In order to appropriately care for photographic collections, it is necessary to:
- identify and document all photographic materials in a collection;
- examine each of the photographs and assess their respective conditions; and
- determine the most appropriate environmental conditions and storage/housing for all photographs in a collection.
The major factors that influence the deterioration of photographic materials are (Robb 2004):
- storage or display in inappropriate environmental conditions;
- the use of poor storage enclosures or housings;
- rough or careless handling; and
- poor processing techniques in the original production of some photographs (e.g. the presence of residual processing chemicals and the use of exhausted processing chemicals).
Unless a particular photograph or photographs are of major significance it is far better to focus attention and resources on improving preventive conservation measures than on costly conservation treatments of deteriorated photographs. Aim to improve the environments to which photographs are exposed, encourage appropriate care and handling practices and use archival quality materials for storing and displaying photographs.
Significant silver deterioration will occur if photographs are not correctly processed and washed. For example, if the image is not well fixed it will eventually blacken, while if residual fixer has been left then the silver image may fade accompanied by yellowing of the image, binder and support (Robb 2004).
In addition to suffering the same problems as other paper-based materials, photographic images have other in-built problems. Some of these problems include:
- the emulsion layer is particularly attractive to insects as a food source;
- gelatin emulsions are very susceptible to mould attack caused by high humidity;
- continual changes in relative humidity may cause silver mirroring;
- images on glass are easily scratched; and
- tinted emulsions may fade due to prolonged exposure to light.
The environment to which photographic materials are exposed is critical to their longevity and preservation. The damaging effects of light are well known. Keep light exposures at 50 lux or less with minimal exposure to UV radiation.
The effects of varying relative humidity levels are manifold. For example, high relative humidity levels may soften gelatin while low relative humidity levels may cause binders to shrink and crack and overall flexibility to be reduced. Dyes deteriorate four times faster at 60 % relative humidity than at 15 % relative humidity and halving the relative humidity at which cellulose triacetate bases are stored doubles their life expectancy (Lavédrine 2003). Many ink jet prints are particularly susceptible to damage if relative humidity levels exceed 50 %.
For a mixed collection of historical photographic prints, slides and negatives, or if the collection comprises only photographs, keep the relative humidity in the range 30 – 40 % with only a 5 % daily variation. Higher levels of relative humidity can be tolerated if lower storage temperatures are maintained (Table 1, Robb 2004).
Table 1: Relationship between storage temperatures and acceptable relative humidity levels:
|Storage temperature||Acceptable relative humidity levels|
|7 °C||20 – 30 %|
|- 3 °C||20 – 40%|
|- 10 °C||20 – 50 %|
Temperature control is also critical as higher temperatures will accelerate degradation processes. For extended storage aimed at long-term preservation, cellulose nitrate, cellulose acetate and chromogenic dye photographs require cool (10 – 16 °C), cold (2 – 8 °C) or freezing conditions (less than 0 °C). Use vapour-proof packaging to avoid increased relative humidity levels and condensation during low temperature storage (Robb 2004).
Air pollutants will damage many photographic materials. Oxidising gases, particularly oxides of nitrogen and ozone, particulate matter and environmental fumes are particularly damaging to silver-based photographs and some ink-jet prints. Separate photographic materials from pollutant sources including industrial pollutants, untreated wood products, varnishes, paints, poor quality storage materials, chlorine-containing cleaning products, bleaches and photocopiers (Robb 2004). Strategies for ameliorating the effects of pollutants are provided elsewhere (see the chapter Preventive Conservation: Agents of Decay).
Storage Systems and Enclosures
Common sense and a knowledge of compatible archival materials goes a long way when considering storage systems for photographic materials. Some general storage considerations include the following (Robb 2004):
- strike a balance between ‘tight’ and ‘loose’ storage of photographic prints. If prints are too loose they may curl while handling damage is increased if they are too tightly packed;
- stainless steel, anodised aluminium and powder coated steel shelving or containers pose minimal risk to photographic materials;
- never use uncoated wood pulp products, glassine or PVC materials to house photographs;
- use buffered archival quality paper or board (in the form of sleeves, envelopes, folders and document boxes) to store silver, acetate and nitrate prints;
- use unbuffered archival quality paper to store chromogenic prints, cyanotypes and architectural drawings;
- use plastic enclosures (polyester, polythene, polypropylene, spun-bonded polyolefins and polystyrene) to store frequently used collections. These provide support and protection against fingerprints;
- do not use plastics that contain large amounts of plasticisers or have fillers, coatings or UV stabilisers;
- keep rubber cement, pressure sensitive tape, paper clips and elastic bands away from photographic materials; and
- if it is necessary to mark a photograph use a soft lead pencil on the reverse side of the image.
The conservation treatment of photographic images is a highly specialised subject and is fraught with potential difficulties for the amateur. Do not attempt to clean photographs other than to brush them with a clean soft brush, proceeding from the centre of the photograph towards the edges. Consultation with a conservator is recommended for almost all conservation problems.
Aspects related to the conservation of specific photographic processes are described in the sections below.
