submitted by James Elwing on 22.02.2006
James Elwing, Conservator, Archives, Powerhouse Museum, April 2003
Views expressed are those of the author and not necessarily those of the Powerhouse Museum.
[The object of this document is to introduce and explain the relatively new international standards for long-term retention of photographic images. International Standards Organisation (ISO) standards apply when there are no Australian standards covering an issue.]
THE CASE FOR LOW TEMPERATURE STORAGE OF MUSEUM OBJECTS:
For many years, standards for the storage of the majority of museum objects and archival records has been around stable 50% relative humidity (RH) and 21 degrees celsius (T) conditions, with set limits for RH & T movements.
While 50% RH is a good general moisture level, particularly for hygroscopic organic materials and mixed media, there has been a general understanding within the conservation profession that 21 celsius had been chosen for human comfort rather than collection longevity.
A modified environment; low temperature and humidity storage, should slow the rate of deterioration of many items, if only on a general application of Arrhenius theory. Arrhenius equation quantifies the relationship between storage temperature and chemical reaction rate (read rate of deterioration).
‘The rate of a reaction or process is proportional to a function involving 2 variables: temperature and activation energy.’ (Thomson).
The reactions causing deterioration will continue, if slowly, at virtually any temperature. In the absence of any other energy (eg light), however, heat will provide activation energy to initiate a reaction which otherwise may not have occurred.
Conservators use elevated temperature testing of materials like paper, to forecast the long term properties of that material. With regard to this activation energy, one of the recognised problems with accelerated heat ageing is that some deterioration reactions are accelerated by heat, others are initiated by it.
On a general application of Arrhenius theory, the lower the temperature of museum storage, the better. With moderate low temperatures, eg. 10° C, this is not a problem. With extended exposure to very low temperatures, I understand, some materials tend to become more crystalline, while in others, such as with the well recorded ‘tin pest’, an otherwise stable material can revert to a structurally unsound allotropic form.
With photographs, film or print, these problems are not an issue. Low temperature storage at stable low humidity has been shown to halve the rate of deterioration rate of photographs for approximately every 6° C drop in temperature. Within limits, low humidity itself slows the rate of image deterioration. Excessively low humidity causes shrinkage, therefore stress, of the gelatin/silver image-bearing layer common to film, glass plates and prints.
Low temperature and relative humidity is of great value to items experiencing rapid deterioration, including colour photographs and film based negatives and transparencies on cellulose acetate. Archives applies this principle for its frost free refrigeration storage, and we understand that some low temperature storage will be incorporated into a new store at Castle Hill, for photographs and other suitable materials including some plastics and appropriate objects (particularly those with short life expectancy.
d ln k = A
dT RT2 (Arrhenius equation)
Within museums, 21°C was a compromise created largely for the creature comfort of the public and staff within exhibition and work areas. The Arrhenius equation would have us operating at lower temperatures in low usage stores for the benefit of collection, and in winter, that is free.
For what it is worth, I would assert that the future of collections as a whole lies with cold storage. The Canadian Conservation Institute (Technical Bulletin 23; ‘Guidelines for Humidity and Temperature for Canadian Archives’ by Stefan Michalski) seems to take this view, as does the American Society of Heating, Refrigerating, and Air Conditioning Engineers, in Chapter 20, Museums, Libraries and Archives in their 1999 ‘ASHRAE Handbook: Heating, Ventilation and Air Conditioning Applications’.
Research based recommendations of this kind, for appropriate long term storage of photographic materials in cold dry environments have been with us for over 20 years (‘Preservation of Photographs’ Eastman Kodak 1979)
ISO STANDARDS FOR PHOTOGRAPHIC MATERIALS.
As mentioned, the object of this document is to introduce and explain the relatively new international standards for long-term retention of photographic images. International Standards Organisation (ISO) standards apply when there are no Australian standards covering an issue.
The standards offer a number of somewhat confusing alternatives for the same kind of material.
It was explained to me by one of the people who prepared the standards that this was to help institutions with existing environment modification (A/C or refrigeration) plant to meet the new standards with little or no modification.
The relevant ISO photographic standards are:
18911 for film (negative or transparency)
18918 for glass plates (negative or transparency)
18920 for reflection prints
18923 for magnetic tape
18925 for optical disk
10356 for cellulose nitrate film (negative or transparency)
14523 Photographic Activity Test (PAT) = 18916 (soon) [for enclosures and other storage materials]
The most important thing to keep in mind is that conventional photographic images are formed by very small particles or particle groups of silver which obstruct the passage of light. The silver is held by some clear material, normally a gelatin emulsion. This sits on a transparent base of film or glass for transparencies and negatives, or on an opaque base, usually paper, with prints.
ISO 18911 FILM
CELLULOSE ACETATE film base, black and white negative or transparency:
B&W silver gelatin on acetate film 2°C at 20-50% RH (relative humidity)
B&W silver gelatin on acetate film 5°C at 20-40% RH
B&W silver gelatin on acetate film 7°C at 20-30% RH
This table applies to the vast majority of black and white negatives and transparencies with an image bearing layer of colloidal silver within a gelatin emulsion, which are on some form of cellulose acetate support, what used to be called ‘Safety Film’ because it is not highly inflammable like the earlier cellulose nitrate films. Acetate film is still being produced, but breaks down with time, generating easily detectable acetic acid, hence the term ‘Vinegar Syndrome’ for such deteriorating film.
It is confusing because it gives three alternative specifications or approaches. Temperature is straightforward, however they are saying RH should be no dryer than 20% in all cases, to avoid physical stress through shrinkage.
