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How and why should I dehydrate my tape cassettes after long storage?

How and why should I dehydrate cassette tapes after they’ve been in storage?

Drying the tape is a big help in eliminating tape squeal, stiction, and stalling.  Without access to a vacuum chamber, we have many tape lovers that store their tapes (all formats) in plastic boxes with tight fitting lids.  The bottom of the boxes have a layer of silica gel.  Above that is an open mesh screen, above that the tape is stored.  Some people seal the boxes with packaging tape to prevent intake of moisture from the atmosphere.  Lacking a hard vacuum, the process of moisture removal is determined by the length of time for moisture within the tape pack to diffuse to the surface of the tape, and then move across the surface to the edge of the tape pack.  This can take many days, to weeks to accomplish.  However, by placing all of your tape into air-tight containers with drying agent, you will already start the process of rejuvenating your tapes – even before you apply the treatment.  In the case of cassettes, the diffusion process progresses much more quickly that in R – R and typical video tape formats.

Magnetic media that has been dried and subsequently treated with the preservative is remarkably resilient and can substantially resist the up take of moisture from the air.  I do not know of any cassette users that have required re-treatment.

For awhile, there were several firms that advocated “Baking” as the sole answer to problem tapes.  The tapes were inclosed in an oven and brought up to a temperture of 135 – 150 degrees F.  This process could mobilize moisture and speed up the diffusion process.  The “unintended” consequence of this process was loss of plasticiser from the tape and matrix (in which the magnetic particles are suspended).  This created a condition called “Gel Blocking” in which cooling that was too quick, shrunk the tape pack from the outside in and welded the inner third to half of the tape pack together.  BAD NEWS and some Very unhappy collectors.