X-Message-Number: 2652
Date: 18 Mar 94 22:14:13 EST
From: Mike Darwin <>
Subject: SCI.CRYONICS Videotape Longevity

Tim Freeman writes that videotape looses its "readability" within a
decade or less.  This is not so.  It takes longer but it does happen.  It
happens primarily because the magnetic material is bound to or "glued" to
the terpthlate substrate with a urethane adhesive.  In fact in some tapes
there may be five or six layers of material attached to the substrate. 
The substrate is more or less indefinitely stable, its the other part of
the tape that aren't.

The primary way that the urethane deteriorates is by hydrolysis so in
theory storing the tape under dessication should do the trick.  Another
possible consideration is liquid nitrogen storage since both audio and
video tape tolerate it well.  Unlike audio tape video tape does not
suffer from the "overprinting" phenomenon (where magnetic information
gets transferred from one "turn" of the tape to another over- or
underlying it) because the magnetic domains on video tape are smaller.  I
do not know if the overprinting effect occurs in audio tape stored in
liquid nitrogen but I would think they would.

All of my information here came from an excellent article by Hugh Hixon
written some years ago (but never published) in response to just these
concerns.  I spoke with Hugh today and urged him to dig it up and post it
here since it contained a great deal of useful information about tape
life, but was too technical to be suitable for publication in CRYONICS.

Recently, video tape manufacturers have started making archival quality
VHS and super-8 tape for taping weddings, babies, and so on.  We plan to
use this on future cases, dupe a copy on small super-8 tapes and store
them in liquid nitrogen along with the fiche of the patient's records
(probably in with the patients).  In the meantime storing the tape under
dry conditions packed in dry nitrogen gas at ambient temperature would
seem a good idea.  One question which remains is what effect DRYING the
tape out would have.  Perhaps we should do an experiment with noncritical
tape -- it would be easy enough to do.

Mike Darwin

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