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The preservation and restoration of photographic materials in archives and libraries
Klaus B Hendriks
INTRODUCTION
The purpose of the present study is to summarise the currently available knowledge on the preservation and restoration of photographic materials that may serve as a reference guide to maintaining, preserving and restoring photographic collections.
The existence of large quantities of photographic records in archives and libraries has created problems of preserving these records that are, to some degree, comparable to conventional paper objects, but also have special requirements due to their particular nature. The present study aims at outlining the principal photographic processes and describing the characterisation and identification of photographic images. It will discuss the factors that affect the stability of photographic materials, the examination of photographic records and the analysis of their deterioration. The duplication of black-and-white negatives and the copying of positive reflection prints will be explained as an important means of preserving photographic images. Such copy photographs must be processed in such a way as to obtain records of maximum possible permanence, and procedures will be outlined to achieve this. A second principal requirement is the prevention of damage and deterioration of photographic materials by keeping them in carefully controlled storage conditions which are conducive to their longevity. Once deterioration has occurred, however, it is sometimes possible to correct the errors of the past through restoration procedures which will also be outlined. Finally, some procedures for testing both the stability of contemporary photographic materials and the suitability of materials used in the conservation of photographs will be discussed.
The emphasis will be on black-and-white photographic materials based on the light sensitivity of silver halides whose image-forming substance, therefore, consists of elementary silver. Monochrome reflection prints made by non-silver processes, such as platinum and other carbon prints, or pictures containing certain metal salts and other pigments will also be mentioned. The scope of this study includes the preservation of contemporary colour photographic materials made by processes such as chromogenic development, silver dye bleach, dye imbibition and dye diffusion transfer (such as instant colour photographs).
11. GUIDELINES
11.1 General
The present study has emphasised the structure of photographic materials and their properties in terms of stability in order to create an understanding of the subsequent recommendations for preservation and storage. After accessioning and cataloguing, the overriding factors determining the longevity of these records are control of relative humidity, air pollution, temperature and exposure to light. The choice of a suitable filing enclosure is of crucial importance to the preservation of negatives and prints, as they are in direct contact with it. Original photographs, when used, should be handled in a sensible way as carelessness can cause irreversible damage. Ink must never be used to write on the back of photographic prints. If unique and valuable photographs are in high demand by users, duplicate or copy negatives should be made for use, while the originals are retired to inactive storage. Several conservation techniques developed for works of art on paper can be applied successfully to photographs. The restoration of discoloured or faded black-and-white negatives and prints remains of an experimental nature, but its further development will surely lead to firm procedures. The destruction of organic dyes in colour photographs through oxidation or hydrolysis is thought to be irreversible, and such pictures can be saved only by placing them into cold storage and by making duplicate copies.
The conservation of photographs is still a young field which does not have the body of knowledge available in older, more established fields, such as fine art conservation. While the testing of materials and experimental work have increased our knowledge of the stability of photographic materials during the past decade, many problems remain unsolved, but which will eventually become understood. Progress in the field is signalled by a number of recent events and developments, such as: the involvement of the American National Standards Institute (ANSI) as well as the International Organisation for Standardisation (ISO) in formulating specifications for the storage of processed photographic records; the investigation of the occurrence of redox blemishes in processed microfilms; the emergence of cold storage conditions as an immediate solution to the dark storage instability of chromogenic colour materials; the thorough investigations into the stability of resin-coated papers; the mechanism of image silver deterioration; the development of emergency procedures for photographs following a natural disaster; and the occurrence of seminars, workshops and conferences on the subject, such as the International Symposium on "The Stability and Preservation of Photographic Images", which was held in the fall 1982 in Ottawa, Canada, and sponsored by the Society of Photographic Scientists and Engineers (SPSE). The majority of problems in the preservation of photographic collections is under control. The quest for solutions to the remaining ones promises to be both exciting and rewarding.
11.2 Specific Recommendations
11.2.1 Identification:
Only complex scientific examination can replace extensive experience in visually recognising types of photographs. Simple and inexpensive experimental techniques include testing the surface in non-image areas of photographs with alcohol and water, and looking at the surface of photographs through a light microscope. While gelatin swells under a water droplet, neither collodion nor albumen react to it. Alcohol dissolves collodion, but leaves a gelatin layer unaffected. Albumen layers react with neither water nor alcohol.
11.2.2 Storage Conditions: Relative Humidity:
Relative humidity is the single most important factor affecting the permanence of photographic records. It must never exceed 60% in storage areas. The optimum storage relative humidity varies with the product type. A level of 35 to 40% is recommended as the value which best accommodates different kinds of photographic materials. Such level should be kept constant, i.e. daily or weekly cycling is to be avoided.
11.2.3 Storage Conditions: Temperature:
Photographic records must be stored at a temperature preferably not above 21°C (70°F). Daily or weekly cycling of more than 4°C must be avoided.
11.2.4 Storage Conditions: Lower Temperature Storage:
Low temperature will provide added protection. For colour film, a storage temperature of 2°C (35°F) is recommended. However, processed photographic materials can be kept at temperatures well below the freezing point of water (0°C; 32°F), provided the relative humidity is kept at recommended levels.
