Photographs consist of an image layer on a base material. The image layer begins life in a light-sensitive form. When it is exposed to light a ‘latent image’ is created; this latent image is then made visible and stable by chemical development. For black-and-white photographs, the image usually consists of minute silver crystals suspended in gelatine. The majority of colour images comprise layers of organic dyes, also suspended in gelatine.
The gelatine can be coated onto paper to produce prints. Early prints were made on high quality, pure cellulose papers. Modern prints (from the 1970s onwards) are made on very thin paper with a polyethylene coating on either side, known as resin-coated, or RC, paper.
Gelatine emulsions can also be coated onto plastic films. This produces either negatives or transparencies, depending on whether the tonal or colour values of the image are reversed or correct.
Negatives are viewed through transmitted light and are used to produce positive images on paper. They come in a range of sizes, the most common being in rolls: 35mm and 60mm; and in sheets: 102 x 127mm (4" x 5"). Today, almost all negatives produced are on film, but during the 19th century glass was used for the base.
Transparencies can be colour (eg 35mm slides) or black-and-white (eg glass lantern slides) and the tones or colours are right reading rather than reversed. They are viewed through transmitted light via a projector onto a screen. Colour transparencies can also be used to produce positive colour prints.
Since photography was invented in 1839, many materials have been used and experimented with in the pursuit of the perfect image.
The earliest processes used a silver-coated copper plate with the image composed of silver and mercury (the daguerreotype) or fine artists papers coated with light-sensitive silver salts (the salted paper print). Subsequent processes included the:
Photographs are therefore varied, complex materials and have their own particular preservation requirements.
Photographs are highly prone to deterioration of various types. Deterioration can affect both the image layers and the support layers. The prime causes of deterioration in photographs are as follows.
If during processing the processing chemicals are not thoroughly washed out, then over time the residual chemicals can cause damage to the photograph in the form of staining, darkening or lightening the image. There is little that can be done about this deterioration except to slow it down with correct storage and handling. A reference copy should be produced before the deterioration gets any worse.
With black-and-white images, the silver that makes up the dark areas of the image can corrode when exposed to pollutants. This changes the shape of the silver crystal, which causes the photograph to reflect light differently and therefore look different. Commonly they will start looking yellow/brown in colour, giving the photo the classic ‘sepia’ tone.
Glass supports can break if dropped or bumped; they will deteriorate over time and become brittle.
There are a number of examples of this:
In all such cases the progress of this deterioration can be sped up by inappropriate storage materials and conditions, and slowed by choosing appropriate ones. See sections on ‘Handling and care’, ‘Protective packaging’, and ‘Storage environment’ below.
See ‘Handling and care of photographs’ below.
See ‘Protective packaging’ below.
See ‘Storage environment’ below.
Nitrate and acetate films pose serious problems for people looking after collections containing photographic negatives.
Both types of film will deteriorate irreversibly over time. The only way to save the images they contain is to copy them before significant deterioration has occurred. Depending on storage conditions, major deterioration can begin after as little as 20 years. Deterioration can be significantly slowed by putting the film into cold storage, ie below 10ºC. Doing so can buy time if it is not possible to copy the film immediately, for example if there is a large quantity of film to be copied.
Nitrate film was the first successful plastic film base, and was used widely from its introduction in 1889 until the early 1950s. It has been used for movie film and still negatives.
Cellulose nitrate film in the early stages of deterioration gives off a distinct acrid smell. At the same time the film sheet will begin to deform and the surface will develop a greasy quality. In later stages dark yellow/brown staining develops, the film becomes sticky, the image bleaches out and finally the film disintegrates. As the film deteriorates, it releases highly corrosive vapours (various nitrous oxides) that attack any objects near by and constitute a health risk for users. The film is also highly flammable, particularly when packed at high density, for example a motion picture on a reel.
Cellulose acetate film or ‘safety film’ was developed to replace the unstable nitrate film base. It became dominant in the 1950s and is still used in some forms today. It has been used for movie film and still negatives.
