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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 Wilhelm’s “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 material’s 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 test’4 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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