11.2.5 Storage Conditions: Air Purity:
Chemically reactive materials pose the greatest threat to the stability of black-and-white photographic collections, especially in the presence of moisture. The source of such chemicals can be the surrounding atmosphere, the photograph itself, residual processing chemicals and materials in direct contact with the photographs. They should be stored in a pollution-free area, i.e. in the absence of sulphur oxides, nitrogen oxides, peroxides and other oxidising agents.
11.2.6 Storage Conditions: Dust:
Photographs of all kinds should be stored in dust-free areas, as fine sand and dust may become embedded into gelatin layers and cause damage by abrasion.
11.2.7 Storage Conditions: light levels:
Well processed contemporary black-and-white photographs are essentially stable to light. By comparison, colour photographic materials are rather sensitive to long-term light exposure in the presence of oxygen and moisture. This should, however, not prevent a problem in storage, as photographs are usually kept in sleeves, boxes or albums and thus are protected from light exposure.
11.2.8 Storage Conditions: Filing Enclosures:
The correct choice of filing enclosures - sleeves or envelopes - can be made of paper or plastic materials. Paper enclosures are more difficult to evaluate. They should have a high alpha-cellulose content (preferably 90%), contain no mobile ashesives or sizes and have an extraction pH of between 6.5 and 7.5. Plastic enclosures should be made of uncoated polyethylene, uncoated cellulose acetate or polyester. For cold storage purposes a heat sealable envelope can be used that consists of a laminate of polyethylene, aluminium foil and paper. As it is not easily possible to write on most plastic materials, photographic negative and prints are placed ideally first into a plastic sleeve, which is then put into a paper envelope. The photograph can be looked at while remaining in its transparent enclosure, while all necessary information can be written on the envelope.
11.2.9 Storage Conditions: Inherently Unstable Materials:
Cellulose nitrate film base is the most prominent example of a material which can itself be the source of contaminating chemicals. As these materials can also be a fire hazard, they are to be stored separately from other photographic records. Apart from using the time consuming and labour intensive technique of emulsion transfer, cellulose nitrate film materials are best duplicated onto safety stock. They can be sealed in appropriate envelopes described earlier and kept in cold storage in order to provide temporary protection.
11.2.10 Storage Conditions: Boxes & Shelves:
Photographs in filling enclosures are normally kept in boxes on shelves. Boxes should be made of stable cardboard materials having properties similar to those described for paper envelopes. Stable boxes are available in North America which are made of stainless steel which is covered with an electrostatically applied polyester coating.
11.2.11 Processing of Contemporary Photographs:
Best results, in terms of permanence of the resultant image, are obtained by following meticulously the recommendations of the respective manufacturer. Certain residual chemicals must not be allowed to remain on the film or photograph. Recent materials may require toning as a post-processing treatment to achieve permanence.
11.2.12 Handling of Photographs:
When handling valuable photographs, white lintless cotton gloves should be worn in order to avoid producing finger-prints. These impressions, unless removed immediately, may cause irreversible damage to a gelatin-coated photograph.
11.2.13 Housekeeping
When working with photographs, no food or drinks should be tolerated in their vicinity. For reasons of keeping a clean, dust-free environment, smoking is not allowed in areas where photographic materials are handled.
11.2.14 Marking of Photographic Prints:
Information about a photographic print must never be written on it in ink, neither front nor back. In the occurrence of a flood, the ink may transfer onto the image side of the next photograph in a stacked pile, and become itself illegible in the process. If any information has to be written at all on a print, a soft pencil can be used. Applying too much pressure, however, may cause the writing to become visibly imprinted on the image side.
11.2.15 Display:
The two important choices which have to be made in the display of photographic prints are the selection of mount board and appropriate light levels. The former should have a high alpha-cellulose content, free of lignin and volatile chemicals. Smith (184) has published specifications for materials to be used in matting and hinging of works of art on paper, as well as techniques for these activities. Light levels for colour photographs on display should be kept around 100 lux. Normal display times, however, ranging from a few weeks to perhaps several months, are not expected to cause excessive damage to most type of colour photographs. Such damage usually occurs when chromogenically developed prints are exposed to light for many years, as often occurs in typical home and office situations.
11.2.16 Emergency Procedures:
If photographic materials become water-soaked following a natural disaster, they can be frozen in order to slow down dramatically further deterioration. Materials can then be freeze dried, or thawed and dried in a vacuum, or thawed and air dried. Glass plate negatives made by the wet collodion process should not be frozen and under no circumstances be freeze dried.
11.2.17 Fumigation:
Recent experiments have shown (185) that most photographic materials can be exposed to common fumigants without suffering changes to image density or gelatin stability. The fumigants include ethylene oxide, methyl bromide, thymol and p-dichlorobenzene.
11.2.18 Printing, Duplication and Copying:
Photographs are collected and preserved in order to be used for many purposes. Negatives are printed to provide positive reflection images. Valuable historical negatives which are in high demand may suffer from continued use and handling. They can be duplicated to produce faithful duplicate negatives for further use. Unique photographic prints can be copied with a camera to give copy negatives from which correct copy prints can be obtained. To perform these activities well, an understanding of photographic technology and sensitometry is required.
11.2.19 Restoration:
Many kinds of deterioration can be corrected through the application of restoration techniques. It is useful to remember, however, that such work should be attempted only be trained conservators. In any experimental work, where the outcome is unknown, only expendable photographic materials must be used.