5. How Can You Prevent Magnetic Tape from Degrading Prematurely?
Magnetic Tape Storage and Handling
A Guide for Libraries and Archives
Dr. John W.C. Van Bogart
The remainder of this document answers this question. Some of the factors to be discussed are more controllable than others. For example, you can normally decide the storage conditions and level of access to an archive collection. However, you do not always have control over the quality of the tape wind, or the brand, type, and format of the tape media on which the information is stored.
Factors affecting the life of the tape over which you have some control are:
- The care with which it is handled and shipped, discussed in Section 5.1: Care and Handling.
- The quality of the conditions in which it is stored, discussed in Section 5.2: Storage Conditions and Standards.
- The number of times the tape is accessed during its lifetime, discussed in Section 5.1: Care and Handling: Frequent Access.
Other factors that affect media over which you have less control are:
- The physical components of the tape, discussed in Section 2: What Can Go Wrong with Magnetic Media?
- The quality of the tape being purchased; for example, standard grade versus high grade VHS.
- Variation in the quality of the manufacturer; for example, a name brand versus a bargain brand.
- Future availability of system technology to play back the tape. For example, quadruplex videotapes still exist in archives; however, the equipment to play them back is considered obsolete, and it is difficult to find working recorders.
5.1 Care and Handling
Magnetic tape should receive the same kind of care that you would give to a valuable book or important photograph. In general, handle the tapes with care, keep them clean, and apply common sense:
- Use and store magnetic tape reels and cassettes in a clean environment.
- Avoid contamination of the tapes by dirt, dust, fingerprints, food, cigarette smoke and ash, and airborne pollutants.
- Take care not to drop tapes or cartridges.
- Keep tapes out of strong sunlight and avoid contact with water.
- Do not store tapes on radiators, window sills, televisions, electronic equipment, or machinery.
- When the tapes are not in use, they should be placed back on the storage shelf, and stored on end. They should not be allowed to lay flat (reel flanges parallel with the table top) for extended periods of time.
Refer to the Ampex Guide in the Appendix for more information. Magnetic tapes do require some unique care and handling precautions. Because they are a magnetic form of storage, exposure to strong magnetic fields must be avoided to prevent information loss. This is generally not a problem, unless the materials need to be transported or shipped.
Tapes that are frequently accessed may have a reduced life expectancy due to wear and tear. The life of the media may not be determined by data error rates, but by the life of the media housing. In one instance, the life of a tape cassette was limited by failure of the cassette door, not because of any fault of the tape media. How many insert and eject cycles will your media be required to handle? This may limit the life of the cassette.
The more a tape or cassette is handled, the more it is contaminated with fingerprints and debris. It is also exposed to less than ideal conditions, especially if the materials are removed from the building in which they are normally stored.
Every time a VHS cassette is loaded into a recorder, the recorder mechanism pulls tape from the cassette. This mechanism can damage the tape if the guide pins are not properly aligned. Debris on the loading mechanism can scratch the surface of the tape. Also, when a tape is removed from a recorder, the tape must properly retract into the cassette, otherwise it will be damaged when the cassette doors close and the tape cassette is ejected from the recorder. Most of us have probably had experience with a VHS deck that has eaten a tape.
Because of potential damage to the tape, it is important that the tapes be inserted and ejected at areas of the tape that contain no recorded information. A tape should NEVER be ejected in the middle of an important recording.
Transportation of Magnetic Tape
Care must be exercised to ensure that tape collections are not harmed when they are transported. When magnetic media are transported, temperatures should not exceed 110° F (43° C). Collections should be transported in the spring or the fall when outdoor temperatures are moderate, if possible. Properly wound tape reels can survive greater variations in temperature and humidity without permanent damage than can poorly wound tape packs.
Tapes and cassettes should be shipped in the same orientation as they are stored - on edge - with the weight of the tape pack being supported by the reel hub. Tapes that are shipped in the flat position are particularly subject to damage from dropping and other forms of shock. This is especially true of tapes that experience large changes in temperature during shipment or tapes that are poorly wound.
Media should be protected from damage due to shock by packing them in materials that will absorb shock (special packages, bubble wrap), using special labeling, and transporting them in appropriate vehicles. Shock-absorbing packaging will often have the added advantage of providing insulation that helps protect the media from large swings in temperature and humidity.
Exposure to strong magnetic fields must also be avoided to prevent information loss. Some of the detectors used to screen luggage in overseas airports have been known to partially erase tapes. Walk through metal detectors and X-ray scanners do not pose a threat to recorded information. Some hand-held metal detectors can cause problems since they use strong magnetic fields. Refer to the section on Stray Magnetism in the Ampex Guide in the Appendix.