However, the RH should not exceed 50% at a stable 2°C , or 40% at a stable 5°C, or 30% at a stable 7°C because each of these balances represent an acceptable, approximately equal, rate of deterioration as explained in my introduction. (eg Arrhenius’ equation)
POLYESTER film base, black and white negative or transparency:
B&W silver gelatin on polyester film: 21°C at 20-50% RH (relative humidity)
Of all the films, this is the only one for which long-term preservation may be served by normal museum standard air-conditioning. This is because the film base, similar to Mylar or Melinex clear plastic film, is stable. As before, RH should not exceed 50%, or fall below 20%. Polyester is a common base for modern sheet film (5”x4” or larger), some 35mm film such as Kodak Technical Pan, but is not commonly used for roll film (ie. 120 roll film), now called ‘medium format’ for Hasselblad and similar cameras.
CELLULOSE ACETATE or POLYESTER film base, colour negative or transparency:
colour, silver gelatin on acetate/polyester -10°C at 20-50% RH (relative humidity)
colour, silver gelatin on acetate/polyester -3°C at 20-40% RH (relative humidity)
colour, silver gelatin on acetate/polyester 2°C at 20-30% RH (relative humidity)
In this category, the most unstable element is the colour image, independent of the film base. Kodachrome transparencies and Cibachrome/Ilfochrome colour prints and transparencies are recognised as being particularly stable in dark storage, but such images have not been given separate attention, as in collections they are regularly interspersed with standard, less stable, colour images.
As before, recommended temperature is straightforward, however they are saying RH should be no dryer than 20% in all cases, to avoid physical stress through gelatin emulsion shrinkage.
In each case, the RH should not exceed 50% at a stable -10°C , or 40% at a stable -3°C, or 30% at a stable 2°C because each of these balances represent an acceptable, approximately equal, rate of deterioration.
ISO 18918 glass plates
B&W silver gelatin on glass plate 18°C at 30-40% RH (relative humidity)
The specification is different from film because the glass plate itself is relatively stable, hence the higher maximum temperature. However, the gelatin emulsion can separate from the glass more easily than from film, if allowed to get too dry and brittle, hence the 30% minimum RH rather than 20% with other film negatives.
The relatively dry 40% maximum RH relates to slowing the rate of deterioration, but also (my opinion) possibly older glass instability in higher humidity environments related to the presence of water soluble compounds such as sodium silicate (‘weeping glass’).
Glass plates with damaged, peeling or mould affected emulsions may not be suitable for this kind of storage.
ISO 18920 reflection prints
Reflection prints are on a variety of surfaces. Most older black and white emulsions are silver gelatin, with an image bearing layer of colloidal silver within a gelatin emulsion on paper coated with barium sulphate (‘fibre based paper’ or ‘FB’). After around 1970, most B&W prints are on resin coated (‘RC’)paper, where layers of polyethylene (‘polythene’) on both sides protect the paper from processing chemicals and speeds processing. Properly processed FB prints are still considered the archival medium because the facing polythene (PE) layer of RC prints includes titanium dioxide as an opaque layer, which catalyses deterioration of the PE layer in extended display .
[Common colour prints are on similar paper again using silver halide / gelatin technology in a number of layers. At some point in the process, the silver has been removed and replaced with a different dye for each layer.]
B&W silver gelatin 18°C at 30-50% RH (relative humidity)
silver dye bleach 18°C at 30-50% RH (relative humidity)
dye/ silver diffusion transfer 18°C at 30-50% RH (relative humidity)
dye imbibition 18°C at 30-50% RH (relative humidity)
pigment 18°C at 30-50% RH (relative humidity)
diazo 18°C at 30-50% RH (relative humidity)
chromogenic dye 2°C at 30-40% RH (relative humidity)
all others -3°C at 30-50% RH (relative humidity)
As before, temperature is straightforward, however they are saying RH should be no drier than 30% in all cases, to avoid physical stress. In this case, both gelatin emulsion (or other image bearing materials) and paper are humidity sensitive. For most, the RH should not exceed 50% at a stable 18°C.
Chromogenic dye black and white prints are chemically similar to colour prints, transparencies or negatives, and thereby require similar storage conditions, 2°C between 30% and 40% relative humidity.
Although I have not had this confirmed, I assume that ‘all others’ includes most early technology prints (eg collodion and albumen prints) from the 19th Century.
A common rider to these standards states that they are not appropriate for deteriorating images which require proactive treatment and/or specialised storage.
ISO 18923 for magnetic tape
magnetic tape on polyester 23°C at a maximum 20% RH (relative humidity)
magnetic tape on polyester 17°C at a maximum 30% RH (relative humidity)
magnetic tape on polyester 11°C at a maximum 50% RH (relative humidity)
BUT not below 15% RH (relative humidity)
not below 8°C
Magnetic tape is thought to have some problem with crystallisation of the subbing layer attaching the magnetic recording strip to the plastic support if tape is stored below 8°C. Otherwise, as with the previous standards, higher humidity storage is at the expense of lower temperature and vice-versa.
ISO 18925 optical disc
optical disc max 23°C at 20-50 % RH (relative humidity)
not below -10°C
not below 10% RH (relative humidity)
ISO10356 for cellulose nitrate film (negative or transparency):
An explanation is not included here.
ISO14523 Photographic Activity Test (PAT) = 18916 (soon) [for enclosures and other storage materials]
Materials available in Australia which have passed this test, and are therefore unlikely to harm photographs by contact are on the National Archives of Australia website ( http://www.naa.gov.au).
See also, Glass Plate Negatives. What are they? How do you clean, preserve, and archive them?
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