If I knew you were coming, I'd have baked a tape.
More info than you ever wanted to know about this.
Information regarding tape baking and why it may be needed.
Excerpts from a Mike Rivers column and other sources
A lot of 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-lengh 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 inevitability 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. Better things for better living through chemistry.
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. To bake a tape, you want to expose it to even heat, ideally at 130 degrees F, with a variation of less than plus or minus 10 degrees. Too cool and the process is ineffective, too hot and you're starting to risk increasing print-through.
There are several kinds of ovens you can use. Controlled humidity Convection ovens are best, but make sure it's large enough to accommodate the size tape reels you 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 good enough temperature control, and a gas flame generates quite a bit of water vapor, exactly what you're trying to get rid of.
Baking time ranges from about 4 hours for 1/4" tape to 8 hours for 2" tape. After you shut off the heat, leave the tape to cool down to room temperature before running it through the deck again.
The technical jargon about the molecules comes from an article by Philip De Lancie in the May 1990 issue of Mix Magazine, where he quoted sources from Ampex. I'm no molecular chemist, just giving credit where it's due (and relieving myself of the responsibility for their errors). P.S. It is really important to let the tape cool down SLOWLY to room temperature, or the stickies will come right back. Turn off the heat, leave the oven closed, come back in 3 or 4 hours. When it's completely cool, then you can play it again.
Polyester Tape + Baking
Polyester or mylar based tape is opaque when held to the light and viewed from the side. Polyester tape will deform permanently before it breaks, so be careful not to stretch it. Acetate backed tape is transparent when viewed this way.
Generally, polyester tape has a superior tape base, and oxide/binder formulations have steadily improved over the years to prevent oxide from flaking off. With one big exception.
Serious problems occurred on tapes manufactured (mostly those made in the US) from roughly 1975 to 1985. The problem is known as "sticky shed syndrome" and arose when tape manufacturers were suddenly forced by the U.S. government to abandon the use of a carcinogen in analog tape. Tape manufacturers hurriedly changed formulations during that period, and thus we face the problem today.
Tapes with "sticky shed syndrome" leave a waxy residue on rollers, heads and guides, which destroys high frequency response and can eventually cause a tape player to stop entirely.
If a tape is suspected of having "sticky shed syndrome," it should be treated before playing. Playing untreated tapes damages them, because it smears part of the binder containing the oxide and reduces high frequency response permanently.
Any tape manufactured (mostly those made in the US) during the period from 1975 and 1985 is suspect. To diagnose the tape, create a roller only path, and rewind the tape briefly. Good tape comes off the supply reel at a near perfect tangent, but sticky shed syndrome will cause tape to stick slightly to the pack as it is pulled away. You can also hold a finger against the oxide side during rewind (be careful - it's easy to cut your finger), and if you detect any friction, or if your finger picks up any residue, then the tape has "sticky shed syndrome." Stop rewinding.
Give the binders time to re-adhere to the base film, and allows residual lubricants deep in the layers of the tape to exude to the surface to make the tapes runnable.
A temperature between 120 and 130 F is recommended by most authorities. When 3M made audio tape, they suggested 125 F for 4 hours for 1/4", and 7 hours for 2" tape.
The most important thing to remember about baking tape is to provide a constant and proper temperature, and for this reason a good thermometer is essential.