1. What are photographic materials made of?
Before understanding the deterioration of photographic materials, it's necessary to know the basic structure of a photograph: In general, a photographs consists of a base, with an emulsion on top. Inside the emulsion are held the image-forming substances. This is true for the majority of types of photographs, be that negatives or positives. There will be techniques that has got more elements, such as binding layers or pigmented inter layers.
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base
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I will here first give a very short intro on the most common elements of photographic materials, and their properties from a preservation point of view. I will mainly concentrate of the major 20th century techniques, as I feel this type of material will be the major substance of photo collections in West Africa.
1.1 Base support materials:
Glass was widely used as base for negatives in the last century and well into the beginning of our. Any historical photographic collection will probably hold big amounts of glass plates. Glass has excellent properties as a photographic base, it's transparent and it's dimensional stabile. However, it can easily break during handling, if stressed or hit. Regarding the chemical stability, a break down mechanism called "glass sickness" or "weeping glass" is promoted by high humidity. Alkalies is washed out of the glass, leaving a whitish fog in the glass surface.
Plastics has been used for negative bases, and movie film bases for about 100 years.
The first plastic was nitrocellulose (also "nitrate" or "celluloid"). This plastic is deteriorating at high RH and temperature, with the release of nitric gases. This gives a very heavy chain reaction of further film deterioration and just about corrosion of everything else nearby. The smell of nitric vapours in an archive will be a warning of this. A totally decomposed nitrate film will crumble to dust. More dramatic is the danger of self ignition, if larger quantities of nitrate is kept together. Nitrate on fire is impossible to stop, the blaze being explosion-like. This is especially a problem for collections of film on spools, and large quantities of nitrate film should always be considered an explosive, and stored accordingly to this. Nitrate was in use until about 1950.
Cellulose Acetate ("safety film") was introduced in the 1940'es, and took over from nitrate because it, opposite to nitrate, can't self-ignite. It has, however, over the years shown that acetate film isn't stable either. Again, a high RH and/or temperature will accelerate a deterioration where acetic acid is released, which then will engage in a further break down. The result is "the vinegar syndrome": the film base shrinks, why the emulsion falls of the base. This is often found out because of a strong vinegar smell in the archive. Often acetate materials are divided into the early type "di-acetate" and the more recent "tri-acetate". "Di" is much less stable than "tri", however, also tri-acetate will deteriorate over time.
Today Polyester is regarded as the most stable plastic base material known today. For some products it's identified as "Estar". For any new photographic negative material it should be emphasized to use polyester materials. Polyester is by the way also regarded as the most stable plastic material for storage enclosures.
Paper is one of the most common print support materials. As a support for positive images it was introduced in 1840 and is still in use today. Paper consists of cellulose fibre, which can be oxidised by atmospheric oxygen, causing deterioration. The products of oxidation can give rise to cracking of the molecular chains. The same occurs if the cellulose is exposed to acid or formaldehyde, which may have been added to the paper during manufacture. However, the same effect can come from air pollution and chemicals from fixing and development processes. A contemporary print product is the resin-coated photographic paper (RC). The encapsulation of the paper with thin resin layers greatly reduce the development process time. However, as an archival material RC-paper is less stable than the old-fashioned Fibre Base Paper.
1.2 Emulsions:
Gelatine: The most common 20th century emulsion binder is gelatine. It is a pure, clear adhesive extracted from animal bones, especially calves feet and ox bones. Gelatine is very sensitive to humidity and swells when it absorbs moisture. Strong alkalies or acids can break it down, originating from air pollution or "bad" housing materials. Since gelatine is an excellent nutrient for bacteria, photographic emulsions can be irreversibly damaged if stored in warm and humid surroundings.
Historic collections might hold other photographic techniques, where albumen (made from hens egg white) and collodium (nitro-cellulose dissolved in alcohol/ether) also will be found used as emulsion binders.
1.3 The image-forming elements:
By far the most photographic techniques are based on metallic silver as the image-forming substances. In modern colour films and prints organic dyes are used to create the images.
Silver will be found as microscopic grains embedded in the gelatine layer. The more silver in a local area, the darker it looks. Silver has great affinity to hydrogen sulphide (H2S). Thus, black silver sulphide is easily formed. The attack manifests itself by the photographic plate or film becoming coated with a yellowish to reddish brown "fog" which subsequently turns black. Early photographic techniques will often be more sensitive to sulphide attacks, as their silver grains are smaller, than at modern silver techniques. But beyond this, in recent years we have come to realise that the silver particles are broken down by other oxidizing gases. The underlying mechanisms have not yet been fully understood. The ensuing damage often resembles the tarnishing just described.