As acetate film deteriorates it releases acetic acid that gives it a vinegar smell (known as ‘vinegar syndrome’). As the deterioration progresses the base starts to shrink, causing the emulsion layer to buckle and pop off the base. It can also develop bubbles under the emulsion layer and white crystalline deposits on the surface. Ultimately the film will weaken and crumble.
Ideally both nitrate and acetate negatives should be kept isolated, as the vapours they produce, particularly those from nitrate film, can damage other collection material. Also, given that nitrate film is flammable, people storing large quantities of this film should ensure that it is stored in an area with adequate fire protection.
Like all archival material, photographs need careful handling to protect them from damage. The following are basic points that should be kept in mind when handling photographic material:
The type of enclosure used for storing photographic material is dictated by the nature of the collection. If the material is going to be referred to regularly, clear plastic enclosures may be best, as they allow the photographs to be viewed without removing them from the sleeve. If the collection is static, and not referred to often, paper envelopes may be the best choice as they allow the collection to ‘breathe’.
If plastic is the preferred option, polyester sleeves made specifically for photographic storage are the most stable choice. Polypropylene (PP) is also suitable and is more readily available to consumers. Polyvinyl chloride (PVC) sleeves and folders should never be used.
Paper envelopes should be made of material that is safe for use with photographic collections. ‘Acid-free’ does not necessarily mean ‘photo-safe’ – photographic collections require higher quality paper than paper-based collections.
The National Archives carries out a test known as the Photographic Activity Test (PAT) to determine the suitability of materials for the storage of photographs.
There is one important rule when choosing a photograph album – never use ‘magnetic’ self-adhesive albums, as they can cause severe damage to your precious photographs.
If it is necessary to place photographs in a display album it is essential that the album be manufactured from materials that are safe. Again the PAT is the best indicator of suitability and a number of albums have been tested by the National Archives and found acceptable. Details of these products are also included in the table at the link given above.
Slides can be stored in the plastic boxes they come in from the processor. If these are lost, archival quality slide storage pages can be purchased from reputable photographic supply shops. Again these should have passed the PAT; details of those products tested by the NAA and found acceptable are also included in the table at the link given above.
Large format material should be stored flat in boxes, folders or drawers.
Ideally photographic collections should be stored at a low temperature and mid-range relative humidity.
For a large collection of high historic value, a dedicated storage area with reliable climate control (temperature and relative humidity) is recommended. Humidity should be controlled slightly on the dry side, at about 35%. The storage temperature should be maintained as low as possible, every degree lower than standard room temperature results in a considerable life extension for photographic material. If it can be achieved the temperature can be reduced to near freezing.
For people without the resources to achieve these conditions, or with small collections, the best that can be aimed for is a stable environment avoiding high temperature and humidity levels. A cupboard inside a building with no contact with external walls is good, as it is buffered from daily temperature and humidity changes. Downstairs rather than upstairs is better, too, as it is usually cooler downstairs. Damp areas such as basements should be avoided.
It is a good general rule to keep storage and work areas clean and never have food or drink in them.
In the home environment:
For larger collections in agencies or institutions:
For frequently-used or significant material, it is a good idea to make reference copies. This means that the photograph can be used and copies shared without the original being over handled. It also means you have a digital scan or negative should anything happen to the original.
Scanning photographic material on a flat bed scanner is a fairly safe procedure. It has been calculated that the light level from a single pass on a typical scanner equates to less than half an hour of exposure to ambient light levels*. Multiple scans should be avoided if possible.
Original photographic material should only be exhibited under stringent environmental guidelines. Some photographic images are highly sensitive to light and can only be displayed in very low light levels. Copies, on the other hand, can be mounted and displayed without concern, as they can always be replaced. For better accuracy, black-and-white images can be copied in colour to recreate the subtle tone of the original.
*See Jean-Paul Gandolfo and Bertrand Lavédrine, Determination of light levels in a flat bed scanner using a gelatin-bromide photographic paper (pdf 227kb).