What's It All About?
By now, every audio-professional has encountered the "sticky recording tape" problem, and has heard that something referred to as "baking" will fix the problem. The truth of the matter is "yes" and "no". Yes, the problem can be temporarily corrected by the "baking" process, but no, the cure is not permanent.
The problem goes back to the 1970's when most tape manufacturers made an ill-advised decision to change the formulation of the "binder" used to glue the magnetic tape particles to the plastic base material. Unknowingly, the new formulation attracted moisture, and eventually enough accumulated to make the tape go "sticky".
The purpose of "baking", is to drive out all the moisture that the tape binder has accumulated, which is what caused it to go sticky in the first place. This will give a few weeks to a few months of "normal" tape functioning... enough time to transfer the affected recordings to a stable medium before the problem reappears when more moisture is absorbed.
Essentially, the process is this... put the tape in an electric oven with an accurate temperature control set to 130 degrees Fahrenheit, for about 4 - 6 hours. DO NOT USE a gas oven... gas produces water vapor when it burns, and that is what you are trying to drive out of the tape. Ensure the temperature is accurate with a lab quality thermometer or use a "known to be accurate" oven. A slight variation (plus or minus 5 - 10 degrees Fahrenheit) above and below this temperature is acceptable.
It is the moisture absorption that caused the problem, and to correct it, you have to drive off the moisture that accumulated. The oven technique is the fastest and probably the easiest way to do this. Handling is pretty much normal after the baking, and will probably last for some time until the tape accumulates enough moisture to go sticky again, whereupon re-baking will temporarily fix it again. It is suggested that you do a good transfer the first time, and file the bad master away somewhere, just in case you ever need it again, perhaps in a sealed plastic bag with some desiccant material inside, like a bag of silica-gel, to keep it dry.
Check out the Issue 12 1995 issue of EQ. There are notes on this subject in the EQ & A column from an Ampex engineer and also from BASF Magnetics.
Author and audio columnist Mike Rivers has compiled the following on the subject:
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.
To bake a tape, you want to expose it to even heat, ideally at 130 degrees Fahrenheit, 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.
I've have used as high as 150 degrees Fahrenheit for this process on many tapes, with excellent results, however new information is appearing that points to a lower "standard" heat being preferable for this process. ... Graham Newton
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" unless you have carefully tested the characteristics of your oven. Most oven thermostats don't go low enough, or don't provide good enough temperature control. Whatever you choose, DON'T use a gas oven... a gas flame generates quite a bit of water vapor, which is exactly what you're trying to get rid of.
It's important that the tape be packed smoothly before baking. Chances are it will be if it's been cared for as a master tape should, but if it needs to be re-packed, this should be done by winding the tape at play speed on to another reel using a tape deck on which the heads can be removed, and with the tape threaded so that it doesn't pass over any fixed (non-rotating) guides.
Baking time ranges from about 4 hours for 1/4" tape to 8 hours for 2" tape. It's not critical. You can't over-bake unless you leave it for a day or so and if you under-bake and the tape is still gummy, you can bake it more. After you shut off the heat, leave the tape to cool down to room temperature before running it through the deck again.If you want a more elegant solution, check your local appliance shop for a Faberware (or equivalent) convection oven, but make sure it's large enough to accommodate the size tape reels you use. These run about $150 and might be a worth while investment if you have a large amount of tape to bake.
Ensure the temperature control is accurate by using a "lab" type thermometer to test the oven's performance BEFORE using it on valuable tapes.
DO NOT ATTEMPT TO USE A MICROWAVE OVEN... they are totally inappropriate for the job, and may be dangerous if used with metal reels.
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 errors).