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On 2015-04-07 3:18 PM, Eric Jacobs wrote:
> 2-inch tapes with sticky shed may not respond as well to baking as
> 1/4-inch.
>
That is true in the sense of in the same or similar time frame.

I don't think there are precise formulae for predicting the time either 
to achieve thermal equilibrium or to achieve moisture equilibrium in a 
tape pack. Vos (1994) inspired me to develop a rule of thumb that 
moisture equilibrium appears to take 1500 times as long as thermal 
equilibrium in a one-inch tape, based on my extrapolations from his curves.

We have long suspected that the width of the tape was a large modifier 
of this ratio. I based my estaimate on Vos's graphs which seemed to 
indicate that a 1-inch tape pack, might achieve thermal equilibrium 
might in 100-200 minutes while it might take 100-200 DAYS to achieve 
moisture equilibrium. I felt that a factor of 1440 implied far too much 
precision in the calculation, so I rounded it to 1500.

Further pointing to this is what Stuart Rohre has reported on the Ampex 
mailing list and elsewhere. He has been responsible for retrieving the 
most information possible from some 1-inch instrumentation tapes which 
are 15-inch diameter tape packs on glass precision Corning reels with no 
windows. The windowless reels further slow moisture diffusion. He had 
originally said they were baking for several days and could get through 
about half the tape and then had to rebake, but they also had to run the 
tape through their Bow tape cleaners. Partially at my suggestion and 
partially on his own initiative, Stuart found that if he baked the tapes 
for 30 days, they would play through without the need for any tape 
cleaning or re-baking and he was getting very clean signals off the 
tapes at that point.

So, are the two-inch tapes not responding to baking or simply in need of 
more of it?

One 7-inch reel of 1/4-inch tape that had been exposed to high humidity 
cycles overnight had a very easy-to-remove mag coat when first 
inspected. When it was stored in my air-conditioned home (minus the 
economizer cycle bringing in Los Angele's famed "Marine Layer" of "night 
and morning low clouds") for 3-4 months, the same test that initially 
showed mag coat removal could not be duplicated and the tape binder 
seemed very secure at that point.

The "more baking" concept pertains to tapes like Ampex 456, 406, and 407 
as well as the instrumentation tapes made by Ampex at about the same 
time. It may also apply to Scotch 226 and 227 and possibly Scotch 250. 
It probably does not apply to Agfa tapes which have some of their own 
nastiness.

This web page attempts to categorize tapes by degradation modality, and 
degradation modalities are currently described more by what can 
ameliorate their effect than by the actual chemical/mechanical failure 
modes. My decade-long goal of a "pool-test kit" for tape degradation 
measurement is farther in the distance than it was when I started the quest.

http://richardhess.com/notes/formats/magnetic-media/magnetic-tapes/analog-audio/degrading-tapes/

Cheers,

Richard
-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada                             647 479 2800
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.