[ARSCLIST] Proper storage of mylar/acetate tapes

[Tom Fine <tflists@xxxxxxxxxxxxxxxxxxx>]

A client of mine is moving to new digs and has the opportunity to set up a better storage area for 
their tapes. This is a collection of 500 or so reels, all have been transferred to a managed digital 
storage system. About 400 of the reels are old brown-oxide acetate/mylar with a few odd cellulose 
base in the bunch. The rest are more modern backcoated types (about a dozen were trouble-era Ampex 
456 and required baking before transfer).

I read through the following:

AES E-Library: Tape Storage Problems by Radocy, Frank
http://www.aes.org/e-lib/browse.cfm?elib=283

Imation document
http://www.imation.com/government/nml/pdfs/AP_NMLdoc_magtape_S_H.pdf

Australian government:
http://screensound.gov.au/preservation/audiovisual_terms/audiovisual_item.php?term=Archival%20Storage%20Conditions

LOC:
http://www.loc.gov/preserv/care/record.html

There seem to be some conflicts about the following issues:

1. what humidity level is too low for acetate/mylar? And, what is the true optimum temperature 
range? The research done in the 50's by AudioTape (ie reaseach done by one of the manufacturers at 
the time the tape was being manufactured) indicates the 40-50 degrees F recommended in the LOC 
document may be too cold, especially at low humidity levels. The humidity level recommendations for 
older tapes seem to be in the 40-60% range, which is easily achievable in the client's new digs, but 
we want to make sure to get it right so the tapes don't curl any worse over time. My own experience 
in standard Northeastern US households is that acetate tapes hold up best (ie don't develop edge 
warp and terrible print-through) if stored in a cool, dry downstairs (with "dry" being, real-world, 
60-70% humidity usually, sometimes lower in winter months and "cool" being temp range of about 55-70 
degrees F). Obviously we are going to avoid a swinging range of temp and humidity because that is 
damaging to tapes, according to all research I've read.

2. the temp/humidity levels appropriate for mylar seem a little different from backcoated tapes, 
but, can the backcoated tapes "live" ok in a mylar-centric environment? In other words, humidity and 
temp may be a bit higher than sometimes recommended for backcoated tapes. The client could store the 
backcoated tapes in a separate area, but my thinking is, we know what tapes have sticky-shed and we 
know they'll need baking before playing in any case. The other tapes are 30+ years stable so 
unlikely not to be fine under the better-than-before conditions (ie from now on more constant temp 
and humiditiy).

I should add, we only had one vinegar tape and it was determined that the content was non-valuable 
so the tape was destroyed. The sticky-shed tapes did have valuable content but I got a very good 
transfer so it's unlikely they will need to be played again. The client is more concerned about 
keeping the analog material as a backstop if the digital archive ever fails.

Thanks in advance for your input. I am particularly interested in what storage conditions are being 
used for large collections of similar tapes.

-- Tom Fine