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A few extra comments and observations:

Humidity:

Polyester-base tape does not have any structural component that requires
water.  Brand new tape will exhibit some moisture content when tested but
water content in the tape can be considered a contaminant.  It is
conceivable that polyester-base tape can be stored at 0% humidity without
damage and no hydrolysis will happen- the tape should not "physically" decay
(Of course, dimensional changes associated with the reduced humidity can
cause serious pack-slip). Acetate-base tape will be damaged by 0% humidity.

Sealing containers:

No standard tape case is either water or air-tight.  This is done on purpose
to allow moisture trapped in the tape or case to escape.  Tapes have been
successfully, hermetically sealed but the process required pre-processing to
insure that the humidity level sealed in the container was very low.  One
process required a few weeks of pre-processing and the tapes were sealed up
under light vacuum conditions.  If you seal up a polyester-base tape that
still has a high moisture content you can actually accelerate the decay.

Cardboard vs. plastic cases:

It is unlikely that archives are going to replace all of their
cellulose-based boxes with plastic cases but, under certain circumstances,
plastic cases are better.  First, we have examined thousands of cardboard
tape boxes that have started to decay and deposit debris on the tape.  This
can cause playback problems, particularly with video and high-data-density
recordings.  Second, plastic cases provide substantially more protection to
the tapes if there is a disaster.  We have been doing disaster recovery of
magnetic tapes for over twenty years and tapes in plastic cases usually show
less than 1/2 the disaster-related exposure exhibited by tapes in cardboard.
The reasons are fairly obvious: cardboard provides little protection from
crushing or piercing damage, it burns fairly easily, it allows water to pass
through, it absorbs and retains water and it promotes fungal growth if it
stays wet for any length of time.

Peter Brothers
President
SPECS BROS., LLC
(201) 440-6589
www.specsbros.com

Restoration and Disaster Recovery Service Since 1983