The organic dyes which are the image-forming element in colour photographs are split in tree layers, a magenta, a cyan and a yellow layer. Together they form all possible colours in an colour image. Photographic dyes are damaged when exposed to heat, moisture or light. The rather complex dye molecules are broken down to colourless substances, why the colours fade away. Some techniques, as Ilfochrome, are more stable than others. Generally speaking new colour photographs will be more stable than older ones. However, no colour photograph will be as stable as a well-processed black-and-white photograph.
2. What deteriorates photographs?
2.1 Bad manufacturing:
While photographic paper and gelatine almost always are of a very high quality, much of the deterioration now observed in our archives origins from the use of other not-so-stable materials in the manufacturing.
Examples are the use of unstable base materials, as nitrate and acetate. Also the early colour photographs might consist very unstable dyes. The early RC-papers were especially vulnerable to light.
2.2 Bad Processing:
But even with stable products it's possible to end up with unstable products. By poor processing either the chemical reactions aren't done through-out (as under-fixing silver photographs), or chemical residues are left in the photographic emulsion because of poor washing. Both will be the cause of brown/dark discolouring of silver images.
2.3 An incorrect climate (high temperature and/or RH):
Standard organizations such as the American National Standard Institute ANSI or the International Standards Organization ISO has outlined storage climates sufficient for different photographic materials. The general pattern are, that the higher the temperature, the faster the speed of chemical reactions (as eg. deterioration mechanisms). Regarding the relative humidity (RH), many types of deteriorations are water dependent, why a high RH will start and accelerate decay. Also a very dry climate could cause deterioration, all though these problems will be physical ones such as brittleness.
The warning signs on a incorrect climate are many. Most of the times occurrence of the various chemical deterioration mechanisms already described will alert that the climate is incorrect. In the tropics, the problems will often be a too high temperature combined with a very humid climate. In negative collections, warning will be the smell of nitric or acetic acid. When single negatives are examined, the base will be brittle and discoloured (nitrate), or buckled with air bubbles under the emulsion (acetate). Glass plates with "glass sickness" will be greasy, possibly with a white fog and a loose emulsion. Prints or negatives kept in plastic enclosures in a humid very climate might stick to the enclosure, causing great damage to the image.
The most obvious warning of a too warm and humid climate is the presence of fungus and mould. Both the paper and the gelatine will be great nutrient for microforms, together with especially a high RH. The same goes for the presence of insects.
In a very dry climate such as desert regions, paper prints will experience serve curling. And glass plate emulsions might loosened from the glass base.
As already said, ANSI and ISO puts out guidelines for storage climate for photographs.
Without going too much in details here, the main distinction is between black-and-white materials and colour materials. For b&w materials an adequate climate is considered being 20°C or below, together with a RH between 20-50%. For colour materials the RH recommendations are the same, but the storage temperature is recommended as low as max 2°C.
This is very strict guidelines, almost impossible to follow without air-conditioning anywhere in the world. On figure 1 is shown how the natural climate from four different places on Earth correspond with the ANSI standard on storage of silver gelatin films (IT9.11). As it can be seen, most places will be too humid, and tropical zones also to hot, as is:
1. Figure borrowed from Padfield & Johnsen, 1996
Despite the very strict ANSI standards, some research has suggested that the safe climate limits could be loosened up. These more lose climate limits are suggested on figure 2, with the red trapez. (Padfield & Johnsen, 1996). Still, as seen on the graphs, very hot and humid places as for example Lagos will just be beyond any safe climate limit, regarding storage of photographic materials.
2. Figure borrowed from Padfield & Johnsen, 1996
The red trapez outlines an acceptable climate for storing b&w photographs. The lines marked "100yr" and "1000yr" marks a climate which should allow at least 100 and 1000 years archival life for new photographic materials stored constantly under these climatic circumstances
So for the intention of keeping photographs for extended-term storage in tropical climates, efforts must be made to lower the RH, and for colour photographs also the temperature.
There are several approaches to this, simple as more advanced. If choosing advanced air-conditioned systems, it's very important that the system actually will be maintained, and that one can rely on a constant power supply.
2.3.1 Below I will list a few different methods for controlling an archive climate:
The local placement of the archive in a house can to a limit give some control of the climate, especially the daily cycles. As the indoor climate will reflect the outdoors, it should be avoided to place the archive to near the biggest "radiator" - the sun. Avoid placing an archive directly under a roof, or directly to the most sunny side of the outer walls. Especially direct sun through a window should be avoided. As the building will buffer the RH more or less, an archive room in the middle of a building, with no outer walls will be a good start.
