Photography can be defined as any method producing a visible image by the inter-action of light with a layer of chemicals. Since the birth of still photography in 1839, photographs have been manufactured employing many different methods. About 40 of these methods have been used commercially and examples of the resulting images can now be found in great numbers and varieties in archives and library collections.

The development of a commercially successful system for recording and viewing moving images was the result of work by many people in the latter part of the 19th century. The first successful public demonstrations were given by Lumière in Paris in 1895. Since then many advances have been made including the introduction of sound and of colour. Many different frame rates and sizes of film were devised before the industry stabilised on to a few "standard" formats. A film collection still has to be able to handle films on many formats.

Microfilm

Microfilm was developed to secure original print and image material with special historical, commercial or scientific value. The use of microfilms can also improve the access to the information carried by the original documents. The use of microfilm for access will, as with other forms of access copy, help preserve the original by protecting it from wear and tear and from theft.

New printing techniques

The most recent developments are as a result of the computer revolution. New techniques have been developed using equipment such as ink-jet and thermal sublimation printers to produce copies of digitised images. These should be considered as printing techniques and not as photographic materials although they can provide a good representation of the original photographic image. Because of the short life expectancy and the sensitivity to light and heat, these printing techniques cannot be considered a substitute for photographic materials.

Original and substitutes

The best practice for photographic materials is to have several sets of images. The Original Image kept in ideal conditions and disturbed as infrequently as possible.

– A Safety Master used as a reserve copy. It should be stored in a separate place to the original in case of the loss of the original in a fire or some other disaster and also kept in good storage conditions.

– A User Copy Master made from the original or the safety master and used to make User Copies. User Copies for routine access to the images.

Though photographic images have been made in a great number of different sizes - from microfilms to large posters - the deterioration and preservation principles are dependent upon the chemical process used to make the image and not the size or purpose of the image.

Synopsis : typology

As the production of photographs has included many different chemical processes in the capture of the image, photographs also have a wide variety of ageing properties. Some materials were made of extremely self-destructive components, others were very sensitive to physical contact and almost every photographic material is sensitive to the environment, not only temperature, relative humidity and air pollution but also oxidising substances found in emissions from some building materials, wall paints and wooden furnishing. The card-board and paper in boxes and envelopes used for protecting the items from physical damage may also contain harmful substances.

Deterioration factors can be categorised in two ways - internal and external.

Internal deterioration factors are dependent on the components of a photographic item and the residual chemicals from developing- and post treatment processes. The speed of the decay processes is related to relative humidity, temperature and oxidising substances.

Cellulose nitrate films

The most commonly known example of a photographic material deteriorating from internal processes is cellulose nitrate film, which during deterioration emits substances that both accelerate the deterioration process as well as attacking materials in the vicinity. The main ingredient of nitrate film is cellulose nitrate which emits nitrous gases. The gases are not only oxidative but also toxic and explosive. In a self-accelerating deterioration process, the support - the film base - and the emulsion are eventually completely destroyed. What is left is a sticky substance. Cellulose nitrate film is flammable at fairly low temperatures and rolls of film, like motion picture films, might even self ignite at a room temperature as low as 41° C when kept for an extended period of time in a badly ventilated environment, for example in the traditional metal film can.

Cellulose nitrate film sheets do not self ignite in the same way because the mass per volume is much less and normally the emitted gases slowly evaporate away from the negatives when they are kept in envelopes and open boxes.

Cellulose acetate films

Another materials group exposed to self destruction is that of acetate film - the first safety film. Until recently, acetate film was considered as very stable but today the problem of the Vinegar Syndrome - the popular name for the deterioration of acetate film with the emission of acetic acid (vinegar) vapour as a by-product that acts to accelerate the rate of decay - is widely known. Still another example, although involving an old process, is the yellowing of albumen prints, where the egg white in the emulsion bleaches the silver image.

Acetate film was introduced in the 1920s as a substitute for the flammable cellulose nitrate film. It was labelled "safety film" as it was less flammable than its predecessor. The early acetate film lacked dimensional stability which made it shrink and loosen the emulsion from the support. The acetate base was improved and was considered more or less stable until the vinegar syndrome was discovered during last decade.

Colour photographs

Colour photographs - negatives, prints and transparencies - generally have bad ageing properties as the colour-components are unstable unless kept below 0°C. Photographic colour materials are not only subject to light fading - fading of the colours and image in the presence of light - but also to dark fading - fading in the absence of light. Transparencies are commonly considered to have better colour stability than colour negatives and prints but ageing properties may differ greatly due to different chemical properties.

Collodion

Collodion, one of the earliest photographic emulsion materials, was used in several similar photographic techniques during the mid-18th century, e.g. ambrotypes, collodion wet plates, pannotypes, ferrotypes and celloidin paper. The collodion emulsion contains cellulose nitrate (also used for the first "plastic-type" film base) and emits nitrous gases, though far less than cellulose nitrate film. These gases may attack other objects in the vicinity and, due to the loss of gas which leads to shrinkage of the emulsion, the emulsion may eventually crack.