5.2. Storage Conditions and Standards
Storing magnetic tape in a clean, controlled environment is the most important precaution you can take to extend the life of the media. High temperatures, high humidity, and the presence of dust and corrosive elements in the air all affect the physical components that make up magnetic tape and can result in loss of readable data through decreased magnetic capability and deterioration of the binder or backing of the tape. Too low temperatures should also be avoided. In some cases, temperatures lower than 32° F (0° C) may actually harm the media and shorten, rather than extend, life expectancies by risking exudation of the lubricant from the binder, which may clog heads. Rapid temperature changes are also undesirable as they introduce stresses in the wound tape pack. Tapes that are to be played in an environment different from the storage environment should be allowed to acclimate to the new temperature.
Temperature and Relative Humidity
For years tape manufacturers have recommended that you store your tapes in a cool, dry place. In Section 2: What Can Go Wrong with Magnetic Tape?, the reasons behind this dictum were discussed in terms of the chemistries of the tape components: Binder hydrolysis is dependent on the moisture content of the tape, and lower humidity results in lower rates of hydrolysis. Furthermore, this reaction will proceed more slowly at lower temperatures. The latter is also true for the magnetic pigments - they will degrade more slowly at lower temperatures. Finally, to reduce unnecessary stresses on the wound tape that could result in deformation of the backing, a limited variation in temperatures and humidities is recommended. (See Figure 6.)
Storage at high temperatures ( > 74° F; > 23° C) increases tape pack tightness. This results in distortion of the tape backing and an increase in permanent dropouts as wound-in debris is forced into the tape magnetic layer. Many layers of tape before and after the debris can be affected by impressions of the debris. Layer to layer adhesion, known as tape blocking, also can result after long term storage at elevated temperatures.
Storage at high humidity ( > 70% RH) results in increased degradation of the binder as a result of the higher moisture content of the tape pack. High humidities will also cause increased tape pack stresses as the tape absorbs moisture from the air and expands, causing distortion of the tape backing and an increase in permanent dropouts.
Fungal growth is also possible at high humidities and temperatures. Molds can live off the binder polymer and added components. This is yet another cause of binder breakdown in high humidities. Hairy growths at the edges of the tape are a sign of mold. The spores that are produced on this fuzz can get onto the tape surface and cause many dropouts.
Changes in both temperature and humidity can also cause mistracking problems on helical scan recordings (See Section 2.4: Format Issues: Helical versus Longitudinal Scan Recording). Substrates will expand or shrink with changing temperature and humidity just as metals do in heat or cold. The substrate films are not completely balanced in their reaction to these changes in temperature and humidity. In other words, they stretch and shrink differently in length and width directions. This causes a change in the angle of the recorded helical scan tracks. Most of these changes are recoverable by returning to a temperature and humidity close to the one at which the tape was recorded. However, heat can also cause premature aging of the substrate in the form of nonrecoverable shrinking and stretching.
Figure 6. Temperature and Humidity Conditions and Risk of Hydrolysis This figure depicts the effects of humidity and temperature and shows that 15 ± 3° C (59 ± 5° F) and 40% maximum relative humidity (RH) are safe practical storage conditions. A similar diagram appears in ISO TR 6371-1989 that suggests even more stringent conditions (RH 20% max.) for long-term storage of instrumentation tapes. (Source: Ampex. Reprinted with permission.)
Variations in Temperature and Humidity
Generally, the temperature and humidity in a tape storage facility are set to specific values, or set points, and infrequently varied or adjusted. This does not mean that the temperature and humidity in the facility are invariant. Changes in the outdoor temperature and humidity will cause the temperature in the tape storage facility to vary slightly.
If the temperature outdoors is higher than the set point temperature in the facility, the actual temperature in the facility will be slightly higher than the set point. If the outdoor temperature is lower than the set temperature, the actual facility temperature will be lower than the set point. The variations in temperature experienced will be larger at larger distances from the thermostat in the facility. The same logic applies to the humidity level in the facility. Larger discrepancies in the set point and the actual temperature will be observed if one of the walls of the facility is an exterior wall, or if the heating/cooling capacity of the environmental controller is less than that required to properly control the tape archive.
The set point in a tape archive may be constant, but the archive will still experience some degree of daily and seasonal variations in temperature and humidity. A tape archivist must have knowledge of the set points in the archive as well as the variations in temperature and humidity to ensure that the archive complies with recommended storage conditions.
Variations in temperature and humidity can cause tape problems. Tape packs are wound under a considerable amount of tension. This is necessary to maintain the shape of the tape pack. A reel of tape can be permanently damaged if the tape pack tension is too high or too low. If the tension is too high, the tape backing can stretch. If the tension gets too low, tape layers can slip past each other, resulting in pack slip, cinching, or popped strands on playback (see Figure 7). Relaxation of the tape backing can also occur if the tape pack tension is not properly maintained. Relaxation, stretching, and deformation of the tape backing can cause mistracking of a videotape or sound distortion on an audio tape. Every time a tape pack is heated or cooled, the tape pack tension will increase or decrease, respectively. The best way to reduce the degree of tape backing distortion is to store magnetic media in an environment that does not vary much in temperature or humidity.