Desiccants: The use of various kinds of desiccants can be a good low-tech solution for keeping the RH down. For small confined spaces like boxes or drawers silica-gel could be an option, however, it should be noted that the silica-gel must be regenerated when exhausted (by heating in an oven to drive out the water). It's possibly to gets silica-gel types embedded in paper or polyethylene. Paper is, by the way, an excellent humidity buffer. In an archive with big bulks of paper the humidity could be kept quite stable by this, although not lowered. If silica-gel is used in larger spaces, the space must be somewhat sealed, because the inflow of moist from the outside will exhaust the silica-gel too quick.
Air condition should only be used in tropical areas if the system also de-humidify together with the cooling. Because of the temperature dependencies of relative humidity of air, humid and warm air will soon reach 100% RH when cooled down, that is: unless de-humidified also. Actually I would say that it's better not to air condition at all, than only to cool the air. If there's regularly power failures in the area, the climate will cycle too much because of the on-off switching of the air conditioning, again why air conditioning maybe isn't always the best idea.
Cold storage: For smaller collections, especially collections of unstable materials as colour slides or negatives, it can be a solution to use an ordinary commercial deep-freezer as storage area. As RH will be almost 100% in a freezer, different systems are invented to keep photographs cool and at a moderate RH. This can be plastic bag enclosures with desiccants, or the use of slightly heated inner boxes.
Please see the reference list for further info on this.
2.4 Pollutants:
Many substances are constantly emitted from building materials, furniture, storage materials, and from some photographic materials themselves. Some of these substances can have a deteriorating effect of photographs, in high concentrations.
Silver images are especially vulnerable to sulphur-containing pollutants, why sources to this should be eliminated in archives. These sources can be fresh oil-paints, natural textiles as wool, carpets, and rubber. From outside sulphur pollution can originate from car exhaust fumes, and burning of oil.
Organic acids and formaldehyde will emit from wood and woodboard products, and will attack gelatine and paper in photographs. Also acetate film is vulnerable to organic acid, as this triggers the vinegar syndrome, with more emission of acetic acid from the film themselves. Nitric vapours will cause a heavy deterioration of photographs. Often the source of nitric pollution is the photographic material it selves, why cellulose nitrate film materials should be stored separately from other photographs. In collections of historic photographs, this also goes for collodium materials, both plates and prints. Nitric vapours can also emit from cellulose nitrate used as adhesive or lacquer on old boxes or furniture, and from car exhaust fumes from the outside.
Peroxides are especially a great concern for photographic silver images, such as resin-coated prints, negatives and microfilms. Peroxides is emitted from fresh paint, and from "bad" storage enclosures. One warning sign of peroxide attack is the formation of red spots in the emulsion of microfilms, the so-called blemishes.
2.5 Storage materials:
Materials used for enclosures must especially be inert, as they are in direct contact with the photographs, and will therefore be a main factor of the possible deterioration of the photographs. As for the storage climate there is standards regarding the properties of a "good" storage enclosure for photographic materials. The outlines of these are:
- That the materials used for an enclosure must be chemical inert
- must be free of acids, sulphur, and peroxides, and have a pH around neutral ( 7)
- should not be able to do physical harm on the photographs
- should pass the Photographic Activity Test or "PAT" (*)
(*) PAT is a standardized test method, which tries if a material is sufficient for storage of photographs. Many manufactures of photographic enclosures will refere to if their products has passed the PAT, as a quality mark.
There are mainly two materials used for photographic enclosures: plastics and paper. The above criteria goes for both types, however, there is off course differences in their properties.
Plastics: Types of suitable plastics are high density polyethylene, polypropylene, and polyester. The above criteria rules out plastic as PVC, unfortunately this sort is very often used as a enclosure material anyway. PVC holds a large amount of plasticizer, which will bleed out during the ageing of the PVC. Common for plastic enclosures are that they are very diffusion tight, why for example acetic acid from degraded acetate negatives gets trapped, causing extra heavy harm. Also the phenomenon "ferrotyping"; the photographic emulsion sticking to the plastic sleeve, is a risk when using plastic enclosures, especially when the climate is very humid.
