Tape shedding oxide?
A lot of analogue tape manufactured in the mid-to-late 1970’s is starting to come out of storage for remixing and re-issue and engineers are finding that in some cases the tape won’t play. In these situations, tapes can exhibit a tendency to squeal or stick to the guides and heads in the recorder’s tape path. In severe cases the tape may be sticky enough to even stall the transport.
Here’s where the stickiness comes from. The binder is the chemical compound that holds the oxide particles together and sticks them to the tape backing. Under humid conditions (which means anything other than controlled low-humidity storage), the polyurethane used in the binder has a tendency to absorb water. The water reacts with the tapes’ urethane molecules causing them to migrate to the surface of the tape where they gum up the tape path during playback, leaving a dark gummy residue. However, this effect is temporarily fixable by a controlled heating (baking) of the tape involving a process pioneered by Quantegy (formerly Ampex).
For the baking process, there are several kinds of ovens you can use. One thing you DON’T want to do is stick it in your kitchen oven and turn the heat on 'low'. Most oven thermostats don’t go low enough, don’t provide a stable enough temperature control and a gas flame generates quite a bit of water vapour which is exactly what you’re trying to get rid of. The tape, after the controlled baking treatment, will remain playable if stored in a typical room environment for approximately 30 days, enabling a transfer onto new medium. Providing the heating process has been applied before too much oxide has been initially dragged from the tape’s surface, there will be no loss of output or spoilt EQ.
It should be emphasised that today’s analogue tapes use state-of-the-art binder systems and formulations that offer much improved archival stability. Even so, the utmost care should still be taken to provide proper temperature and humidity control in areas where valuable master tapes are stored.
