Magnetic Tape

Contributions by: Kelci Martinsen

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Magnetic Tape

Magnetic tape is a type of electronic media that was used for audio/video recording as well as data storage.

Technical Details

Magnetic tape is a type of electronic media that was used for audio/video recording as well as data storage. Gibson (1996) explains that magnetic tape can be between 1/8 of an inch to 2 inches wide and can measure between 1/4 mm to 1.5 mm thick. Magnetic tape is composed of multiple layers. Most often, tapes are composed of two or more layers. The first layer of film is called the base film and commonly is approximately 350 micro-inches thick. It has been most commonly fabricated from polyethylene terephthalate, also known as PET, since the 1960’s. Cellulose acetate and polyvinyl chloride have also been used to manufacture the base film layer. Wheeler states that PET will respond to temperature and humidity slightly but overall is an extremely stable compound. Although, it is important that film made from PET be stored in an area that will not expose the film to ultraviolet light.

The second layer is the magnetic layer that is applied to the tape and this layer is the recording surface. The layer is generally 100 micro-inches thick and forty percent of this layer is comprised of magnetic particles. Originally, iron oxide particles were used to give a film its magnetic properties. Chromium dioxide was also used but today, the magnetic particles are often comprised of ferric oxide. The remaining sixty percent is composed of the binding agent and occasionally includes fungicides and lubricants to reduce friction. These are preventative measures take by the manufacturer to prevent damage and degradation.

The carbon backcoat layer can be an additional layer applied to magnetic tapes but it is not present on all magnetic tapes. This layer is often only 50 micro-inches thick. It is added in order for the tape to be wound around a reel more easily. Additionally, the backcoat layer can prevent potential damage caused by shipping. The back coat is also included in order to decrease the amount of static charges that are created when the film is played.

Conservation Practices

Magnetic tape is a fragile media and can be susceptible to harm. Damage can affect the base layer as well as the binder layer and there are various methods to repair damaged tape. The first suggested technique is environmental conditioning or staging. This technique is used on tapes that are suffering from Sticky-shed Syndrome. This syndrome occurs when the polyurethane binder begins to absorb moisture. An increase in moisture causes the structure of the binder to change and eventually break down. Large changes in temperature and humidity can cause an increase of moisture, which in turn causes the tape to display a gummy or powdery residue. If this residue is not removed from the tape, the residue will cause poor playback and may even jam the playback machine. In environmental conditioning, the sticky tape is placed in a controlled area that is set to forty degrees Celsius and has a relative humidity of 20-25% (Gibson, 1996). The tape can stay in this environment for multiple weeks depending on the amount of damage the tape has experienced.

The next method that can be used to repair magnetic tape is rewinding. Rewinding is a less forceful technique and should be used before mechanical cleaning, chemical cleaning or splicing. However, rewinding should never be conducted before baking because it can cause print through as well as a ghost signal ((Gibson, 1996). Print through and ghost signals are images or sounds unintentionally copied from one part of the tape to another part of the tape.

Mechanical cleaning is also a technique that can be used to conserve magnetic tape and cleaning should be conducted with a lint free cloth. Wheeler suggests using 3M Pellon or #610 Tap Cleaning Fabric (Wheeler,2002). Mechanical cleaning should not be conducted using forceful pressure. Buffing is not a method that is accepted in conservation because it can rub magnetic particles off of the tape, which can cause a loss of information.

Baking is another method that is often used on tapes with Sticky-shed Syndrome. Before starting the baking process, the flanges should be left on the reel and the tape should be played all the way through. The tape should be baked between forty-five to fifty-five degrees Celsius for up to thirty-six hours. The tape should then be allowed to cool before it is played. The tape will eventually return to its sticky structure, so a copy of the tape should be made within one to two weeks after baking occurs (Wheeler, 2002).

Increased friction is another factor that can cause increased wear on magnetic tape. Increased resistance can be caused by a loss in lubrication. A silicone solution or Krytox should be applied to the tape in order to prevent further wear and should be applied with wicking or a cotton ball (Gibson, 1996).

Further Reading

• Magnetic Tape Storage and Handling: A Guide for Libraries and Archives by Dr. John W. C. Van Bogart (The Commission on Preservation and Access, Washington, DC, 1995)

References

Gibson, Gerald D. "Magentic Tape Deterioration, Recognition, Recovery and Prevention." Edited by Helen P. Harrison. IASA Conference . Perugia: UNESCO, 1996.

Wheeler, Jim. Videotape Preservation Handbook. Hollywood: Association of Moving Image Archivists, 2002.