Top tips for recovering oral history and sound collections from failing magnetic audio tape.

by Scott Temby

A vast amount of oral history and other sound collections are still kept on audio cassette and ¼-inch open reel audio tape. Like many forms of magnetic tape media, analogue audio tape will often suffer from various forms of degradation as it ages. Degradation can occur while tape is “left in a drawer” (for some, these storage conditions are considered reasonably good!), and is exacerbated by poor storage environments, such as open shelves in a basement in a city with a hot and humid climate. If the media is left to its own devices for too long, collectors run the risk of diminishing sound quality, or even worse, the content may be completely unrecoverable when the decision is made to migrate the collection.

In this post, I want to share with you an overview of the two most common afflictions of magnetic audio media - “Sticky Shed Syndrome” (where the tape binder degrades causing stickiness) and mould/fungal growth. I also wanted to provide some guidance as to how these conditions can be remedied.

Firstly, sticky shed syndrome (or “Binder Hydrolysis”) is caused by the absorption of moisture (hydrolysis) by the binder*. This causes the tape to stick to parts of the tape path (usually the heads), resulting in squealing or screeching sounds and tape damage. Sticky shed is not always visible to the naked eye, and may only be detected when the item is played. If left too long, eventually the recording surface will completely shed from the tape, rendering the media useless.

There are specialist techniques to temporarily rejuvenate failing media to enable the replay and digitisation of the content. The most common approach is incubating (“baking”) the tapes at a moderate temperature in a carefully controlled scientific chamber, which also manages pressure and humidity. This often relieves the symptoms, by forcing the water out of the binder, temporarily allowing playback. It is important that such tapes are migrated (in the past, to another analogue tape, but now to a digital file) as quickly as possible after baking, since the fix is not permanent. The time it takes for a tape to revert to an unplayable condition varies with the tape type and nature of degradation.

Sticky shed is so common in professional analogue audio recording tape, several standard metadata schema for this medium include fields for “Tape Baked”, “Date Baked”, “Baking Temperature”, “Hours Baked” and “Hours Cooled” alongside the transfer/digitisation date**. This gives a reference for the nature of the digital object in context of  the condition of the original analogue artefact. It should be noted that there are a few side-effects to this procedure, notably print through, magnetic field weakening, and shortening of overall life-span. Of course, these side-effects should be weighed against the alternative, which is to do nothing while the tape condition deteriorates further.

Mould or fungus infestation of ¼-inch and audio cassette is another common cause of several replay issues, largely due to the disruption of tape transport. For example, mould on the recording surface of the tape can lead to an increased gap between the replay head and the tape, which results in a loss of treble response. It can also contribute to speed irregularities and alignment errors. In addition, if left too long, mould attacks the materials which make up the tape, and can result in the “scarring” of the tape surface so that even once cleaned the recovery of the content can be affected. Mould can be seen on the media as white spots (or lumps or hairs depending on the degree of infection), however, mould may also appear on the flat recording surface and escape detection on a visual assessment. 

Like sticky shed, there are specialist techniques that can be used to rejuvenate infected media, and unlike sticky shed, if done properly, the results can be permanent. Importantly, mould needs to be removed before playback is attempted, both to optimise sound quality and prevent spreading mould spores to “sterile” equipment.

Clearly, it is important for both curators and digitisers to identify mouldy tapes quickly so that they may be isolated to prevent the spread of spores. If there is any indication that mould spores may still be active, it is best to first deactivate all spores by placing the tape in a sealed plastic bag with several packets of silica gel. By removing all moisture, the spread of active mould is deterred and the dormant mould may then be removed.

Mould can be removed from the tape surface and reels using lint-free wipes, however, extreme care needs to be taken so as not to wipe the oxide face excessively. This can be done by hand, or achieved by using specially designed tape cleaning devices. In either case, care should be taken to capture any mould particles which come free from the tape as it is cleaned, so as to reduce the chance of contamination of controlled audio environments. Of course, OH&S is vital, so for operator safety, ensure a face mask and gloves are worn whenever the media is being handled.

For particularly stubborn mould on reels, isopropyl alcohol may be used as a cleaning agent. If there is a large amount of mould present, or a large volume of tapes to be processed, performing the cleaning under an extraction hood is advisable. Often, cardboard tape cases encourage the growth of mould or fungus, so these should also be thoroughly cleaned, and clean tapes transferred to alternate storage (plastic, sealable bags are ideal, though do not offer protection from mechanical damage). The original cases should also be bagged and stored, as they often form a valuable part of the physical artefact, as far as auxiliary information is concerned***.

Not all audio cassettes or ¼-inch audio tapes were created equal, and different brands will endure the test of time better than others. At the same time, not all media will have been treated uniformly, and unfavourable conditions will create challenges for a collection. It is therefore important to first understand the issues your collection may be facing, and then to understand what can be done to address those challenges and optimise recovery as you move to digitise the collection. Finally, if attempts are made to rejuvenate the media, all of the collection stakeholders should be made aware of the consequences of intervention. Some collections only get one chance at recovery, so that one chance should be the best chance.

* The binder is the layer which holds the oxide particles on the backing, and facilitates playback.
** For an example of this scheme, see A Workflow Study of Migrating Analogue Multi-Track Audio Recordings to Digital Preservation File Sets by Toby Seay, available in IASA Journal no. 39, June 2012.
*** Note, in certain workflows, it may be desirable to scan or photograph these cases and include them with the digital audio files.