Magnetic Tape Recording: Forever?

With that in mind, we will review what we know, what we think we know, and offer some recommendations for the long term storage of prerecorded magnetic tape.

1. The magnetic signal.

2. The binder system integrity.

3. The backing integrity.

1. The heat required to destroy the magnetism in a magnetic material is known as its curie temperature. The curie temperatures of the materials used as the magnetic media in tapes range from 250°F and higher. The lowest one being the chromium dioxide pigments at the 250°F level while the others are above 800°F. In either case, the other materials involved in the tape construction would be rendered useless before the 250°F temperature was reached. Basically this means, unless exposed to a fire, there is little danger of failure of the magnetic signal due to temperatures expected in storage conditions.

2. Obviously external magnetic fields can affect the signal on the tape or it never could have been recorded or erased in the first place; however, what is known as the tape's coercivity must be taken into consideration when trying to assess what levels would be a factor. The coercivity is a measure of the magnetic field strength required to affect erasure or polarity reversals in the magnetic material. Coercivity is expressed in oersteds. Virtually all commercially made video tapes have a coercivity of 250 oersteds or higher, with current day video tapes being 500 oersteds and above. This means an external magnetic field of 50 oersteds or less will have very little erasing effect on the tape.
Because the strength of a magnetic field falls off by the square of the distance from the source, the mere spacing between the tape and the source offers considerable protection. Examples of some field strengths for reference purposes are as follows:

               The earth's magnetic field 0.6 oes.

               The field strength directly on the case of an electric hand drill 10.0 oes.

     The field strength 3" away from a 1500 oersted bulk deguasser under 50.0 oes.

Even after long term exposure, the signal deterioration effects from the stray magnetic fields likely to be found in a tape storage area, or on a home user's shelf, would be negligible.

One signal stability problem, resulting from magnetic fields, is one that could be called an unwanted signal gain and is more commonly referred to as "print through." This is a phenomenon whereby the long wavelength recordings, as found in the audio recording portions of the tape, will cause magnetic particles to become magnetized by the recorded signal on an adjacent layer. In effect, the recording on one layer is imprinted onto the adjacent layers in the tape pack. Although of low level, the effect is noticeable on playback as a pre-echo or post echo. This can happen because within the tape pack are some magnetic particles with a coercivity lower than the average value, and as a result they are more easily magnetized by weak signals from the other layer. The level of the "print through" signal is dependent on the number of low coercivity particles, distance between layers (backing thickness), recorded wavelength, and time. The ratio of the wanted signal level to the print signal level with time is known as the signal to print characteristic of the tape. The higher the number, the better. The signal to print characteristic of a typical video tape is shown in Figure 1. As you can see from the graph, there is about a 2 to 3 dB loss per decade of time. With the tape shown, the print would not be considered objectionable until well beyond 100 years and still very usable out to 1000 years.

3. Based on tests that have purposely exposed magnetic recordings to microwave radiation and X-rays, the intensity levels below those that adversely affect a human being will have no adverse effects on the recorded signals.

4. A magnetic tape when exposed to high mechanical stresses can exhibit signal losses, but the stress level that could be reached in a storage situation would have no more effect on the signal retention than that experienced by one playback pass on the machine.

The strength of this plastic comes from the design of the individual molecules making up the heart of the material. These molecules may be considered long chains of atoms that give the molecule a rope like shape. If the binder material could be seen with the naked eye, it would probably look like a bunch of long intertwining spaghetti. Like most plastics, this material is very stable and will weather the tests of time, but unlike the permanent siding on a house, this application requires that the material maintain an integrity beyond mere looks.

The binder material is hygroscopic, which means it will absorb or attract moisture from the air. When this occurs to a large enough extent, a hydrolysis action begins to take place. This means the binder molecules will react with the moisture, and at some point the long chains of spaghetti will break down into shorter chains, thus losing some of their strength. The degree to which this happens is dependent upon the moisture level, temperature, and time. When degradation has proceeded far enough, the tape will be susceptible to shedding and head clogging during playback on the machine, and at some point could reach a stage of complete breakdown to the buzz saw action.

The time and tolerance levels that will ensure complete protection are not theoretically clear due to many variables involved, but the studies that have been done in this area, relative to magnetic tape, have been valid enough to offer advice on setting some guidelines. These studies indicate that from a pure binder integrity point of view, the exposure to relative humidities below 50% RH and temperatures below 70°F will afford a very high probability of successfully retrieving the recorded information after very long term storage.

The polyester is a plastic material made from resins with, most likely, some additives; and, as with the binder, it also is hygroscopic in nature and subject to degradation due to high humidity arid temperature. This degradation reduces the strength of the backing and, along with the binder deterioration, could increase the potential of adhesion failure (separation of coating from backing). Again, with temperatures below 70°F and relative humidities below 50% RH, the probability of successful long term storage is very good from this point of view.

