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Storage of cellulose acetate collections
Storage of cellulose acetate collections
The life span of cellulose acetate film is determined by a
number of factors. Some of these are intrinsic factors, such as the quality of
the cellulose acetate base (i.e. the presence or absence of residual solvents
or acids), the storage history, and the quality of the photographic processing
the item underwent. These factors cannot be controlled after the
event.
The storage environment is the major external factor that
affects the life span of cellulose acetate materials. It is widely accepted
that cool to cold low humidity storage, in combination with duplication or
copying programs, will be required to manage cellulose acetate collections. The
following section considers how adjusting storage conditions may assist in the
preservation of cellulose acetate collections.
Temperature
Lowering the temperature has the effect of slowing down any
chemical reactions that may take place inside the film, thus increasing the
life span of the film.
The most recent extended term storage
recommendations for processed safety photographic films2 are quoted by Adelstein (1999, p. 16):
| Image
|
Base
|
Max.
temp |
Relative humidity range |
| B&W silver
gelatin |
Triacetate
|
2ºC
5ºC 7ºC |
20-50%
20-40% 20-30% |
| B&W silver
gelatin thermally processed silver vesicular silver dye bleach |
Polyester
|
21ºC
|
20-50% |
Colour
(Chromogenic) Diazo |
Triacetate
Polyester |
-10ºC - 3ºC 2ºC |
20-50%
20-40% 20-30% |
However, Adelstein points out that
|
this table only applies to films that
are currently manufactured. Older films may have already undergone some acetate
or colour degradation
Consequently, more stringent conditions should be
used for such materials (Adelstein, 1999, p.16-17). |
Research indicates that as storage temperatures are lowered,
the life expectancy of film increases. A decrease in the temperature from 10
ºC to -12 ºC increases the life expectancy of the film by a factor of
100 (Adelstein, 1992, p351-352). Therefore, the lower the temperature, the
greater the benefit. However, maintaining a cold store at low temperatures is
expensive.
Australian institutions are currently using cool storage for
cellulose acetate collections at temperatures ranging from 2 ºC to 16
ºC. More details about cool stores in Australia are available in Appendix
1.
Relative Humidity
Relative humidity, or moisture, is a very important factor
when considering storing cellulose acetate film. Deacetylation of cellulose
acetate requires the presence of water. Water also combines with at least one
degradation product of plasticiser deterioration (diphenyl phosphate) to form a
strong acid:
triphenyl phosphate + H2O > diphenyl
phosphate + phenol diphenyl phosphate + H2O > strong
acid
|
| (Edge, 2000, p.40) |
As the polymer degrades, its affinity for water increases
and degradation accelerates. As the pH of the base decreases, the pH of the
gelatin emulsion decreases also. As the pH falls, the gelatin swells and may
absorb more moisture at any given relative humidity (Sheppard, 1927, p.708;
Rose, 1977, p.64). Gelatin is very susceptible to physical damage and mould
growth under conditions of high relative humidity. However, the gelatin
emulsion requires the relative humidity to stay above a minimum level of about
20%, otherwise it is subject to excessive and irreversible drying and
subsequent embrittlement. There is also a danger that the emulsion will shrink
and delaminate from the base.
Relative humidity is an important factor even at very low
temperatures. If film is stored in high relative humidities it will slowly
equilibrate to that high humidity irrespective of the temperature. (Wilhelm,
1993, p.701). As the moisture content of the film increases, the temperature at
which the getatin will become sticky lowers, with the possibility that at
humidities of 70-75 %RH the gelatin will become sticky at room temperature. At
these high humidities it is also likely that the gelatin will be susceptible to
physical damage and mould growth (McCormick-Goodhart, 1995, p.68).
|
It has been suggested that at temperatures below
freezing, the moisture content of the film and the relative humidity of the
storage environment may not matter. In fact, there could be serious problems.
If a film were stored at 0º F (-18º C) with an ambient relative
humidity of 95%, the film would eventually reach equilibrium with the 95% RH
air. The moisture content of the film would then be essentially the same as if
it were stored at room temperature at 95% RH. Upon removal from the freezer,
the film would stick together, swell, and likely support fungus growth.