Paper consist mainly of cellulose fibres from plants, an adhesive, and some fill-out material as clay or chalk. The best quality cellulose for archival enclosures are pure cotton fibres. Care should be taken with paper made of wood-pulp, as cheap ground wood-based paper will hold lignin, a substance which makes the paper acid over time. Also the glue used for holding an enclosure together should meet the above requirements, therefore rubber glue should for example be avoided. There are envelope designs which requires no glue at all, as the four-flaps-envelope. Also the "glassine" type of paper made transparent should be avoided, because of it's acid content.
Despite of the disadvantage of paper; that you can't see through it, I would still recommend paper rather than plastic enclosures because of the risk of the emulsion sticking to the plastic, and because of the much higher permeability of paper. This risk is especially high under tropical conditions.
Damages from "bad" storage materials on silver images will show as either blemishes (peroxides from non-archival paper), or fading of the image, the silver image becoming faintly yellowish. Also the so-called silver-mirroring can often be directly related to bad enclosures, such as the glue at the edge of an envelope. In very serve acid attacks, the paper base and the gelatine will also be brittle, and probably discoloured (yellow-brown).
Finally, all other items that very well might damage photographs should be removed. This can be item as rusty paper clips, or rubber bands.
2.6 Light:
When exhibiting photographs, an extra deterioration factor should be considered: Light radiation. Light is energy, why it will be able to break down molecular bonds in the photographic materials. Where silver images are considered quite light stable, the organic dyes in colour photographs generally are not. This will result in fading of the image, often the yellow areas first giving the image a bluish tint.
Also organic materials as gelatine and cellulose can be deteriorated when exposed to large amounts of light, and especially modern prints coated with plastic layers might experience problems, such as cracks and yellowing.
The basic rules for mitigating light induced damage is:
- Lowering the UV-radiation. Therefore avoid direct sun light, use curtains for the windows.
- Low light levels: No more than 50 lux for fragile materials, and 300-400 lux the most for other materials.
- Limit the exposure time. As light deterioration is accumulated, it's not recommended to exhibit photographs for unlimited time.
2.7 Biological attacks:
A relative humidity above 75% will promote the growth of fungus and mould. As the spores are everywhere, this growth will always occur. The damage this can cause is irreversibly. The only sure method to prevent micro organic growth is to maintain a moderate RH. However, if this is not possible, it seems that some mitigation can be gained by circulate the air in the archive, eg. with a fan. Microorganisms often grows at places with very little air movement, as behind cabinets, shelves, and boxes. Surveys should be done quite regularly in very humid regions, for detecting growth of fungus or mould.
Insects will especially be a danger for paper, such as silverfish, bookworms and termites. The warning signs are quite obvious, such as parts of the photographs simply is eaten away. As for the microorganisms, high temperature and RH will be good living conditions for insects, why air condition to some extent can lower the risk of attacks. However, a very effective and basic prevention method is good housekeeping, as removal of dust will remove food and hide-places for the insects. Mechanical prevention as nets in front of windows and air-inlets will also stop some insects.
If fumigation or extermination with insecticide is considered necessary, it must be emphasized to try to limit the use of insecticides, so the treatment goes specific towards the species of insects found in the archive. This gives the best effect, and the lowest risk of creating a resistence in the insects. If there is a problem in an archive it is therefore important to collect insects and get the species identified, and deliver this information to the extermination company which is in contract.
Besides this, great care should always be taken with fumigation and use of insecticides, as they will migrate from the archival materials to us, the users, when handling the materials. This might not be a problem for the one-time users, but for staff working everyday in an archive the exposure to poison can be quite high.
2.8 Disasters is a threat which must be taken very serious, as the result of disasters often are immediate and complete deterioration of a collection. I will not go into detail of the various kinds of disasters; fire, water damage, natural disasters, theft or vandalism, etc. However, on a general plan it must be emphasized that it's vital to think through the possible disasters an archive or museum might face in the future, and then take actions to prevent the possible effect of such factors, so then if a disaster one day happens, the staff are prepared to act. A well-planned disaster plan can be the difference between failure and success if a disaster strikes.
3. Preservation strategy
As photographic collections often hold vast amounts of materials, the right preservation strategy is not single items conservation as much as it's preventive conservation actions, being beneficial for the lot.
I must say that my experience tells me that maintaining a moderate, suitable climate in the archive together with the use of proper housing materials are the main factors to delay deterioration.
Then, it's important to survey the collection on a regular basis, to identify "outbreak" of deterioration and finding the source.
If photographs are deteriorated, action can then be taken to either conserve the material or to preserve the image by other means. Methods besides traditional conservation can be making 1:1 duplicates, microfilms, or possible digital work copies.