Another factor that must be taken into account is the dimensional stability of the backing. In the manufacture of the backing, the polyester is put through a stretching process in arriving at its carefully controlled caliper. This produces a memory effect that, along with the thermal and hygroscopic expansion characteristics, is an important factor in what the dimensions of the tape will be after long term storage. When the backing is exposed to higher temperatures than that found in manufacturing, the material will tend to shrink with time and the result is not reversible. This dimensional change is of little consequence before a recording is made; but, after recording, a dimensional change could cause tracking or timing error problems upon playback. This is particularly true of helical scan formats.

As a result, 3M incorporated a stabilizing process into the manufacturing of all their helical tape backings which significantly improves the tolerance to time and temperature effects. Thus, there are two types of backing that may possibly be encountered in the field; namely, stabilized and non-stabilized. Based on laboratory tests conducted at 72°F over a 10-month period in comparing the two backings, the non-stabilized backing tapes reached a 0.03% dimensional change in the 10 months, while it is projected that the stabilized tapes will not reach that level until after 100 years. The combination of times and temperatures that will result in giving the same 0.03% dimensional change on the stabilized backing is shown in the following table:

The degree to which this phenomenon will affect the information retrieval after long term storage is dependent on the recording format and the machine's designed-in ability to handle tracking and time base errors. In the quadruples format this presents no tracking problem, but will increase head banding effects. In the 1 " and other formats with auto scan tracking and time base correction, this should not present a problem.

At temperature and humidity conditions above 75°F and 70% RH, the chances of fungus growth developing on the tape during storage is very high.

There have been reports of some audio tapes made with acetate backing that were considered unuseable after 10 years storage due to the backing becoming brittle and cracked. The storage conditions were known to have been very dry for most of the period. A few tapes, known to have been under extended periods of 80°F and 80% RH or more, exhibited blocking (sticking between layers) and adhesion failures (separation of coating from backing) when taken out of storage after 15 years.

Adhesion and blocking failures in the areas of physical splices have been reported on some tapes that presumably had been stored for approximately 25 years in good environment.

Actual signal measurements on tapes before and after many years of storage are difficult for reasons of accuracy, and are often in question as to whether the heads, the electronics, the test equipment, or the tape itself has changed. However, when two signals of the same frequency are measured for level changes between them after a long period of storage, the results are meaningful. This is the case when making signal to print measurements. Several reports of the print through levels found on tapes after long term storage have confirmed the initial tests used for predicting the time vs. print characteristic.

Storage reports received from various operations since that time, involving pre-recorded quadruplex tapes ranging in age from 10 to 20 years, have been similar in nature and most have involved color recordings. Of course, the magnetization on the tape cares less whether the recording is in color or black and white.

Early 1 " tapes, utilizing non-stabilized backing and recorded on one of the older 1 " formats, exhibited a relatively high percentage of time base error problems after only 2 or 3 years of storage. The storage conditions were believed to have exceeded 80°F on occasion and the resultant time base error was beyond the machines ability to correct via the tension control. This prompted 3M to take corrective action. This led to their introducing the stabilized backing in the early 1970's.

Pre-recorded U-Matic tapes over 10 years old have been evaluated at 3M and judged to be of acceptable quality with only minor problems. Numerous 1 " Type C recordings up to 8 years old have also been evaluated and found to be of high quality with virtually no problems that can be attributed to storage.

1. Recorded with a quality meeting the machine manufacturer's specifications and complying with the appropriate standard.

2. A smooth, even, and full length uniform wind at proper machine tensions.

3. No bad edges.

4. Free from excessive moisture absorption.

5. Free from external contaminants.

6. In the case of reel to reel tape, the end properly secured with hold-down tab material.

7. In its original clean container or equivalent.

8. No visible physical distortion in the tape pack.

9. In the case of cassette tape, a properly functioning and clean cassette.

10. Proper labeling and identification.

11. No physical splices.

In order to meet some of the conditions just outlined, those tapes that have been exposed to considerable usage and have been in suspect environments should be given a cleaning pass through a properly operating tape cleaning device if possible.

Having arrived this far, there is one big advantage working in everyone's favor; the tape merely has to sit there. The only further attention needed is controlling the atmosphere in which it will rest. We recommend one of the following be adopted, depending on the situation:

1. If only video tape is to be stored, and in an isolated area where no access to the area is expected or required during the life of the storage, we recommend maintaining a reasonably constant temperature and humidity below 70°F and 50% RH.

2. If video tape is to be stored along with mag film, photographic film, or any other type of media, and no access to the area is expected or required during the storage life, we recommend maintaining a reasonably constant temperature below 70°F and humidity between 30% RH to 50% RH.

3. If the storage area is to be made available to frequent access, we recommend the temperature and humidity be maintained within the same limits recommended for the operations area; namely, 70°F plus or minus 5% and humidity 50% plus or minus 20%.

One further consideration is the video tape machine itself. With the rapid development of video recording technology, the possibility of the current machines being readily available in 50 to 100 years is highly unlikely. It therefore requires the archivist to store and maintain the machines necessary for playing back the recorded format. The particular machine manufacturer involved may have recommendations in this regard, but the same storage and operating conditions outlined in item 3 should also suffice for the machines.