Films and prints must always be protected from excessive humidity, regardless
of the temperature (Wilhelm, 1993, p.701). |
Geographical location may also be important since local
climate affects the internal conditions in buildings which do not have
temperature and humidity control systems. Many institutions have
air-conditioning systems to moderate outside temperatures. However, unless
relative humidity is also controlled, internal relative humidities during the
summer months could be higher than outside levels. As the warm outside air is
cooled, the relative humidity of the air rises. This is because cool air holds
less moisture than warm air.
For example during January in Sydney, the mean minimum
relative humidity may be 62 % and the mean maximum daily temperature may be 26
ºC. If air at 26 ºC and 62 %RH is cooled to 21ºC, the relative
humidity rises to just over 80 %. During January in Melbourne, the mean minimum
relative humidity may be 50 % and the mean maximum daily temperature may be 25
ºC. If air at 25 ºC and 50 %RH is cooled to 21 ºC, the relative
humidity would rise to about 63 %. Additional dehumidification of the air is
needed to bring the internal relative humidity within acceptable storage
limits.
In practice, the relative humidity inside a building is
influenced by a number of other factors as well as outdoor temperature and
relative humidity.
|
The final indoor relative humidity depends on a
complex set of factors including the outdoor temperature and relative humidity,
ventilation rates, building insulation, internal heat load
, moisture
added to the air by people and other sources, solar heat load, type of air
conditioner and many others
It is not uncommon, however, for the indoor
relative humidity in an airconditioned building to actually be higher
than the outdoor relative humidity (Wilhelm, 1993, p.549) |
Horvath found that collections which had been exposed to
excessive relative humidities at some point in the past were much more likely
to exhibit signs of deterioration than collections which had not been exposed
to humidity stress.
|
Many of the collections faced with serious
degradation were produced and/or stored in geographic areas with a high average
relative humidity or the specific storage history of individual collections has
included some degree of temperature and humidity trauma at some point in their
lifetime before arriving at an institution (Horvath, 1987, p.
50) |
Temperature and humidity averages for Australia can be found
at the Bureau of Meteorology website at http://www.bom.gov.au/.
A psychrometric chart3 is
commonly used to predict how relative humidity will rise or fall as temperature
changes.
| The risk that relative humidities may rise
dramatically when air is cooled is also the reason why cold storage facilities
should be designed to fail safely. If dehumidification equipment fails, the
cooling system should automatically shut down. Otherwise, as the air cools, the
contents of the storage area may become saturated with water. (Wilhelm, 1993,
p. 711-712) |
Fail-safe automatic shutdown systems are further discussed
in Henry Wilhelms The Permanence and Care of Colour
Photographs, Ch. 20, p.711-12.
Fluctuations in environmental
conditions
Film materials have low thermal mass and equilibrate
relatively quickly to changes in temperature. However, moisture equilibration
is much slower, especially at low temperatures. Cyclical variations in
temperature and relative humidity, which are typical for both refrigerators and
freezers, are not a problem as long as the cycling is moderate and fairly rapid
the materials equilibrium moisture content will not change quickly
enough to be affected, but will remain more or less stable at the average.
Enclosures further retard moisture equilibration, and
therefore help to maintain a stable environment. Long-term variations, such as
seasonal fluctuations in an uncontrolled relative humidity environment, are
more serious because they will cause fluctuations in the equilibrium moisture
content of the material. They should be avoided as much as possible. Sealed
enclosures may be used to protect the materials from these changes (This is a
general recommendation for hygroscopic materials, see Bigourdan & Reilly,
1997, p.7).
Enclosures
Choice of enclosure materials is an important aspect of
providing storage which will improve the life expectancy of cellulose acetate
film.
|
Open or permeable enclosures may offer a small
benefit to film that has already started to degrade, but they can only be used
when the macroenvironment is optimized. (Bigourdan & Reilly, 1997 p.
i) |
This is because they permit evaporation and diffusion of
acetic acid, thereby slowing deterioration.
