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ScreenSound Australia
ScreenSound Australia
Screensound Australia is constantly undertaking research in
areas relating to cellulose acetate materials. Below are descriptions of four
such projects they are currently working on. For more information, please
contact Mick Newnham, email Mick.Newnham@screensound.gov.au.
The most common method of preserving film is by controlling
both temperature and relative humidity. It is high cost operation requiring
expensive to run and maintain plant. At best, this slows down the
decomposition. However, without benchmarks it becomes theoretical as to how
successful the established conditions are at controlling the rate of
decomposition.
The National Collection maintained by ScreenSound Australia
holds over 300 000 reels of film. Assessment of each object is clearly
impossible due to time and resourcing implications. A statistical approach was
needed, as was a non-destructive and repeatable method of assessment for the
selected objects.
A project was established to provide a bench mark from which
the effectiveness of the current storage conditions could be measured. The
project tested a statistically useful number of objects in specific populations
of our collection for a pass/fail result in relation to Vinegar Syndrome by
using IPI AD Test Strips. Then by using statistical analysis the test results
were extrapolated to the whole collection.
Once a benchmark had been determined an ongoing program of
repeatable testing over time to monitor the rate of change in these populations
was established. This provided management information regarding the relative
risks and rate of change of these risks.
One of the major issues in film preservation is the
decomposition of the cellulose ester polymers used as the film base. In short
acid by-products formed during decomposition tend to catalyse and accelerate
the reaction. Apart from using controlled low temperature and relative humidity
storage to slow the reaction, passively removing the acid by-products from the
decomposing film is another approach. In particular encouraging diffusion of
the acids away from the film base is considered an effective control method.
Film cans are comparatively well sealed and provide a
buffering effect for the internal microclimate. This sealing also reduces the
ability of the decomposition acids to diffuse from the film. Punching holes in
the sides of film cans to allow ventilation of the diffused gases is often
practiced. ScreenSound undertook research to determine how effective this
strategy is for the long-term storage of motion picture film.
Results showed that the "leading edge" of the can slows the
airflow and increases the pressure and the "trailing edge", with higher
airspeed, creates a proportionally lower pressure area. The air flows through
the can to equalise this pressure differential. The simple arrangement of two
holes aligned in the airflow proved to be the most efficient location for
creating the desired airflow through a film can and thus aid in diffusion.
In investigating the airflow through a ventilated film can
it was observed that in order to maximise airflow around the film, and thus
diffusion, space was required underneath the film pack. DanCan International,
in consultation with ScreenSound, undertook a project to design a structure
that would both raise the pack and improve airflow. The result of this project
is the "Danweb". ScreenSound is currently undertaking research to determine
wether this approach is effective in reducing the acid content within the
film.
Acid by-products are known to accelerate the decomposition
of cellulose acetate films. One of the methods believed to aid in the removal,
or increased diffusion, of these acids from the film is that of a low tension
or "preservation" wind.
It is commonly believed that such a wind allows for an
increased airflow and thus diffusion of acid by-products from the film.
Previous research has shown that airflow, even in a ventilated can, is minimal.
An almost saturated steady state concentration of acetic acid within the film
can may be reached, impairing the ability of the acetic acid to diffuse from
the film.
Other research suggests that gelatin acts as a buffer to the
acids escaping from the deteriorating base. The stabilisation of the base then
can be influenced by the degree to which the gelatin can absorb these acids. It
is hypothesised that as shrinkage of the film occurs over time even the current
preservation wind standards still place a great deal of pressure on the gelatin
which is believed to restrict the emulsion's ability to absorb the acid as it
is formed.
ScreenSound is investigating whether it is the restrictions
in the uptake of acetic acid by the emulsion due to pressure on the emulsion or
the ability of the film pack to more readily diffuse acidic decomposition
by-products in more loosely wound films that has the greatest impact in
lowering the rate of decomposition.
Last updated 17 June
2002
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