Deterioration likely to affect daguerreotypes includes:
- tarnishing of the silver image;
- damage of the fragile image by poor handling when out of its frame;
- mechanical damage from the brass mount or broken glass; and
- corrosion due to high relative humidity levels.
Exercise great care when handling these images and do not attempt to treat them. Consult a conservator for advice and note the following points:
- the silver-mercury amalgam image is formed on the surface of the plate and can be wiped off if carelessly handled. Wear gloves and handle the plate by the edges only;
- protect the case by placing it in a four-flap paper enclosure;
- physical damage such as scratching is irreparable. Make photographically restored prints from damaged images;
- store daguerreotypes in a low relative humidity environment (45 - 50%); and
- if condensation is present on the inside of the glass cover, disassemble the mount and clean the glass with alcohol. Hold the daguerreotype plate by the edges when reassembling. If inexperienced, only carry out this process in consultation with a conservator.
Although thiourea silver dips have been used to remove tarnish from daguerreotypes, this treatment is not recommended. It is extremely hazardous as surface coatings may have been applied and the image may have been hand tinted.
Unless the tarnish is so bad as to almost obscure the image, do not attempt to remove it. Seek advice from a conservator.
Deterioration of ambrotypes involves:
- mechanical damage to the emulsion layer by scratching; and
- breakage of glass.
Methods of conserving ambrotypes include:
- minimising mechanical damage to the emulsion by placing a black acid-free card behind the image;
- protecting the case by placing it in a four-flap paper enclosure;
- if the image bearing glass is broken, do not attempt to glue it. Instead copy it and sandwich it between two sheets of modern glass; and
- removing dirt by using a soft brush or a moistened cotton bud. Do not use organic solvents as they may dissolve the emulsion.
Factors most likely to cause deterioration of tintype plates include:
- not housing them in a case or mount so they become bent, causing the collodion emulsion layer to crack;
- high humidity which causes the iron plate to rust, resulting in further blistering of the emulsion; and
- scratching, tarnishing and dirt that will damage unmounted tintypes.
The simplest way to conserve Tintypes is to mount them. A mount, comprising unbuffered acid-free backing board, spacer, window mount and a glass or perspex cover will protect the image.
All prints will suffer damage if:
- handled carelessly;
- exposed to inappropriate storage or display conditions such as high light, temperature and relative humidity levels; and
- exposed to insect attack.
Staining and foxing of prints disfigures them. Lignin in paper causes it to discolour and become brittle, while metallic impurities and localised mould growth can result in foxing. Light and high humidity will accelerate these forms of deterioration.
Silver prints are susceptible to damage from:
- sulphide formation;
- oxidation and reduction; and
- deterioration or staining from poor quality mount boards.
Sulphiding of the silver may result from impurities in the mounting material or the presence of residual processing chemicals such as the fixer (sodium thiosulphate). The result is a general fading and yellowing of the print or image.
Oxidation of silver is caused by the reaction of oxidant gases with particles of silver, converting them to highly mobile silver ions. These silver ions can migrate through the emulsion layer and if reduced back to metallic silver be redeposited in a new area. This results in fading of the image and colour changes. Sources of oxidant gases include industrial pollutants and poor quality mount boards.
The extremely small particles of silver in printing out paper are more susceptible to oxidative damage than the coarser particles present in developing out paper. In printing out paper the image tends to become more yellow and red whereas developing out paper tends to exhibit a bluish-silver mirroring effect.
Platinum prints and pigment prints do not usually fade. The main damage to these images is usually caused by the support material becoming stained or brittle. For platinum prints high temperature and relative humidity or pollution can cause image transfer with close contact mounts or covers.
Conservation of photographic print images is enhanced by:
- careful control of temperature, relative humidity, light and pollutant levels;
- the use of unbuffered, acid-free storage containers; and
- the use of archival quality, acid-free mounting materials.
Specialist conservators and some professional photographic organisations carry out photographic restoration work. Some problems that can be tackled include:
- the removal of mounts and remounting. Many photographic prints are brittle or may suffer if exposed to aqueous treatments. Removal of mounts is extremely difficult in these cases;
- the repair of physical damage such as that caused by insect attack or tearing. Repairs can be made using infill paper fibres, retouching and then coating with gelatin or methyl cellulose as the binder; and
- restoration of silver mirroring. Silver mirroring can be tackled by applying a gelatin layer over the top of the mirrored emulsion. This masks the effect and makes for easier copying.
Conservation specialists are required for these tasks, particularly in the case of important original photographic works.
Make copies before beginning any restoration work and use retouched copies for display if the originals are badly deteriorated.
Because of the catalytic effect of the platinum in platinum prints store these print types in polyester or polypropylene sleeves to separate them from direct contact with other photographic materials.
Some of the chemical treatments that can be used to restore images include:
- redevelopment; and
Glass Plate Negatives
The most common problems encountered include:
- scratching of the emulsion;
- breakage; and
- silver-gelatin may exhibit silver mirroring or the effects of inadequate processing.
Store plates on edge in an enamelled metal box. Other materials that can be used for boxes are unbuffered acid-free board, heavy duty polyethylene and foam core. Fit polyethylene channel of the same width as the glass along two adjacent sides of the plate, to allow them to stand separately and to avoid crowding. This allows an airflow between the plates and prevents them sticking together. Do not stack glass plates.
