RE: [AV Media Matters] cassette tape shelf life?

[Jim Wheeler <Jimwheeler@aol.com>]

I have never been concerned about floor vacuum cleaners because the motor
would always be several inches/feet away from the tapes.  A backpack vacuum
cleaner is something that should NEVER be allowed in a tape vault.

The U.S. National Bureau of Standards (now NIST) studied magnetic tape
erasure and found that a tape had to be less than one meter from a large
junkyard magnet before it had a noticable effect on low-coercivity tape.
The
report was NBS-500 as I recall.

I am more concerned with the RECORD button on tape recorders.  ALL tape
recorders in an archive should have a record lock-out of some type.  Only
the
archivist should have the key to enable the record function.  Also, some
headphones and microphones have magnets in them and should never be laid on
a
tape.  Sounds silly, but it could happen.

I use powerful rare earth magnets to perform tricks for kids.  These magnets
would have to be within a couple of inches of a tape to have an affect but
that is possible if tapes are lying on a table with the magnet.

Jim Wheeler
A magnetic engineer

Moderators Comment:
One last thing about this "magnetics" thread. There are really three things
to be concerned about - the strength of the field , the proximity to the
field, and the time of exposure to it. We have talked about the strength of
the field and there have been other emails over time on this list discussing
it - essentially it takes a very strong field to do any meaningful damage to
a modern high coercivity video or audio recording. Older recordings on low
coercivity media will become effected with a less strong field, but it still
is pretty strong. As mentioned elsewhere the inverse square law applies
here - and to us regular folk that means that the strength of the field
decreases with distance VERY quickly. Essentially an inch or two makes a big
difference, a foot or more makes a HUGE difference. The time that the media
is exposed to the field also makes a difference. A vacuum for instance would
presumably only have the chance of being anywhere near the tapes for a very
short time - maybe a second or less - this is very different from a tape
being stored on top of a stereo speaker for 10 years. While the motor MAY
have a stronger field even at close proximity the length of exposure will be
extremely short. A weaker speaker magnet could do much more damage over
time.

In the practical world there has only been one vault that I have ever
visited that I would have a concern about, and only one other that I know of
that I would have a concern about - and I have been in MANY vaults. The one
I visited had used shelving that was formerly in use in the rare book
collection. In that collection they used very strong magnetic bookends to
secure the books on the shelf. These magnetic bookends were used on the
shelving for many YEARS - and as a result magnetized the shelving - so the
shelves actually were magnets in certain areas - and actually they appeared
to be pretty strong ones. If tapes of LOW coercivity (early audio tape) were
stored on these shelves for a LONG time, it is possible that some damage
could be done. It is unlikely that they would be completely erased - but it
is possible for other types of damage to occur. The solution? The client
replaced the specific shelves - a very minor fix that probably cost them a
couple of hundred dollars.

The other vault that I know of but have not visited has a powerful
electrical isolation transformer for the entire company inside the vault
area. These transformers are used to regulate the power for the computer
equipment. In this specific case it is a rather small one - but in fact on
the transformer itself there is a warning from the manufacturer that there
are strong magnetic fields in close proximity to the transformer. This
clearly is not a good plan - but in this specific case the transformer is
more then 3 feet from any piece of media, and the media types it is near are
high coercivity media. Clearly not a choice location for many reasons - but
that is the limitation that must be worked with. Even in this situation I
doubt that any real damage will be done, and if it is done the length of
time that it would take there are other concerns such as obsolescence of
format, heat, dirt, binder breakdown and so forth. The strategy in this
case? Do the best you can - keep the masters as far away as possible - "use
copies" should be the ones closest (the least valuable tapes) preservation
masters and originals as far as possible from the field. At night the moving
shelves are cranked so that they are as far away as possible from the
transformer - and ultimately let management know that another site has to be
found at some point in time.

So as a rule of thumb I suggest - if there is a VERY strong field (like
maybe from an electrical transformer or a MRI machine or speaker magnets),
if the media is low coercivity and it is CLOSE to the field, and if the
media will be there for a LONG time - then be concerned - otherwise there
are other issues to consider that are probably more pressing.

Jim Lindner
NOT a magnetic engineer!