As John Pytlak suggested "venting" the film during handling may alter the
AD Strip readings. Even the act of opening the film can may affect the
results as we found out in our first AD measurement project. I'd better
explain how this observation arose!
We tested a number of cans and measured the results as per the
instructions. For some reason I don't fully recall, we then went back a
few
weeks later and retested some of the original cans. All of the retests had
noticeably lower readings. None of the films were removed from their cans
or handled in any way other than opening the lids (a total of four times).
Returning to Kara's posting, when the films were shrinkage tested I would
assume that the films were fully rewound at least once. Air passing over
the surface of the film during a winding operation is a very efficient
method of assisting the diffusion of the acetic acid from the film. Hence
the handling did wonders for lowering the acid content of the film. So
what
about the shrinkage????
The formed acetic acid occupies a finite space within the film base, this
maintains some degree of dimensional stability (even though it is a bad
thing in the longer term). When the acid is removed by whatever process
the
cellulose structure contracts to occupy the vacated space (*sigh* howls of
protest from polymer scientists over this dodgy explanation!). This is in
addition to the loss of the residual solvents manufacturing agents as
described by John P. in his posting. So we have a situation where on a
second testing after handling the acid content is measured as lower (let's
hear it for Fick's First Law of Diffusion) but the shrinkage has increased
(is it correct to say "shrinkage increases"?!!).
I probably need to point out that the rate of this acid removal induced
shrinkage is hard to predict. Factors such as the wind tension and
internal
friction within the film pack, temperature and handling (and probably
others I can't think of at the moment) will all play a role. It may take a
long time or it may be very quick.
This raises a contentious issue of shrinkage with regard to storage
strategies that remove or help the diffusion of acetic acid from the film.
Firstly let it be stated loudly that the best strategy for long term
storage of film is low temp and RH. When this cannot be achieved due to
cost etc, then other options may be useful, vented cans, molecular sieve
etc. The question is - is it better to try and remove the acid and lower
the rate of deterioration AND suffer from shrinkage; or is it better to
leave the acid in place and reduce the impact of shrinkage BUT have a
potentially faster rate of deterioration?
My opinion (which I believe is sound) is - reduce the acid and try to deal
with shrinkage. I feel this is justified by the fact that the higher level
of free acid will, apart from increasing the rate of deterioration, wreak
havoc among the other components of the film (e.g. colour dyes,
plasticisers and ultimately gelatin stability). But I would love to hear
other points of view, and I am sure that there are very valid reasons for
this.
To Kara's final question re: a relationship between shrinkage and AD
results. As I mentioned before no - I don't think a definite relationship
can be determined. This is for resons such as wind tension, potential for
interaction between the internal film can environment and external
environment, the frequency of handling/access etc. Nevertheless I am a big
fan of AD Strips (well done IPI!) for the management of film collections
(the latter comment is not necessarily endorsed by NFSA, nor paid for by
Jim, Jean-Louis or any of the other gurus!).
mick
Mick Newnham
Senior Researcher
Preservation and Technical Services Branch
National Film and Sound Archive
phone: +61 2 6248 2118 | fax: + 61 2 6248 2165
McCoy Circuit Acton Canberra ACT 2601
www.screensound.gov.au
The National Film and Sound Archive collects, preserves and provides
access
to Australia's historic and contemporary moving image and recorded sound
culture and is a Division of the Australian Film Commission.