The term 'magnetic media' is used to describe any record format where information is recorded and retrieved in the form of a magnetic signal.
The common types of magnetic media are:
Magnetic tape consists of a carrier of plastic film coated with a matrix containing magnetisable particles. The matrix also contains a plastic or resin binder, and other ingredients such as lubricants and fungicides. Sometimes the tape is coated on the reverse side with an antistatic material to reduce the build-up of static charges, and to improve its winding capability.
Magnetic hard disks have a metallic base, usually of aluminium. The base is coated on both sides with a matrix similar to that of magnetic tape.
Disk packs, which have a wide application in computing, consist of a number of hard disks stacked together around a central spindle. They require a special recording and playback system with many pairs of read/write heads.
Floppy disks and diskettes consist of a plastic base with a magnetic matrix on one or both sides. They are enclosed in a rigid, plastic protective jacket, which does not easily flex or bend. A slot in the jacket allows the read and write head to make contact with the disk.
All materials degrade over time. We cannot control this inevitable deterioration, but we can slow it down. Some materials are inherently prone to deterioration, while others will only significantly degrade if they are stored in poor environmental conditions.
Below are examples of the types of deterioration to which magnetic media are prone.
Because magnetic media store information by the alignment of magnetic particles, even a small external magnetic field can cause information loss on a tape or disk if it is in close proximity for long enough. Magnetic fields can be generated by items such as fridge magnets, magnetic screwdrivers and most machines with electric motors.
The degree of risk depends on several main factors:
The effect of a magnetic field decreases with distance. This means that running a vacuum cleaner past the shelves will probably not cause any damage, whereas storing tapes or disks close to a large electrical generator could result in serious loss.
Paper and cardboard enclosures are unsuitable for the storage of magnetic media, as they tend to generate dust.
Tapes should be stored in cases made of non-magnetic material, preferably an inert plastic such as polypropylene. Polyvinylchloride (PVC) is unsuitable because it contains substances that may be damaging. Cases should have fittings to hold the tapes in position by the hub. They should be strong enough to protect the cassettes from physical damage, and they should close tightly to keep out dust particles.
Reels or cores used for winding tapes should be clean and free from cracks or sharp edges. There should be slots in the flanges of the reels to prevent bubbles of air from being trapped between the layers of tape on the reel. Reels should be made of aluminium or a stable plastic such as polypropylene (not PVC).
Floppy disks and diskettes should be stored in protective envelopes that have a non-abrasive surface and are resistant to the build-up of static electricity. Tyvek envelopes are widely available and are suitable for this purpose.
Qualified staff should check storage areas to ensure the absence of magnets or magnetic fields that exceed acceptable limits. Walls, floors, wiring, and all storage and electrical equipment within the area must also be checked.
Storage areas should be free from potential sources of dust, such as typewriters, paper shredders, printers and carpet. Measures, such as the installation of an air-lock, or the maintenance of positive internal air pressure, should also be taken to prevent dust entering from the outside.
Magnetic media should ideally be stored in closed metal cabinets to provide extra protection against heat and dust. However, if adequate environmental controls are in place, storage on open shelves and racks is acceptable. All storage equipment should be sturdy, allow tapes and disks to be stored vertically, and most importantly, be electrically grounded.
Magnetic media should be stored in a temperature and relative humidity range of 18–20°C, and 35–40%, respectively. Under these conditions the natural deterioration of the objects can be slowed. In some instances deterioration can be further slowed by storage under lower temperatures. It is important that these environmental levels are stable. Mould will start to grow at around 60% relative humidity, and if the humidity fluctuates more than 10% in 24 hours or the temperature is too high, the items will be subjected to physical stresses that will accelerate their deterioration.
Exposure to ultraviolet (UV) light will also hasten degradation. Fluorescent tubes with UV-filters should be used wherever possible in storage areas, and turned off when not in use. UV light can be easily measured with a light meter, and levels should not exceed 75µW/lumen. An ideal storage area would have no windows, but if windows are present they should be covered with curtains or blinds.
Cleanliness is very important in records storage areas, both for the sake of the records, and from an occupational health and safety perspective. Never allow food or drink to be taken into a records storage area, and ensure the area is cleaned regularly. Insects and rodents, once attracted to a records storage area by food, may begin to eat the records.
Dust, heat and moisture can cause irreversible damage to magnetic media. Therefore storage areas should be fitted with special alarm systems, such as VESDA (Very Early Smoke Detection Alarm). Use of these systems can provide much earlier warnings of fire or high dust levels than conventional detection systems, and also minimise the need for large amounts of water to enter the storage area in the case of a fire. Fire detection and suppression technology is rapidly developing, and advice should be sought at the time the system is required to ensure the best method is employed.
The information held on magnetic media can only be processed or read by mechanical means, therefore it is essential that equipment is maintained in good condition: the use of poorly maintained equipment can actually cause damage to records. The heads, disk drive and tape drive elements of playback and recording equipment should be cleaned regularly according to manufacturers' recommendations.
Some tape manufacturers also recommend the exercising of tapes to improve their life span. Problems such as creases or folds in the tape may build up as the tape pack sits in storage. Exercising can reduce the stresses that cause these problems and may also reduce the danger of print-through.
Exercising involves winding the tape slowly through its entire length at playback speed, without stopping. The process should be carried out in the same environmental conditions in which the tapes are to be stored. Tapes which are to be moved to a different environment for exercising should be allowed a period of 24 hours to acclimatise to the new environment before exercising them. It is generally recommended that exercising be carried out at least every three years.
To minimise deterioration due to handling and use, copies of important and frequently used tapes should be made for reference purposes. Ideally, a preservation master copy, a duplicating copy and a reference copy should be produced, and clearly labelled as such. As a disaster preparedness measure, the preservation master copy should be stored in a different location to the others. The duplicating copy may be used to produce further reference copies when required.
Long-term preservation of magnetic media is affected by two major factors: the intrinsic instability of the media; and the likelihood of the hardware required to read the media becoming unavailable. Even if tapes or disks made today are in excellent condition in 30 years time, the machines required to play them will almost certainly have been superseded long before, and for all practical purposes the records will be unusable. Beta format videotapes are a good example of this problem. Once very common, they have now been entirely superseded by VHS format tapes and it will soon be very difficult to view a Beta video.
The main prospect for long-term retention of the information held on magnetic media seems to be in regular copying or data migration, thus maintaining a good quality signal that can be read using available equipment. Copying can either be to fresh tape or disk, or to some other machine-readable format such as CD-ROM.
Copying to analog tape will involve some loss of signal quality at every copying stage. This may be significant after as few as two or three copies. The problem may be overcome by copying to a digital format such as digital tape (DAT for audio tapes) or optical disk. The tape used for digital recording is no more permanent than the tape used for analog recordings but the information can be copied many times without a significant loss of quality. The problem does not arise with computer tapes as they are already recorded digitally.
Please contact the Agency Service Centre if you require further advice on protecting and handling magnetic media.
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