If macroclimatic humidity control is not available,
enclosures, in combination with adsorbents, such as silica gel or zeolites, can
be used to create beneficial microclimates, to buffer and protect materials
from fluctuations in temperature and relative humidity, and from condensation
during warm-up, and/or to segregate deteriorated material.
Enclosures should be composed of inert materials that will
not release chemicals that may affect the photographic materials. A
photographic activity test4 has been
devised which can be used to evaluate materials under consideration (Adelstein,
1999, p.17).
To protect film from excessive moisture, it should be sealed
inside a vapour-proof enclosure. Two methods which have been used to achieve
this are the FICA system and the CMI system. The FICA system uses enclosures
that have a very low permeability to moisture vapour, consisting of fused
layers of polyethylene, aluminium, plastic, and sometimes paper. The enclosures
are heat-sealed, and a moisture preconditioning procedure is generally
recommended. Disadvantages listed by McCormick-Goodhart are that the packages
are not reuseable, it is difficult to monitor the condition of the material
because the packaging is opaque, and preconditioning the material increases
time out of storage5.
The CMI system was developed by McCormick-Goodhart to
improve on the FICA design. He uses two zip-lock low-density
polyethylene (LDPE) bags, one inside the other, with a moisture buffer between
the two (eliminating the need for preconditioning). A cobaltous chloride
humidity indicator, which is visible though the outer PE bag, indicates when
humidity levels are getting high (McCormick-Goodhart, 1999, p.20). He estimates
that the cardboard buffer will need to be replaced every 15-20 years inside a
freezer environment6.
Adsorbents: Zeolites and silica
gel
There are three major factors that accelerate the
degradation of cellulose acetate film: temperature and humidity, which have
both been discussed, and the amount of acid present. Initially, free acid is
generated slowly, the rate of reaction being governed primarily by temperature
and moisture. However, as the film degrades, the amount of acid present becomes
a more important factor in the reaction. The film will eventually reach its
autocatalytic point, where the reaction will continue by feeding on itself,
producing more and more acid (Reilly,1993). This autocatalytic reaction can
still be slowed if the temperature or moisture of the film are reduced. The
reaction can also be slowed if the amount of free acid around the film is
reduced.
Zeolites have been reported to have the advantage of
trapping both acid vapours and moisture. Silica gel may also exhibit similar
acid trapping properties (Bigourdan et al, 1998, p. 156 & 161) as well as
its well know ability to adsorb moisture. The adsorbants trap the acid by
holding the water that is bonded to the acetic acid. This, combined with the
adsorbtion of moisture in the sealed enclosure and the film itself, slow the
autocatalytic reaction.
Active ventilation (Section 8) is another way of reducing
the amount of acid available for the catalytic reaction.
(2) Photography
Processed safety photographic films Storage practices. ISO 5466.
International Organisation for Standardization, Case Postale 56, CH-1211,
Geneva 20, Switzerland.
(3) An on-line psychometric
chart can be found at
http://www.chemicalogic.com/download/psychrometric_chart_metric.pdf
(4) Photography
Processed photographic materials Photographic activity test for
enclosure materials. DIS 14523. International Organisation for Standardization,
Case Postale 56, CH-1211, Geneva 20, Switzerland; or American National
Standard for Imaging Media--Photographic Activity Test, ANSI Standard
IT9.16-1993. New York: American National Standards Institute, 1993. 1430
Broadway, New York, New York 10018.
(5) According to an earlier
paper by McCormick-Goodhart, sealing material in vapour-proof packaging at
moderate relative humidities before placing it in a freezer is an effective
method of ensuring that the relative humidity inside the package will not be
too high:
for humidity controlled
vaults which operate at 18ºC, a 30 to 35% RH environment does not
actually extract any more moisture from photo materials previously conditioned
to 45 to 50% RH at room temperatue. Thus, the micro-climate of a package sealed
under moderate humidity conditions at room temperature and then stored at
subzero temperature is essentially equal to the macroclimate found in a
traditional humidity controlled cold vault operating at 18ºC and
30%RH(McCormick-Goodhart, 1995, p.68).
(6) CMI packages are
available commercially from Metal Edge, Inc.; further details are available at:
http://www.wilhelm-research.com/Cold_Storage/cold_storage.html.
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