4. Bibliography and Internet resources
4.1 Printed matter:
American National Standard for Imaging Media: Photographic Processed Films, Plates and
Papers -- Filing Enclosures and Storage Containers, ANSI/PIMA IT9.2-1998. New York: American National Standards Institute. (1430 Broadway, New York, New York 10018.)
American National Standard for Imaging Media: Processed Safety Photographic Films - Storage, ANSI/NAPM Standard IT9.11-1993. New York: American National Standards Institute. (1430 Broadway, New York, New York 10018.)
American National Standard for Imaging Media: Photographic Activity Test, ANSI/NAPM Standard IT9.16-1993. New York: American National Standards Institute. (1430 Broadway, New York, New York 10018.)
American National Standard for Imaging Media: Reflection Prints - Storage Practice, ANSI/NAPM Standard IT9.20-1996. New York: American National Standards Institute. (1430 Broadway, New York, New York 10018.)
Helen D. Burgess & C.G. Leckie (1994): "Evaluation of paper products to be used in the storage of photographic materials". Environnement et Conservation, de l'Écrit, de l'Image et du Son. Actes des Journees Internationales d'Etudes de l'ARSAG, 16 au 20 mai 1994. Association Pour la Recherche Scientifique sur les Arts Graphiques, Paris. pp. 29-35.
George T. Eaton (1985): Conservation of Photographs. Kodak Publication no. F-40, Eastman Kodak Company, Rochester, ISBN 0-87985-352-2. 156 pp.
Klaus B. Hendriks et al (1991).: Fundamentals of Photograph Conservation: A Study Guide, Lugus Publications, The National Archives of Canada and The Canada Communication Group - Publishing Supply and Services, Canada, ISBN 0-921633-80-7.
Jesper Stub Johnsen (1997): Conservation Management and Archival Survival of Photographic Collections. Göteborg Studies in Conservation vol. 5 (Jan Rosvall, ed.). ACTA Universitatis Gothoburgensis. ISBN 91-7346-318-3. 106 pp. + 8 extra papers.
Tim Padfield & Jesper Stub Johnsen (1994): "Preservation of Photographs in a Humidity-Controlled Freezer" in: Preventive Conservation Practice; Theory and Research, Summaries of the Posters at the Ottawa Congress, 12-16 September 1994. The International Institute for Conservation of Historic and Artistic Works (IIC). 1 p.
Tim Padfield & Jesper Stub Johnsen (1996): "The Breath of Arrhenius: Air Conditioning in Photographic Archives". in M.S. Koch et al (ed.)(1996): Research Techniques in Photographic Conservation, Proceedings of the Conference in Copenhagen, 14-19 May 1995. The School of Conservation, The Royal Academy of Fine Arts, Copenhagen. ISBN 87-89730-18-6. pp. 59-64.
Also found on the WWW: http://www.natmus.dk/cons/jsj/arrh.htm
James M. Reilly (1993): IPI Storage Guide for Acetate Film. Image Permanence Institute, Rochester. 24 pp. + 3 charts.
James M. Reilly (1998): Storage Guide for Color Photogarphic Materials. The University of the State of New York, Albany. 48 pp + 1 storage guide wheel.
4.2 Internet references:
"Caring for your Photographic Collection" by Debbie Hess Norris, at the homepage of Library of Congress, USA: http://lcweb.loc.gov/preserv/care/photo.html
"Care of Photographs, Technical Leaflet" at the homepage of Northeast Document Conservation Centre, USA: http://www.nedcc.org/phocar.htm
"Archival FAQ" and "Glossary of Terms" at the homepage of Light Impressions (Company of archival materials), USA: http://www.lightimpressionsdirect.com/
"The Image Permanence Institute" homepage (USA), among other things one sub-page explaining the Photographic Activity Test: http://www.rit.edu/ipi/
Monochrom (Company of archival materials), Germany: http://www.monochrom.com
"Wilhelm Imaging Research, Inc." a private company mainly researching in light fastness of photographic dyes. But also investigating the use of non-humidity-controlled freezer units to the storage of photographs and movie films: http://www.wilhelm-research.com/
"Conservation On-Line", the most comprehensive homepage on conservation and preservation issues. Also the sign-up place for the Conservation Dist List Internet discussion group: http://palimpsest.stanford.edu/
Thanks to Mogens S. Koch, The School of Conservation, for discussion and advice while writing this text.
E-mail: Morten Ryhl-Svendsen
From the roundtable discussions during the seminar
Africa links:
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