Figure 7. Bad Tape Wind Examples This figure shows schematic examples of popped strands, pack slip, and a flange pack. The illustrations show a cross-section slice of the tape pack through the hub.
Dust and Debris
Dust, smoke particles, and tape debris present in the environment can get wound into the tape pack as the tape is played, resulting in dropouts when the tape is subsequently played. The lost signal is generally greater than expected from the size of the particle. The record and read heads must maintain very close contact with the tape. A particle of dust on the tape causes the head to ride up over the particle and lose contact with the tape. For perspective on the size of various debris particles compared to the normal head to tape spacing, see Figure 8.
Figure 8. Size of Tape Debris Relative to the Tape/Head Spacing This figure shows the relative size of debris commonly found on tapes and on recorders relative to the tape-head spacing. It is clear from this diagram that even the smallest airborne particles can result in a dropout if the debris gets between the head and the tape.
Polluted air is known to cause problems with books, photographs, and works of art. Airborne sulfides, ozone, and nitrous oxides can cause accelerated deterioration of these objects. Silverware and black and white photographs are blackened by airborne sulfides produced by the degradation of wool fibers, the burning of coal, and bioeffluents. Magnetic tapes are no exception. They, too, are susceptible to corrosive gases in the environment.
Exposure to very low levels of corrosive gases representative of urban office environments has been known to cause corrosion on bare metal particle (MP) and metal evaporated (ME) tapes. In general, these tapes are contained in cassettes, and the cassette shells have been shown to be an effective armor against pollutants in the environment. This corrosion problem is limited to the metal based MP and ME tapes and is not a significant factor in the deterioration of oxide tapes (iron oxide, chromium dioxide, barium ferrite).
If a tape archive is known to contain MP or ME based magnetic tapes, and the tape archive is situated in an environment characterized by high levels of pollutants (e.g., downtown Los Angeles), some precautions may be necessary to ensure that the level of chlorine and sulfides in the archive are at a sufficiently low level. Air conditioning systems may require special filters to remove pollutants if the archive is located in an urban environment.
Current industry standards recommend that materials be stored around 65 - 70° F (18 - 21° C) and 40 - 50% relative humidity (RH) (Table 1). Unfortunately, these recommendations are based, in part, on what is best for recording and playback, and what has historically proven to be good for film and paper storage. They may not be the best conditions for the long-term storage of magnetic media. Standards committees are beginning to consider storage conditions specific to magnetic tape and are recognizing that magnetic tapes benefit from storage at temperatures and humidities lower than those recommended in the past.
|Cuddihy||1982||65°F ± 3°F
18°C ± 2°C
|40% ± 5%|
|SMPTE (RP-103)||1982||70°F ± 4°F
21°C ± 2°C
|50% ± 20%|
|NARA||1990||65°F ± 3°F
18°C ± 2°C
|40% ± 5%|
Table 1. Current Recommendations for Magnetic Tape Storage Note: These are general recommendations that were being made in the 1980s. Standards committees are beginning to recognize the benefits of lower humidities and temperatures for the long term storage of magnetic tape. The above conditions may not be optimal for preserving magnetic tape for as long as is physically possible.
AES, ANSI, NARA, and SMPTE standards committees are coming to recognize that organizations have different storage needs and requirements. In some cases, information older than five years is considered obsolete. In other cases, information needs to be preserved in perpetuity. The optimal storage conditions for each of these requirements differs (Table 2). In the case of short-lived information, storage conditions can be at or near the room ambient conditions of the facility in which the tape collection is housed. No special storage facilities would be required, assuming that temperatures stayed between 68 - 76° F (20 - 24° C) year round and humidity never exceeded 55% RH. For the indefinite storage of information, special storage facilities would be required to maximize the lifetime of the media. No medium lasts forever, so transcription of information from old, deteriorating media to new media would eventually be required; however, storage conditions can be optimized to preserve the current media copy of the information for as long as possible.
