My previous blog post was all about the Vinegar Syndrome that can be found in some types of acetate audio tapes. An even more pervasive issue with open reel tapes is “Sticky Shed Syndrome.” The following will discuss what it is, the cause of it, and how we deal with it from a preservation standpoint.
Sticky Shed Syndrome is a problem in high-output tapes manufactured from the mid 70’s to mid 80’s. One can detect that a tape is suffering from this issue if, upon playback, the tape produces a squealing noise, leaves behind sticky residue on the contact points of the tape machine, and/or slows the playback speed of the tape. This is a serious problem because not only does it make the tape unplayable in most instances, but doing do causes severe physical wear to the tape.
Tape is constructed in layers, consisting of a layer of magnetic oxide particles that contain the audio recording, held to a substrate layer of polyester (or some other material) with a binder material. There is sometimes a back coating layer which consists of carbon particles held to the substrate with another binder layer.
The cause of the shedding issue is a reaction called hydrolysis. Basically, the tape binder is absorbing water from humidity in the air, causing the magnetic oxide particles to separate from the substrate. The loose oxide particles easily shed off the substrate upon playback in this condition.
To explain this reaction in more chemical terms, I will reference a recent publication by Sarah Norris in the ARSC Journal, Vol 41, No. 2:
“The polyester polyurethane binder used in audio tape is formed by a reversible chemical reaction, esterification. In esterification, a carboxylic acid and an alcohol react to form an ester, with elimination of water. Hydrolysis is the reverse of esterification. In this reaction, water breaks the ester linkages to reform the carboxylic acid and alcohol. In the hydrolysis of polyester polymers , not all the ester linkages are broken. Instead the reaction produces smaller molecule segments with both acid and alcohol ends.”
In dealing with this issue from a preservation standpoint, there is a short-term solution to the problem used by audio engineers called baking. Baking involves putting the tape in a controlled, low-heat oven or food dehydrator for a period of time. This process removes moisture from the tape, and effectively makes the tape temporarily playable again. It allows us to get a successful digital transfer without damaging the original tape.
To explain the chemical reaction of the baking process, I will again reference the Norris article:
“One study says that baking reverses hydrolysis, thus re-forming ester linkages and reassembling binder molecules (Bertram & Cuddihy, 1982). The elevated temperature and relatively low humidity, RH, inside a hot oven drive excess moisture from the tape, creating an environment in which binder hydrolysis can be reversed, stickiness reduced, and playback performance improved. These effects are understood to be temporary because hydrolysis resumes after baking, presumably proceeding for several weeks until a critical mass of binder re-degrades and the tape again becomes unmanageably sticky. High temperature and low RH work together to reduce the tape’s water content through processes of evaporation and equilibrium.”
Now for a walk-through of the baking process, with pictures:
Finally, the tape is left for a 24-hour period to return to control room temperature and is transferred back to the correct size reel before being played back. If problems persist, the tape will go back into the food dehydrator.