Store at 30 – 40 % relative humidity to minimise glass decomposition and flaking (Robb 2004).
Do not store cellulose nitrate near any other material as it is inherently unstable. In addition to giving off noxious gases as it deteriorates, cellulose nitrate has been known to ignite spontaneously. Seek specialist advice if you have a substantial collection of cellulose nitrate materials (see the chapter Modern Materials).
If stored in the correct environment modern negatives should last for hundreds of years. Careless handling is the most common cause of damage.
Although colour photography has been around since just after the turn of the century the initial additive process was cumbersome and did not gain widespread popularity. Colour photography gained popularity in the 1930s with the introduction of the chromogenic system, a subtractive process. In the 1960s the more stable silver dye bleach was reported but was not used for negative print material.
Digital output prints are probably the most common form of photographic imaging used by the general public. As such, ink jet, electrostatic and dye sublimation prints are likely to form the bases of many future collections. Ink jet prints are prepared when very small ink droplets are deposited on a paper or plastic support. The inks may be either organic dyes or pigments. Dye sublimation prints involve the use of a thermal transfer process to create images. Dyes are made gaseous and then condensed onto a coated paper that is specifically designed to receive the dyes. Dye sublimation prints are expensive in comparison with ink jet and electrostatic prints. Electrostatic prints are produced when heat is used to fuse a toner (containing resin and pigment) onto paper. These sort of prints are generally of lower quality than either ink jet or dye sublimation prints.
It is important to differentiate between true colour photographs and hand-tinted black and white images or pigment prints. Most pigment-based systems are quite tolerant to light exposure.
Colour photographs are inherently unstable. This is because the image is formed using organic dyes rather than metallic silver. Organic dyes fade in response to inappropriate light, temperature, relative humidity and pollutant levels.
Until the advent of digital photography, the most popular types of photographs were the colour negative and print system. These photographs are the least stable. This system, the chromogenic process, uses colour couplers during the processing to produce the dyes. As well as fading at uneven rates this process can exhibit yellow staining even when stored in the dark.
Colour slides are particularly susceptible to fading. Kodachrome materials fade in strong light but have excellent colour stability if stored in the dark (Robb 2004). The light fastness of inks used in digital printing has improved but there is still concern at the moisture sensitivity of ink jet inks and of their susceptibility to damage from pollutants.
A more stable system is the silver-dye-bleach system in which dyes are incorporated in the emulsion during manufacture and selectively removed during processing. This process is often used for making prints from colour slides.
While modern colour material has improved stability, images from the 1940s and 50s are often found in a severely faded or stained state. Photos from the 1960s and 1970s may also show severe deterioration, depending on their method of manufacture and their storage and display conditions. Some dye systems, especially those used in the early to mid-1990s, are very sensitive to light (Robb 2004).
Descriptions of the full range of colour material from the 1940s to the present, is too extensive for inclusion here. Obtain advice from photographic conservators and by consulting appropriate specialist publications.
Some basic guidelines do apply however:
- restrict light levels to no more than 50 lux for displayed items and limit the duration of exposure to light sources;
- exclude sources of UV radiation if possible. Otherwise restrict levels as much as possible;
- maintain relative humidity levels in the range 30 - 40%;
- as long as relative humidity is controlled, store colour materials in a cool place (16 – 20 °C). The lower the temperature, the longer colour material will last;
- mount prints with unbuffered acid-free board behind perspex or glass. Glass is cheaper but perspex does not break as easily. Never use chipboard, hardboard, plywood and particle board to mount prints;
- avoid direct contact with glass or perspex to prevent any transfer of the image; and
- maintain a high standard of cleanliness in display and storage areas to decrease the attractiveness of these areas to insects and rodents.
Correctly storing photographic prints, negatives and slides will increase their durability (Figure 6). Ideally each photograph should have its own enclosure to protect it against rapid changes in relative humidity, dust, insect attack, pollutants and excessive handling. Archival products are available from conservation supply companies and high quality photographic albums can be obtained from most photographic stores.
The following storage guidelines will increase the life of colour prints:
- never use self-adhesive or ‘magnetic’ albums for storing prints as they are harmful to the image;
- store slides in their boxes or in uncoated polypropylene or polyester sleeves;
- store negatives in uncoated polypropylene or polyester sleeves. Glassine or paper envelopes are not recommended;
- only use acid-free boxes or those manufactured from polyethylene, polypropylene or polyester and store these on enamelled metal cupboards;
- wrap large prints with unbuffered acid-free tissue and store them in an acid-free cardboard box, which should then be stored on a shelf or in a metal cabinet;
- if using paper envelopes to store prints, ensure they are unbuffered, acid and lignin-free;
- never use PVC plastics, rubber bands, paper clips and poor quality adhesives;
- do not store glass plates and cellulose nitrate material in plastic enclosures; and
- regularly inspect and clean storage areas to ensure there is no insect attack. Change tissue regularly as it will become contaminated with impurities from original mount boards;