PE or Resin coated papers

PE or Resin Coated papers are made from paper fibres covered with polyethylene with the gelatine emulsion outside the polyethylene layer. Until about the mid 1980s this photographic print paper had bad ageing characteristics. The paper base contained optical whiteners which absorbed light energy. An oxidising substance was formed which attacked the resin coating resulting in cracking. The oxidant also attacked the silver image and bleached it. During the last decade an anti-oxidant has been introduced and thus the resin coated papers now have improved longevity.

Microfilms

Microfilms have been and are produced using a variety of processes but the silver-gelatine developing-out film is considered to have the best long-term stability. Diazo- and vesicular processes are commonly used for making access copies but they do not have long-term stability and are not recommended for preservation copies.

Envelopes

External deterioration factors are harmful substances in the preservation environment. Among the many contaminants, a few should be particularly mentioned. Lignin, alum rosin sizing and oxidative residual chemicals in paper and cardboard used for envelopes, boxes and mounting boards as well as plasticisers in PVC-folders and similar storage media are the most common together with air pollutants.

Furbishing

Furbishing in repositories should not consist of materials emitting oxidising gases. Oxidising gases react with photographic materials in a similar way as common air pollutants.

High temperature and relative humidity accelerates these processes.

The external deterioration factors may co-operate with the internal factors to increase the reaction speed of the internal deterioration factors.

Materials with good initial ageing properties - i.e. with few internal deterioration factors - may last longer in a bad environment than an object with bad ageing properties - i.e. with many internal deterioration factors - kept in a good preservation environment.

Good storage conditions will counteract deterioration of materials with bad ageing properties to a certain point, while bad storage conditions will always accelerate deterioration processes.

The best way to preserve photographic materials is to emphasise measures on preventive care. The necessity of proper storage materials - envelopes, boxes, archive and library furbishing, etc. - and storage climate cannot be over estimated.

Climatic conditions

If possible a photographic collection should be divided and stored as two sets ; an active and a passive. The active set is for frequently used material - mainly copies of originals - and the passive set is for long term keeping of the originals. The passive set should have a stable climate with low temperature and relative humidity

A number of recommendations exist but they do not differ significantly from the requirements listed in the following table. These are weighted for a good cost/effectiveness ratio. The requirements can be difficult to achieve but must always remain the target. The target temperature and humidity readings can be relaxed provided that the conditions are kept stable and with the proviso that the humidity level is kept above 25 % and below about 60 % - the level above which moulds are encouraged to grow. The penalty in most cases is, however, a shorter life expectancy for the carriers.

A range of humidity levels are quoted for still images and microfilm. The humidity must not move outside this range. Any variation must not exceed the change of RH figure for 24 hours. For moving images, a target humidity level and figures for the maximum movement from this figure over periods of 24 hours and one year are quoted.

Basements and attics are usually not suitable for storing photographic materials. Basements are usually very humid and often accommodate plumbing which, if it starts to leak, may cause irreversible damages. Attics, if not properly insulated, will have an uncontrolled climate affected by the out-door conditions.

High temperature and high relative humidity (RH) accelerates most deterioration processes. The cooler the temperature, the slower the deterioration rate. The control of relative humidity is even more important in an archive or library with photographic materials. These types of damage may occur when the RH is too high :

• mould and fungi start to grow when RH rises above 60 %,
• the emulsion swells and get sticky,
• residual chemicals will accelerate deterioration processes,
• glass plates might start to deteriorate and the glass may turn foggy,
• deterioration processes caused by air pollutants, paints, etc, may accelerate,
• photographs on metal support, ferrotypes, may start to corrode.

The following damages may occur when RH is too low :

• the emulsion dries out and might flake,
• dry emulsion may fall off the support,
• film support may lose its flexibility.

Pollution

It may be difficult to keep the air in an archive or library clean since most major collections are usually situated in the centre of major cities. But it is nevertheless of the utmost importance to keep the areas free from air pollutants as possible. They are very reactive with substances in both b/ w and colour photographs. Listed in the following table are the requirements for clean air in photographic collections.

Other harmful substances exist in the air but good chemical filters customised for the substances listed in the table will control these as well.

If the collection includes any nitrate moving films, seek advice from the local fire authorities about the storage requirements, the maximum quantity of film that can be kept in one storage area and any other restrictions that they may require. This action is not merely good advice - it is essential. Nitrate movie film is considered to be an explosive by the fire authorities in many countries.

Photographic objects belong to a very delicate category of our cultural heritage which need special attention by trained personnel. Materials are susceptible to air pollutants, both fuel generated and emitted from furbishing and protective materials in repositories, as well as high humidity and temperature. It is important, therefore, to be in control of the preservation environment. It is also important to be able to identify the photographic methods represented in a collection and thus be aware of specific preservation problems.

Specifications, methods and measures for improving the preservation environment for photographic materials can be found in special literature and standards. Some of these are listed below.