Analog Tape Restoration Service.
We have been very successful using a process of baking tapes at low temperature
Using non gas ovens to prevent moisture build up. We use convection along with dehydrators
over a 8-10 hour period for best results.
Audio tape manufactured in the mid-to-late 1970's is starting to come out of storage now, for remixing and re-issue, and engineers are finding that it won't play. The surface of the tape has become gummy and it sticks to the heads and fixed guides of the tape transport, squealing, jerking, and, in extreme cases, slowing down or stopping the tape transport. This problem has cropped up on all brands of tape, but is nearly always fixable, at least temporarily.
Tapes can exhibit two different problems as a result of long term storage; binder breakdown and lubricant breakdown. Lubricant breakdown, which is fairly rare, leaves a white residue when the tape is run over the heads. Binder breakdown, the more common failure mode, leaves a dark, gummy residue, and is fixable by gentle heating ("baking") of the tape. Fixing lubricant breakdown requires careful cleaning of the tape and possibly applying fresh lubricant. Baking will not solve the lubricant breakdown problem and may make it worse. Make sure you know which problem you have before you put a tape in the oven.
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 but controlled low-humidity storage), the polyurethane used in the binder has a tendency to absorb water. The water reacts with the urethane molecules, causing them to migrate to the surface of the tape where they gum up the tape path during playback.
Short strings of urethane molecules are particularly prone to water absorption, while long strings make the coating mixture too viscous to produce good tape. Middle-length strings are the best, but the tape manufacturers didn't know this at the time, and didn't always know what they were getting.
In the case of Ampex tape, tapes most likely at risk are 406 and 456 manufactured from approximately 1975 through 1984. During those years, Ampex tested the goop they got from their binder suppliers simply by measuring viscosity. Unfortunately, the long and short strings average out, viscosity-wise, to a viscosity about the same as the ideal medium strings, so some tape was inevitably manufactured with an overly great proportion of short urethane strings in the binder. In the worst cases, as little as 3 days exposure to 70% relative humidity can cause a tape to become gummy, but typically, it takes 2 to 15 years under normal, people- friendly ambient conditions. In 1984, Ampex started doing it's incoming inspection with a high pressure gas chromatograph (that's when it was invented), and was able to more accurately determine the molecular makeup of it's binder, and control production much more carefully.
The good news is that the "sticky shed syndrome" resulting from water absorption by the short urethane molecule chains is almost always fixable. The process for repair is commonly know as "baking a tape". The fix lasts about a month under normal storage conditions, and Ampex claims that a tape can be re-baked any number of times without ill effects. Best advice, though, is to make a copy of the tape on first playing, and work with the copy.