Information stored at room ambient conditions would be readily accessible and playable. On the other hand, information stored in deep archive conditions would require a period of time to acclimate to the conditions of the facility in which the information would be played back. As such, the storage condition recommendations are generally referred to as access storage and archive, or preservation, storage.
|Key Feature||Access Storage||Archival Storage|
|Function||To provide storage for media that allows immediate access and playback.||To provide storage that preserves the media for as long as possible.|
|Acclimation required prior to playback?||No.||Yes.|
|Media Life Expectancy||At least 10 years when stored at the indicated temperature and humidity conditions.||The maximum allowed for the particular media type.|
|Temperature set point.||At or near room ambient. In the range: 60 to 74°F (15 to 23°C).||Significantly lower than room ambient. As low as 40°F(5°C).|
|Humidity set point||At or near room ambient. In the range: 25 to 55%RH.||Significantly lower than room ambient. As low as 20%RH.|
|Temperature variations||Difference between maximum and minimum value should not exceed 7°F (4°C).||Difference between maximum and minimum value should not exceed 7°F (4°C).|
|Humidity variations||Difference between maximum and minimum value should not exceed 20%RH.||Difference between maximum and minimum value should not exceed 10%RH.|
Table 2. Key Features of Access and Archival Storage of Magnetic Tape Information represents a general summary of conditions being proposed in drafts of storage recommendations by SMPTE, ANSI, AES, and others.
Access storage conditions are recommended for those materials that need immediate access for playback purposes and for information that has a functional lifetime of ten years or less. Access storage conditions are close to the temperature and humidity conditions of the playback facility - generally room ambient conditions. The single, one-size-fits-all storage condition recommended for magnetic tape in the 1980s and early 1990s generally fit the category of access storage.
Archival storage conditions are recommended for materials that need to be preserved as long as possible. The conditions are specifically designed to reduce the rate of media deterioration through a lowering of the temperature and humidity content of the media. The temperature and humidity are also tightly controlled to reduce the deformation of the tape pack as a result of thermal and hygroscopic expansion/contraction.
Considerable cost is normally involved in maintaining a temperature/humidity controlled archive. However, as mentioned elsewhere in this report, the quality of care a magnetic tape receives should be commensurate with the perceived value of the information contained on the tape. If the information stored on the tape is of great value and must be preserved indefinitely, this could justify the cost of purchasing and maintaining the recommended archive facility. See Section 4.1: Tape Costs and Longevity for more information.
Removal of Magnetic Tapes from Archival Storage
Tapes cannot be immediately removed from archival storage conditions and played on a recorder. Time must be allowed for the tapes to equilibrate to the temperature and humidity of the recorder environment prior to playback. This allows the stresses in the pack to equalize and the track shapes (helical scan) to return to normal. In the case of very low temperature storage, it may be necessary to place the tapes in an intermediate storage environment first to prevent condensation of moisture on the tapes and reduce stresses on the tape pack that would be introduced by rapid temperature changes.
In general, it is the width of the tape that determines how rapidly it will come to equilibrium. A tape that is twice as wide will take four times as long to stabilize to the new environment. Table 3 indicates the amount of time that should be allowed for the tapes to come to equilibrium after significant changes in temperature and/or humidity ("Heat and Moisture Diffusion in Magnetic Tape Packs," IEEE Transactions on Magnetics, 30 (2), March 1994: 237).
|Tape Format||Time for Temperature Acclimation||Time for Humidity|
|Compact audio cassette
8mm video cassette
Table 3. Acclimation Times for Magnetic Media Removed from Archival Storage
A tape that is stored at a temperature or humidity that is significantly below that of room ambient conditions must be allowed to acclimatize prior to playback.
5.3 Refreshing of Tapes
In order to maximize their useful life, tapes may require periodic refreshing. This is a nonstandard term in the tape recording trade that can refer to the retensioning or rerecording of the tape, depending on the community of tape users. To avoid confusion, the terms retensioning and rerecording are preferred to refreshing.
Retensioning is normally recommended where prolonged tape pack stresses could cause damage to the tape. Some manufacturers have recommended that tapes be unspooled and rewound at regular intervals (often three years) to redistribute tape stress and prevent tape pack slip, cinching, and tape backing deformation. For example, retensioning was often recommended for large diameter tape reels, such as the old twelve-inch quadruplex videotape reels, so that tape stresses near the hub of the reel could be relieved. Some tape user communities refer to the process of retensioning as exercising the tape.
Rerecording requires that data be read from and written to the same tape periodically to refresh the magnetic signal and prevent data loss. Rerecording was employed primarily with some older nine-track computer tapes used in the 1960s and 1970s that were susceptible to print through.
Transcription, the copying of one tape to another, has also been referred to as refreshing. Transcription is the preferred term for this process. Tapes purchased today generally utilize small diameter tape reels and high coercivity magnetic pigments so that they often do not require retensioning or rerecording on a periodic basis. In some specific instances, tape manufacturers may still recommend the periodic retensioning of tape (see Ampex Guide in the Appendix, for example). It is best to check with the manufacturer to determine if tape retensioning is necessary.
Finally, refreshing should not be confused with restoration. Refreshing is a preventative maintenance procedure. Restoration refers to the reconditioning of a damaged or degraded tape in order to allow playback. Restoration is a repair or damage recovery procedure.