Baking Audio Tapes Which Have "Sticky Shed Syndrome"
Copyrighted by Dave Luepke
Many audio tape formulations from the 1970s suffer from a phenomenon commonly known as "sticky shed syndrome". Such tapes were manufactured into the 1980s, until the problem was discovered. They were typically made with a polyester base and often with a "backcoating", and generally fall into the category of "high output tapes" designed to maximize signal-to-noise ratio and minimize distortion.
The primary problem with these tapes is that the polymer binder used to adhere the oxide to the base slowly absorbs moisture over time. The result is that the binder becomes somewhat viscious, and as the tape is pulled through the transport, the binder (and magnetic oxide) is stripped from the tape and adheres to the machine's parts, notably the heads and stationary guides. Unfortunately, a person cannot tell if a tape will have this problem simply by looking at it or running a cotton swap on it. Generally, the first indication is that the tape machine rapidly slows down and possibly begins to squeal, as a result of the deposits which quickly build up on its guides and heads.
There are two critically significant problems which result: 1. The stripped material includes magnetic oxide (not just binder), which is the material which holds the recorded information; thus, the "goo" which is deposited on the machine parts is in fact, lost signal; and 2. Parts of the tape machine, notably the heads and stationary guides, but also rotating parts over which the tape passes, become contaminated ("caked" would be a better word) with the stripped off material, interfering with their performance. While tape transports can be cleaned, magnetically stored signals which are stripped away from a tape cannot be replaced, and are lost forever.
Some years ago, it was found that there is a temporary "cure" which would allow these tapes to be played safely (if only for a couple of weeks). This solution is to "bake" the tapes at a low temperature for some period, as a means of evaporating the moisture out of the binder. This temporarily restores the integrity of the binder, allowing the tape to be played without significant amounts of oxide and binder being stripped away. It is important to re-state that this is a temporary solution. Also note that repeated playings may still result in unacceptable signal loss and deterioration.
I have researched this topic for a long time, and along with my own testing, I have developed a reliable approach to baking affected (or potentially affected) tapes, so that they can be safely played and transferred to another media.
During this research and testing, I found five primary factors which are critical to a successful result:
1. Do not re-wind or play the tape. Some sources recommend that you have a good, fairly loose tape pack (as opposed to a tight re-wound condition). While a loose (played condition) pack may be desirable, re-packing the tape has a high probability of damaging it. This is discussed further below.
2. Avoid stray magnetic fields. It is clear that whatever "oven" is used for baking, or where it is placed, it must be free of constant magnetic fields which can adversely affect the quality of the finished tape. Clearly, exposing a magnetic tape to a 60 Hz (power line) AC field for several hours will be detrimental to the recorded signal.
3. Baking temperature. A too low or too high temperature can cause undesirable results. At too low of a temperature, the moisture in the binder will not evaporate. At too high a temperature, problems may develop which affect the quality of the tape or the reel which holds it.
4. Baking time. By controlling the temperature, the tape can be safely baked for an appropriate amount of time without concern about over-baking. The lower temperature, the longer the tape can be baked without damage.
5. Window of opportunity. There are actually two windows involved here. One is the baking environment, meaning that it is not desirable to bake a tape when the air is too humid or too cold, etc. The other is in regard to when the tape can be transferred to another media. It does no good to bake a tape if it cannot be transferred until several weeks or months later. A reasonable schedule of events must be planned in advance. Ideally, the tape should be played and transcribed within a couple of days after baking, but certainly within a couple of weeks, depending on humidity conditions.
Re-winding or playing the tape. As mentioned above, it is critical to note that some sources of information about the process of baking tapes include advice with regard to the tape "pack". Specifically, it is sometimes recommended that the tape should be in a good "played condition" before baking it, to help ensure that the tape doesn't become stretched or pinched during baking.
While an even, loose pack is desirable, you should not wind the tape in order to achieve it. It is quite possible that there are layers of tape which are stuck together, and winding the tape is likely to cause irrepairable damage as sections of oxide are pulled off the base and remain stuck to the adjacent layer of backcoating or base, thus ruining that part of the tape. I advise people to leave the pack alone Here is an example:
Even if you "hand pull" some of the tape, or maybe play a minute or so to see how much oxide is shed or stripped, it is critical to be aware that the condition of the tape is likely to vary throughout the reel. In one example, the worst condition was near the hub, so even running most of it through the machine (bypassing the heads and non-rotating guides) would not have uncovered that hidden disaster.
All-metal reels. A second recommendation which you may have read about is the need for all-metal reels. In my experience, this is not necessary. I have baked tapes for up to six hours on plastic reels, with no adverse effects. In fact, if you have a tape on a plastic reel, and then wind it onto a metal reel, you may irrepairably damage the tape, as described above.