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Re: [AV Media Matters] cassette tape shelf life?
The shelf life of any tape media, or even disk media, is a function of its
storage environment, its environment before it was stored, and in some
cases the formulation of the media.
In disks, (CDs) of the recordable type, there were the problems of green dye
vs. the gold dye. Early CDs used printing on the recorded side, that
affected the playability over time, and thus the unprinted recorded side
became the norm. (The problems were colors affecting the laser reflecting
off the recorded surface.)
In tapes, there was a formulation change that created a sticky tape syndrome
in the 80's, with tape manufactured for the instrumentation market and
possibly others. Originally natural components such as whale oil were part
of the formulations, and this attenuated moisture effects on the binders
between the base film and back coating, or oxide. When whaling became
politically incorrect, and the supply of the natural lubricant ceased,
synthetic formulas were tried that failed to prevent hydrolysis breakdown of
the binders over a 5 year period we have documented in our own collections.
Although the magnetic layer could survive, bleed through of binder products
of hydrolysis would clog guides and heads. In extreme cases, layers of
oxide would peel or flake off the base film as the tape was played.
One of the most easily implemented repairs, is to hand spool off any suspect
tape and see if you have layer to layer stiction, which can be a sign of
hydrolysis damage. If that is found, the tape is a candidate for the baking
process. Of course, if the tape has been played, and seen to shed right at
the beginning, it should be removed from a drive and treated before it jams
on the drive guides and heads and damages oxide.
Baking is straightforward if you have a consumer convection oven, or access
to a circulating air laboratory oven with electric heat that can be
accurately controlled by thermostat. 8 hours at 130 degrees Fahrenheit, (53
degrees C) will create a remarkable recovery of binder adhesion, and allow
the tape to be played at normal speeds and copied to another medium. After
baking, the polymers are allowed to re cross link for 24 hours before
playing the tape. We also take the warm tape into a sealed, moisture free
environment, to avoid readsorption of local humidity after the treatment.
Other industry users of baking have reported hundreds of tapes successfully
baked. Long term storage of tapes can be somewhat protected from local
humidity by use of solid sided reels for reel tapes, and tape seal belts.
In the cassette media, Library shelf storage boxes are available. Sealed
cabinets are another aid to tape/disk protection.
The makers of modern small cassette formats for digital backup are now
predicting 30 year or more lifetimes for their products in 4mm, 8mm and
other formats. Sony and other Japanese brands are listing that in the small
print with their product. I would expect that good quality backcoated,
audio and videotape formulations should also attain that lifetime with
careful storage and use. Poor handling, leaving a tape in a drive after
use, out of its box, etc. can defeat the best air conditioned and humidity
As to effects of the earth's magnetic field, it is only about 400 milli
gauss, which is well below the harmful level to modern higher coercivity
tapes. It is far more likely that any perceived effects of fields are due
to local fields from electronic and electrical equipment. One case I
recently investigated was somewhat unique. A computer lab had moved into a
facility that formerly had an electrolytic metal processing purpose, where
electric processes had used high currents and voltages in refining metal. A
large overhead craneway was found above the new computer room ceiling.
Its tracks had become magnetized over the years and was a source of a
magnetic field that disrupted computer screens in the facility. (The same
can happen from AC fields from electric metal bus ducts going to high
voltage transformer primaries.) If tapes had been stored there for some
time, possibly they could have been affected.
I conducted a number of magnetic storage room surveys in the days of 7 track
computer tapes, and found that a four inch sheetrock and wood framed wall
provided enough protection by spacing alone, to resist a 10 gauss field
magnet applied to the outside wall surface. It may be that very old audio
tapes are lower coercivity than the older computer tapes, but I would expect
local electrical problems to be a factor before the earth's field. Even
thin galvanized steel wall sheeting reduced the effects of 60 Hz electrical
switch gear on an outside wall of one storage facility below the earth field
component by our measurements. Mu metal is the industry standard shielding
material, but is not necessary with attention to archive construction with
sufficient wall thickness. Metal cabinets with doors will protect tapes and
magnetic disks from most environmental fields.
We do have audio cassettes that are in good condition in a residential
storage environment (no extra humidity control other than air conditioning),
after 30 years. (And these were not top of the line tapes in their day.)
Although my sample size is small, my observation is that users do the most
damage to cassettes, by not storing them in cool dry locations.
It is amazing to me that the hot humid environment of the automobile has
allowed cassette audio tapes to remain a viable entertainment media for
mobile use. The dust that can build up in a mobile player can be extensive.
Yet many users have successfully had cassette audio tape collections used in
their cars over time.
The short answer to cassette tape shelf life is IT DEPENDS on the total
environment during recording, use, and archiving.
Univ. of TX Applied Research Labs, Analog/Digital Lab