tapes from the 1950s and 1960s (Emmerson 1999), which used natural animal and vegetable oil lubricants. From the mid-1970s onwards, tapes commonly used long chain synthetic fatty acid esters (Klaus and Bhushan 1985). For example, pre-1970s tapes that used whale oil were found to exude lubricant crystals onto the tape surface that did not reabsorb into the binder with warming (Lindner 2008).
If the lubricant crystals remain on the surface of the tape, there will be dropouts and overall poor signal levels when the tape is played. The equipment heads could also become clogged. Cleaning the tape to eliminate the lubricant on the surface may allow the tape to play properly, but this will depend on how much lubricant has been lost.
The melting point of a lubricant and the chemical changes it and the tape binder experience with ageing will likely figure into whether or not freezing initiates a problem. It has also been stated that contaminants on the surface of tape appearing after cold storage may not be lubricant at all, but rather residual processing components (such as plasticizers) from the tape manufacturing process (Telemetry Group Range Commanders Council 2004, Brothers 2006).
Another issue that is unclear is the lower limit for cold storage of optical discs and magnetic tapes.
The ISO standard (ISO 2008) for optical discs states that storage below -10°C and 10% RH is not recommended, but:
Specific manufacturer’s recommendations, when available, should take precedence over the above general recommendations.
The manufacturer’s recommendations for several brands and formats of optical discs are examined in Table 1. As can be seen, storage temperatures as low as -40°C and RH levels as low as 3% are occasionally quoted. The discrepancy between these recommendations and the ISO standard specifications creates some confusion as to what conditions are acceptable and safe.
Recommendations for temperature and RH storage conditions for optical disc formats as provided in the product literature of several different manufacturers of disc media. Note the broad ranges and, in some cases, a temperature limit that is lower than the ISO standard specifications.
Other recommendations for environmental storage conditions for optical discs, summarized by Byers (Byers 2003), are much more conservative than the manufacturer’s recommendations. Several sources advocate storage above freezing (Brown 2003, National Archives of Australia 2004) and most recommend only cool storage. Some suggest following the lower limit of the ISO standard (i.e. -10°C and 10% RH), but only one indicates storage at temperatures as low as -20°C — and that only for DVD-R or DVD-ROM discs. With no experimental data to support these figures, there is uncertainty on how to proceed.
The situation is just as confusing for magnetic tape, for which the ISO standard (ISO 2000) does not recommend cold storage, but does not rule it out:
Storage of tape below 8°C can cause lubricant separation from the tape binder. The manufacturer should be consulted to determine if this is a potential problem.
Unfortunately, most of the manufacturer’s literature that accompanies different formats of magnetic tape provides very little information on storage temperature. A survey of some of this literature indicates that only very general statements about storage temperature are made, e.g. store at room temperature; avoid extremes in temperature; store cool and dry. Also, when dealing with older tape, it is often difficult to identify the manufacturer and many manufacturers have since disappeared from the scene.
It has been stated that problems become evident only with long-term exposure to sub-zero temperatures (Telemetry Group Range Commanders Council 2004, Lindner 2006). The lack of problems with short-term sub-zero exposures is supported by widespread use of tapes over many years in cold environments with very few reports of problems. Although the short-term versus long-term cold storage theory has not been proven, it is interesting to note that standards for the transportation of tape allow tape to get very cold, as long as the time interval is short. For example, SMPTE RP-103 (SMPTE 1995) indicates that the temperature for transporting professional videotapes can be allowed to get as low as -20°C.
Considering all this information, it appears that, for good-quality optical discs, the lower limit of -10°C as specified by the ISO storage standard is more than sufficient to extend the media lifetime beyond the life of the technology. For poor-quality discs, the lower storage temperature of -20°C will ensure this is the case. The ISO lower limit of 8°C for magnetic tapes will increase their longevity, but poor-quality tapes will benefit more from much lower temperatures.
The ISO storage standard also states that the fluctuation limits for temperature and RH when acclimatizing discs should be a maximum of 10°C and 10% RH in any 1-hour period. No specific acclimatization rates are provided in the ISO storage standard for tape, likely because the rates depend on the amount of tape and whether it is in a reel or cassette. For storage conditions of tape, the allowable cycling is ±2°C.