Demonstration of the ‘sticky-shed’ syndrome exhibited by Ampex (and other) tapes that were manufactured during the 1980s and 1990s.
The polymer that was used to bind the tape’s magnetic oxide dust particles to its substrate had been found to absorb moisture over time, becoming sticky, and shedding oxide/binder goo over tape heads and transport elements. This causes squealing, which modulates the audio signal and renders it unusable. Sticky tapes may even completely stall the tape recorder’s transport, running the risk of motor damage.
According to the manufacturer, baking the tapes at 55 degrees Celsius for a few hours dries out the binder sufficiently for the tape to be played. Opinions vary over how many times a tape can be baked before its mechanical structure and/or audio content become degraded, but most people simply want the tape to play reliably for a single pass, while they capture the recording to a modern digital format.
This video shows me baking tapes in a conventional domestic fan-assisted electric oven, while monitoring temperature with an external meter and thermocouple.
UPDATE 2018: I now ‘bake’ tapes using a food desiccator – this is a very simple and popular solution, and I figure it’s less likely to ‘fry’ tapes due to momentary operator inattention!
CAUTION: This process has worked for me, but if you are considering baking your own tape collection I recommend that you read all the literature available on the Internet before deciding whether or not to proceed. A good starting point is Wikipedia’s article on Sticky-shed syndrome.