The ISO recommendation not to go below 15% RH is based on contraction of the
tape thickness that can lead to loose packs and slippage during handling of
material removed from low-humidity storage.  It is a "shot in the dark" RH
measurement based on anecdotal evidence and extrapolated tension concerns
derived from shrinkage formulas from a limited test base.  The low-end RH
limit has nothing to do with decay vectors in magnetic tape.  There is no
known component of polyester-base magnetic tape that requires any water
content in the tape at all.  The ISO technical commission had no
experimental evidence that storage of polyester-base magnetic tape at 0% RH
would have any negative effect on the tape other than the above mentioned
possibility of pack slippage.  Of course, there may be additives such as
lubricant or abrasives that could be effected but have not been tested for
low-humidity chemical changes. It should be noted that, from a
chemistry/decay point of view, no binder hydrolysis at all will occur if
polyester-base magnetic tapes are stored at 0% RH- an interesting
consideration for long-term storage.

Recommendations in the ISO document are a combination of "best case" figures
modified by practical handling concerns.  This means that, within the
guidelines given, tape should remain in good condition and should be safe to
handle.  It does not mean that the recommendations given are the absolute
best for any one specific concern- such as binder hydrolysis decay.

Note: the above mentioned opinions are my own as one of the primary authors
and editors of the ISO storage document and are not intended as representing
the commission as a whole.

Peter Brothers
(201) 440-6589

Celebrating 20 Years of Restoration and Disaster Recovery Service

> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:[log in to unmask]]On Behalf Of Walter Cybulski
> Sent: Tuesday, October 28, 2003 9:39 AM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] environment controls
> Thanks for the detailed response, Joe.   Lots of people are
> looking at that Technical Bulletin.  Walter
> >>> [log in to unmask] 10/28/03 08:40AM >>>
> Cooler and drier storage has been cited numerous times for extended term
> storage conditions of magnetic tapes and other archival materials.  ISO
> 18923 Polyester-base magnetic tape - Storage Practices is one source.  For
> a more thorough discussion on the benefits of lower RH and temperature, I
> would recommend the CCI Technical Bulletin #23 - Guidelines for Humidity
> and Temperature for Canadian Archives.
> As for 25% being the "ideal" target, it is not written in stone.  I guess
> 20-30% RH is the ideal range, but because the ISO standard does not
> recommend going below 15%, then 25% seems to be the best target
> (a bit more
> of a buffer to avoid going below 15% because of fluctuations).
> Joe Iraci
> Canadian Conservation Institute